restoring an rca avr-11 ‘airport receiver’ – gerry o’hara

Restoring an RCA AVR-11 ‘Airport Receiver’ – Gerry O’Hara

The RCA AVR-11 receiver was introduced in 1937 by

the RCA Manufacturing Company Inc.1 especially for

airports and airline ground station services. It covered

‘Long Wave’ from 140kHz, through the ‘Standard

Broadcast’ band, ‘Medium Wave’, and the ‘Short

Wave’ bands up to 23MHz. Air navigation, weather

reports and some airport communications at that time

used the longer wavelengths and airports therefore

required receivers that includes these wavelengths.

However, higher frequencies were also starting to

come into general use, so coverage up to 23MHz was

also useful. Very few AVR-11 receivers were produced

and they are therefore considered rare and collectible.

The sales brochure for AVR-11 is reproduced in the

Appendix to this article which provides some useful

technical information as well as the usual sales ‘hype’.

The AVR-11 was supplied in a tall metal cabinet that

also housed the speaker mounted on a separate

panel above the receiver. The speaker sported the

most imposing and magnificent art-deco winged

RCA emblem I have ever seen on a receiver – a true

‘statement’ that you are using a high-end receiver

that RCA was very proud of (image on sales

brochure, above).

In addition to the AVR-11 model supplied in the

single (dual height) metal cabinet, it could be

supplied in two separate single height cabinets

(page 2 of the brochure – see Appendix). Two

other variants were also available: the AVR-11A,

designed for rack mounting, with a grey painted

front panel, the chassis of which did not have a

dust cover, supplied with a matching panel-

mounted speaker, and the AVR-11B, with a black

painted front panel, with a dust cover over the

chassis, and again supplied with a matching panel-

mounted speaker.

1 The RCA Manufacturing Company Inc. was a division of RCA that built communications and broadcast equipment

Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara

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The AVR-11 Receiver

The AVR-11 is a 16-tube single conversion

superheterodyne receiver that included many of

RCA innovations from the mid-1930’s, including

metal envelope tubes, a ‘Magic Eye’ tuning

indicator, a dynamic noise suppressor, automatic

noise limiter, and a separate RF sub-chassis (rather

esoterically marketed as the ‘Magic Brain’) that

included a ‘band-in-use’ dial mask and a slow

motion logging scale that enabled accurate visual

resetting of the dial to a particular frequency. Up

to 8W of push-pull audio was available, fed to an

8” electrodynamic speaker. The receiver covers a

140KHz to 23MHz2 frequency range in four bands,

omitting 420KHz – 530KHz due to the 460KHz IF

frequency:

Band ‘X’ (‘Long Wave’) – 140KHz – 420KHz Band ‘B’ (‘Medium Wave’) – 1.8MHz – 6.4MHz

Band ‘A’ (‘Standard Broadcast’) – 530KHz – 1.8MHz Band ‘C’ (‘Short Wave’) – 6.4MHz – 23MHz

The circuit is reasonably conventional for a high quality receiver of the period, comprising two tuned RF

stages (2 x 6K7), mixer (6L7), local oscillator (6J7), two IF stages (2 x 6K7) operating at 460KHz3,

detector/eye tube rectifier/noise limiter (6H6), two stages of AF amplification (2 x 6C5), push-pull audio

output (2 x 6F6), noise suppressor (6J7), AGC detector/amplifier (6R7), BFO (6J7), eye tube (6E5) and

rectifier (5Z3). The schematic for the receiver is provided in the Appendix.

The front panel controls (photo, below) comprise ‘Frequency’ (tuning), ‘Signal Input’ (RF gain – rather

optimistically ‘calibrated’ to 1uV sensitivity), ‘Volume’ (AF gain), ‘Fidelity’ (tone), ‘Range’ (bandswitch),

Power (Off/On), ‘Beat Frequency’ (BFO pitch), ‘A.V.C.-C.W. Selector’ (mode/standby), and ‘Noise

Suppression’ (threshold level adjustment). A single 600 ohms headphone jack is provided on the front

panel. Standby mode is indicated by a front panel light disguised to look like a second ‘Magic Eye’.

2 There are minor discrepancies in the claimed frequency coverage on some bands between the sales brochure and the technical manual 3 An optional crystal filter unit could be supplied. Instructions on how to install and set it up are included in the Technical Manual


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The complete AVR-11 receiver/speaker combination when house in its cabinet weighs in at 78lbs and

measures 23.125” high x 21” wide x

16.125” deep – quite an impressive table

top receiver by any standard.

The circuit and physical design of the

AVR-11 is similar to the

contemporaneous RCA ACR-111, which

was aimed at the well-heeled amateur

radio market, and, to a lesser extent, the

RCA 15K-1 high-end domestic receiver –

indeed, it could be considered as a

partial hybrid of these two chassis.

Articles that provide a technical

description and detail refurbishment of

these two RCA models can be found here

(ACR-111) and here (15K-1):

The circuit and main chassis layout (IF stages, detector, audio stages and power supply) of the AVR-11 is

almost identical to the ACR-111 with the exception of the crystal filter (standard on the ACR-111 and

optional on the AVR-11) and a few minor changes in component values, eg. the cathode resistors in the

IF stages, and switching arrangements. The main chassis of the 15K-1, being designed for high-end

domestic use, incorporated more ‘fidelity’ features, including adjustable IF bandwidth and more

sophisticated tone control, a push-pull 6L6 output stage delivering up to 35W of audio power feeding a

12” speaker unit, and the ‘Ultra Short Wave’ band (23 – 60MHz) was included to cover the ‘HiFi’ (wide

band) ‘Apex’ band transmissions4;

The RF sub-chassis (‘Magic Brain’) of the AVR-11 is very similar in design and layout to that in the 15K-1,

though with an additional RF amplifier stage and omission of the ‘Ultra Short Wave’ band5. The RF sub-

chassis of the ACR-111 was a more sophisticated unit that incorporated both the additional RF amplifier

stage and the ‘Ultra Short Wave’ band, together with electrical bandspread tuning using a separate 4-

gang tuning capacitor.

The AVR-11 could therefore be considered as a compromise design that RCA considered to best meet

the needs of the aviation industry of the period, ie. good sensitivity and image rejection provided by the

two RF amplifier stages, omission of the ‘Ultra Short Wave’ band that was not needed for aviation work,

4 In the mid-late 1930’s the ‘APEX’ stations were introduced in certain large US cities that provided wide-bandwidth, uncompressed, high quality audio AM transmissions. More on Apex stations here (“….Finally, starting in 1937, several radio manufacturers began to introduce models that could tune all the way up to the Apex bands. The Raco R-S-R Clipper and several McMurdo Silver models were among the first. That same year, RCA introduced its “Magic Brain” series of receivers which had a top band that tuned up to 60MHz. These early radios proved to be insensitive and unstable at those rarified frequencies.”), and here 5 The dial on the AVR-11 is identical to the dials used on the ACR-111 and 15K-1 receivers, including the ‘Ultra Short Wave’ band, though on the AVR-11 this section of the dial is never exposed through the mask as only four positions of the band change switch are available

https://sparcradio.ca/wp-content/uploads/2020/10/RCA-ACR-111-refurbishment.pdf

https://sparcradio.ca/wp-content/uploads/2020/04/RCA-Model-15K-restoration.pdf

https://en.wikipedia.org/wiki/Apex_(radio_band)

http://www.theradiohistorian.org/Apex/Apex1.htm

https://www.radioworld.com/news-and-business/a-detroit-apex-station-in-1936


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reasonable IF selectivity that was satisfactory for phone transmissions

used by aviation, as provided by the three double-tuned IF stages, with

an optional crystal filter available to provide additional selectivity

and/or heterodyne/adjacent signal rejection only if needed/specified by

the purchaser, noise suppression and limiting for potentially noisy

airport situations/atmospheric ‘static’, and fairly basic audio circuitry

feeding an 8” speaker well-suited to voice reproduction.

Comparison photographs of the main and RF sub-chassis of the AVR-11,

ACR-111 and 15K-1 are provided in the Appendix.

Restoration Strategy

The answer to one fundamental question needs to be answered prior to starting work on a radio: “What

standard/effort of work is wanted (or warranted)?” – this can range from simply repairing the chassis to

operational condition, through to a full restoration, the latter comprising a complete strip-down and

rebuild such that it looks and works much as it did when it left the factory. The usual level that I will

extend to for a radio is what I term ‘sympathetic’, which is around 75% of fully restoring the chassis.

By ‘sympathetic’, I mean that the set be working close to its original specification, and made to appear

generally as it would as it left the factory, but with some ageing/wear through use, and perhaps

retaining a few ‘battle scars’ acquired through a few years of use – both above and below chassis and,

perhaps, also the cabinet. This level of work would generally entail re-stuffing/replicating paper and

electrolytic capacitors, and replicating/replacing out of tolerance resistors where these components are

visible, replacing broken parts with suitable parts resembling the originals where possible (or recovered

from ‘donor’ chassis of the same or similar type), thoroughly cleaning the chassis and cabinet, repairing

broken mechanical parts/mechanisms, and then aligning/testing/troubleshooting the finished set.

‘Sympathetic’ restoration requires significant additional effort to that needed simply to render the set

operational , ie. ‘repair-level’ work (replacing only those parts that have failed to the point that the set

no longer functions properly), or to render the set operational and more reliable long-term, ie. including

a level of preventative maintenance, such as ‘shotgunning’ (full replacement) of all paper and

electrolytic capacitors and any out of tolerance resistors (but not by re-stuffing or reproducing these to

give an original appearance), in what I would consider ‘refurbishment-level’ work, which may or may not

include some level of cabinet repair/touch-up or refinishing.

In a ‘sympathetic’ restoration, however, I would not expect that every replaced/re-stuffed or

reproduced component, lead dressing and other cosmetic nuances to be indistinguishable from the

original, or that every blemish in the cabinet finish had been removed such that the set looked ‘factory

fresh’ as would be in a full restoration in the strictest sense. Rather, the work would be ‘sympathetic’ to

the look of the original, ie. the chassis would receive thorough cleaning, the mechanical components

cleaned and lubricated (where appropriate), capacitors would be re-stuffed where possible (the originals

may not be present), reproduction parts, eg. dog-bone resistors, would closely match the general

style/colour code method of the originals, and replaced parts, eg. transformers or chokes, should be

chosen, or adapted/modified, to resemble the style or form of the original, though with some ‘artistic

license’, eg. in the design of reproduction labels if the originals are unavailable or unknown. Also, a

judgement call would be made regarding any modifications that had been made to the set, ie. whether

to reverse them or not.


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In the case of this AVR-11, the owner, given the rarity of the set and its existing condition, (quite rightly)

requested a level of work that I would consider to be ‘high end’ sympathetic, bordering on full

restoration.

Background

This AVR-11 receiver was purchased in the greater

Vancouver area (BC, Canada) by its present

owner. Its condition was very rough, but a good

price was paid, reflecting how difficult this

receiver is to find today. It sat for several years

before the decision was made to bring it to a

condition close to as it was over 80 years ago. It

duly arrived in Victoria for this work in April, 2021

but had to await completion of several other

projects before work could start in June, 2021.

Preliminary Inspection

Some comments from initial visual inspection of

the radio on arrival (photo, right):

- The chassis metalwork on the upper side of the

main chassis and RF sub-chassis was in poor

condition and extensively pitted with rust (photo,

below). Although I could clean this up to look much better than was, I noted to the owner that it would

not be ‘perfect’ – I recommended to

the owner that I remove several

components, mask the remainder and,

after cleaning and degreasing, spray

the chassis with a steel-finish paint;

- The RF sub-chassis screen cover

(‘box’) was missing. The owner agreed

to ask a friend to check if there is a

spare one at the SPARC museum that

could be suitable, eg. off a scrap RCA

15K-1 chassis of the same period;

- The chassis metalwork on the

underside was in good shape,

especially in the RF sub-chassis (and its

lower screen cover was present);

- Most tubular paper capacitors on the

top of the RF sub-chassis (photo, left)

and underneath the main chassis


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(photo, right) had been

replaced with modern

plastic film capacitors,

likely within in the last

decade or two given the

type of replacements

used;

- Three electrolytic

capacitors had been

replaced on the main

chassis;

- No capacitors had been

replaced on the

underside of the RF sub-

chassis;

- No resistors had been

replaced on the main or

RF sub-chassis with the

exception of part of the

‘Candohm’ multi-section metal-clad power resistor (extreme right of the photo, above);

- The tubes were all in very poor cosmetic condition (I recommended a set of NOS RCA tubes be

obtained to improve visual appearance, even if the existing tubes tested ok);

- The workmanship associated with the capacitor replacements was poor, though the replacement

components used were likely ok. I agreed with the owner that I:

• remove all the replaced tubular and electrolytic capacitors and re-use them where

possible/appropriate in reproduction part bodies, or re-stuff the original bodies/cans

where these were still in place; and

• reproduce replacement resistors where possible and required, ie. when the original part

was out of tolerance and/or in a critical position in the circuit where an out of tolerance

value would likely adversely impact the receiver performance.

- Some rubber-insulated wiring was present and I proposed to replace this with reproduction rubber

insulated wire where visible above the chassis (tube grid

connection wires) using scuffed/’grimed’ PVC insulated wiring,

and cloth-covered wiring elsewhere where the rubber

insulation had degraded significantly and risked shorting to the

chassis or other wiring/components.

- The slow motion tuning shaft wobbled laterally in its bearing

(photo, left) due to considerable wear and tear over its service

life, say airport service for a decade or two, likely followed by


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radio amateur/shortwave listener use for a few decades. This issue would have to be resolved as it

would significantly detract from the operating experience of the set.

- Apart from the dial escutcheon being

absent and the dials being discoloured

and scuffed (photo, right), the front

panel cosmetics were reasonable, with

only minor paint scuffs and one small

area of missing paint exposing the bare

metal (steel). The owner had sourced a

replacement escutcheon and was

sending this to me, and I confirmed that

a replacement (reproduction) dial set

was available from Radio Daze. The

rear of the front panel was bare steel

and had areas of rust present. The

control fingerplates were generally in

reasonable condition, though the paint had worn/chipped off in some areas. Three of the knobs looked

like replacements, looking newer and having larger pointers than the others.

- The speaker grill cloth (felt) was in poor condition –

threadbare in places and looked like glue had been

spattered against it (photo, left). The speaker panel

paint was in generally good condition, though the

rear (bare steel) was badly rusted (photo, below), as

was part of the speaker frame and magnet clamp.

The speaker panel had been fitted with a switch

marked ‘Cabin’/’Deck’, with a flying lead and plug to

connect to a remote speaker. The switch wiring is

such that the ‘Deck’ speaker is the one fitted to the

panel and ‘Cabin’ is the remote speaker. The switch

is an unusual design, and switches all

the connections to the set, including

the high voltage field coil winding.

Restoration

Initially there seemed to be a dearth

of technical information on the AVR-

11, however, I finally found a copy of

the ‘Technical Information and

Service Data’ for this set here.

However, this is not the best of

scans, and while the text is all

https://www.radiodaze.com/

https://www.manualslib.com/


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readable, most of the component numbering and values are

illegible on the schematic and parts layout diagrams. I found

it necessary to cross-reference more legible scans of the

diagrams for the ACR-111 and 15K-1 , as well as the parts list

for the AVR-11 to ‘fill in the gaps’.

First Steps and Chassis Painting

RF Sub-Chassis

- I removed the front panel from the chassis and the RF sub-

chassis. It was soon apparent that the sloppy tuning action

was due to a worn bushing on the slow motion drive/tuning

control shaft: the inner concentric shaft of the slow motion

drive was a bit worn, but the outer bushing the main culprit.

- I removed the worn tuning shaft bushing and slow motion

drive shaft from the RF sub-chassis and a friend offered to

take the mechanism to a machinist he knew to see if a new

bushing could be made.

- I removed all the coil screening cans and

dial light shade from the RF sub-chassis

(part of the preparation for preparation for

spray painting) – photo, left;

- Neutralized the rust spots on the RF sub-

chassis and than polished it with

microcrystalline silica – this almost brought

the finish to being acceptable if it wasn’t for

three larger rust patches and some

persistent smaller ones;

- De-greased the chassis (again) with IPA;

- Masked-off coils, slugs, tuning capacitor, tube

bases etc.;

- Sprayed with two coats of non-etch grey

primer;

- Sprayed with two coats of chrome finish paint

(photo, right);


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- Removed the masking tape/paper;

- Worked on the tops of the coil screening cans with

ultra fine steel wool and polishing compound to remove

pitting;

- Left the RF sub-chassis alone for a few days for the

paint to cure before trimming any minor overspray,

plus some cosmetic improvements around the tuning

gang and front of the sub-chassis (behind the dial area),

etc. and then temporarily refitted the coil screening

cans (though not screwed down) to check on how the

finished sub-chassis would look - photo, right.

Main Chassis

- Tested the power transformer on the main chassis. It

tested ok: 350-0-350vAC HT and the 5vAC and 6.3vAC

windings also both seemed ok, however, I noted that

the transformer was drawing around 20W with no load

- a little higher than I would expect, though there was

no continuity between any of the windings and the

laminations, or between windings. I left it connected and monitored the transformer temperature over

several hours and it was still at room temperature, so I concluded that the power transformer was ok;

- Strangely, the power transformer primary had been disconnected from the power switch by someone,

and there was no line cord to the set, or even any

remnants of one – a bit of a mystery;

- Removed the ‘RCA’

plate from the rear

apron of the main

chassis – the chassis

was badly rusted

beneath (photo,

left);

- Cleaned and degreased the main chassis with

naphtha/IPA using Q-tips and cloths. Sanded badly-

rusted areas and then rubbed with wire wool;

- Neutralized the rust spots then polished the top

and sides of the main chassis with microcrystalline

silica;

- De-greased the chassis (again) with IPA;

- Masked-off the IF transformers, power transformer,

flying leads, tube bases, etc (photo, right);


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- Sprayed with two coats of acid-etch

grey primer;

- Sprayed with two coats of chrome

finish paint;

- Removed the masking tape/paper;

- Removed the power transformer top

shroud, removed/neutralized the rust

present on the shroud, primed it and

spray-painted it gloss black (photo,

below). I left the main chassis for a

couple of days for the paint to cure

before progressing the work.

I then worked on the tops and sides of the IF screening cans with

ultra fine steel wool and polishing compound to remove pitting,

and cleaned up any overspray I found on wiring or components on

both the main and RF sub-chassis. Next, I cleaned and replaced

the ‘RCA’ plate on the rear apron of the main chassis, and

replaced the (painted) power transformer shroud.

Electronic Restoration

Main Chassis – Part 1

First, I re-stuffed the three can electrolytics - the

two larger ones had an unusual construction

with the densest and most removal-resistant

innards (a dense, fibrous sticky mass sealed in

with tar) I have ever come across (photo, right).

I gave up trying to empty the cans of this

material and removed just the tar and enough of

the sticky fibrous material to install a suitable

modern electrolytic (22uF 500vw) beneath the

innards, most of which remains in place. I

installed a plastic diaphragm to separate the

innards from the ‘compartment’ at the base of

the capacitors where the new electrolytics are

installed and sheathed the electrolytics in heat-shrink as additional environmental protection. The cans

of these larger electrolytics are held together with metal tape, the cardboard sleeve and the metal

chassis clamp, so they are really solid. There are a few small dings on the capacitor bodies that I could


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not remove as I could not extract the innards to allow

these to be worked out (think of them as ‘patina’…). I

removed most of the pitting on the cans using super-

fine steel wool, and managed to carefully polish the

cans leaving the lettering in place. The smaller ‘spigot’

type can electrolytic was straightforward – I used PVC

pipe/epoxy to support it internally after re-stuffing - the

photo, right, shows these capacitors prior to

reassembly. I then re-installed the three can

electrolytics onto the painted chassis (photo, below).

The 1uF paper case

chassis-mounted

capacitor was then

re-stuffed – this

was

straightforward,

with the old

capacitor ‘guts’

pulling out easily after warming the case slightly (photos, right).

The three smaller electrolytics located under the main chassis had all been

replaced by someone previously and the originals removed. In order to find

out what type of capacitor body was used, I found a photo of an ACR-111

chassis that had these in place (follow the arrows in photo, below left) – they

were single-ended cardboard box

types fixed to the rear apron of

the chassis. I found that the

mounting holes for these were in

the correct place on the AVR-11

rear apron, thus allowing me to

size reproduction ones correctly.

Most modern capacitors have lead

wires of 0.6 or 0.7mm diameter. The owner asked for

thicker connecting leads per the originals. I measured an

original RCA capacitor’s wires as 0.9mm diameter. I found

that the wire removed from the (perished) rubber-insulated

wiring removed from the chassis as part of the restoration


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work was of the correct diameter and was already partly oxidised (ie. ‘looked the part’ perfectly), so this

proved to be suitable for this purpose.

Next, I inspected the Candohm resistor – one section (4Kohm) was open circuit, and had been replaced

by two large brown power resistors mounted on a bent piece of steel strip bolted to the chassis by

someone. Two other sections of the Candohm measured incorrect values, and one of these was

intermittent, with the resistance fluctuating slightly. I would normally replace a faulty Candohm with

multiple power resistors mounted on a tagstrip, however, the owner wanted the underside of this

chassis to look as original as possible, so this solution would not work well given this constraint. I

decided to think about what could be done and progress other aspects of the work in the meantime.

I wired the new electrolytics into the power supply section, replaced the rubber-insulated wiring under

the chassis (blue, yellow and green) as it was perished and crumbling - I replaced it with cloth-covered

wire. The yellow cloth-covered wire was a little ‘bright’, so I dulled it using a rag that I had used to clean

the chassis with (yes, even recycling the dirt!).

Next, re-stuffed the two metal-clad tubular chassis-

mounted paper capacitors (photo, right), and then

tacked in some replacement capacitors in place of the

few remaining tubular paper ones before powering-up

the main chassis to see if it worked and, if so, check

voltages etc. before I start the ‘re-re-cap’ (replacing the

capacitors someone had replaced previously with the

reproduction ones I would be fabricating later).

Speaker Panel

Before powering-up the main chassis, I installed a power

cord and tested the power switch - this worked ok, so I have no idea why it had been disconnected.

Next, I checked-over the speaker panel:

- The speaker was loose and two of the chamfered-head screws either side of the RCA emblem were

missing. I removed the RCA winged emblem and removed the original chamfered-head screws from

underneath and installed new (bright) chamfered-head screws in their place as these would be hidden

when the RCA emblem was reinstalled. I

then used the removed original screws in

the screw holes that were visible;

- There were some wires disconnected and

the ‘Cabin/Deck’ switch (photo, left) was

not working correctly – I repaired the wiring

and re-set/burnished the switch contacts to

regain continuity through each pole;

- On checking continuity through the

connectors/wiring/switch, I then found that

there was significant corrosion on the

connectors and cleaned those as well;


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- One half of the output transformer primary was found to be faulty, reading around 27Kohms DC

resistance (the other half has a DC resistance of around 700 ohms). Typical 6F6 push-pull output

transformer primaries are between 200 and 500 ohms DC resistance (each section). Unfortunately the

resolution on the schematic I have (and on the ACR-111 schematic) is not good enough to determine the

correct value. To prove it was a transformer problem and not further corrosion in the connectors,

switch or wiring, I disconnected the

transformer and tested it independently

and confirmed it was the transformer. I

had some push-pull output transformers

and I selected a suitable ‘period’ one as a

replacement (~10Kohms plate to plate

impedance);

- The original output transformer is housed

(potted) in a small metal box, however, no

replacements I had to hand would fit into

this. As the speaker panel had already

been modified (with the ‘Deck/Cabin’

switch), I figured that it probably was not a

problem to leave the replacement

transformer ‘exposed’ (being a part from

the same period it looked like it had always

been there) and, if a better replacement was eventually sourced, it could be easily replaced;

I fitted the replacement output transformer (photo, above) and powered-up the main chassis. It was

drawing around 120W and the HT voltage was around 275vDC (a bit on the high-side), but only silence

from the loudspeaker. The fault was traced to an open-circuit primary on the output stage driver

transformer, ie. in the plate circuit of the 6C5 audio

driver tube. This was very unfortunate, and

unusual, as the only three possible failure modes

for this transformer are: 1) corrosion on a

connection or wire inside the transformer, 2)

internal short in the 6C5 tube, or 3) an accidental

(temporary) short-circuit by someone, as there is no

capacitor or other component to cause a fault

(short) on the plate side of the transformer that

would overload the primary sufficiently to cause it

to act as a fuse. Its rather unlucky to have both a

failed driver and output transformer on the same

chassis!

I had three split-secondary inter-stage transformers in my ‘junk box’: two of the possible replacements

(photo, above) would fit inside the original transformer enclosure, the third was too large to fit. I

temporarily jury-rigged one of the smaller driver transformers under the chassis (a NOS one of Stancor


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manufacture), held in by its leads6. I powered-up the main chassis again and it was now working –

plenty of noise from the speaker. A video of this initial function test of the main chassis can be viewed

here.

The IF, AGC, BFO and audio stages were all working reasonably well, with plenty of IF and AF gain. The

driver transformer I used had a 3:1 turns ratio, which is typical, but the DC resistances of the windings at

581 ohms primary and 2.14Kohms (centre tapped) secondary, are significantly different to the original

spec. (722 ohms primary, 291 ohms secondary). The higher DC resistance of the secondary may affect

the bias condition of the 6F6 output tubes, but I figured that I would investigate that later if there was

low output or distortion problems (there wasn’t).

Next, I manufactured the repro tubular paper capacitor bodies. I experimented with a new

manufacturing method for these, using stiff brown ‘Kraft’ paper for the (slightly oversized) labels with a

supporting ‘roll your own’ cardboard tube underneath. The 'artwork' for this style of RCA capacitor is

very simplistic, comprising only the part number

designation, ie. capacitor value, voltage rating and another

number (purpose unknown to me!). They don't even sport

a ground indicator band like most tubular paper capacitors

from this era, instead, the ground end of the capacitor

(outer foil) appears to coincide with the end of the part

number (as this is how all the remaining original capacitors

were orientated). I added an outline rectangle to the

artwork to facilitate cutting the paper labels to the correct

sizes (photo, right).

The Kraft paper I had was on a large roll, so I had to cut it

into letter size sheets to fit my laser printer paper tray.

Once the labels were printed and cut from the sheet, they

were secured with a piece of masking tape at one end to,

and then wrapped around, a fabricated cardboard tube of the correct diameter. The label was

positioned so the seam was on the opposite side to

the label, and then held in position with stick glue.

The ‘surplus’ width of one side of the label was then

tucked into one of the ends of the cardboard tube.

The new capacitor was then installed into the

cardboard tube, having the larger-diameter wires

attached (soldered on – photo, left) if needed, eg.

the 0.01uF capacitors I used have 0.6mm diameter

leads that look odd emerging from a repro capacitor

body, so these were replaced with the wire

recovered from the rubber-insulated wires removed

6 I had left the original driver transformer enclosure in place when I painted the main chassis – mainly because it is secured using bent-over

metal tabs and these are difficult to get back in place correctly (and tightly). I figured that removing and replacing this would not be easy,

especially retaining the above-chassis cosmetics if I re-stuffed the driver transformer enclosure

https://youtu.be/1GjMIEs51cc


15

from the chassis. However the 0.1uF parts I used are fitted

with 0.9mm diameter leads and these look ok as they are.

The surplus width on the other side of the label can then be

tucked into the other end of the cardboard tube, and the

open ends then stuffed with a crumpled slip of paper as

packing and heat protection for the new part (photo, right)

before squirting in some brown hot-melt glue to seal the

ends. The finished capacitors were then coated with IPA-

diluted

amber

shellac, giving an appearance very close to the

originals (the lower capacitor in the photo, left, is the

reproduction part). The photo below shows the

tubular capacitor ‘production line’…

Once completed, all the (16) reproduction tubular

paper capacitors were installed into the main

chassis, along with the three repro carboard box

electrolytics and two re-stuffed metal-clad tubular

paper capacitors. The main chassis was then re-

tested and found to function much as it did as

before.

RF Sub-Chassis

Before further undertaking any further work on the RF sub-chassis, I

thought it would be a good idea to re-install the five RF coil screening

cans permanently to avoid any damage while handling the unit.

Before I did that, I carefully checked the rubber-insulated wiring from

the coils to the tube bases and band change switch – surprisingly, all

but one (a green one) still had supple insulation and were generally in

good shape. I replaced the one where the insulation was showing

some degradation with scuffed PVC wiring of the same colour (looks

almost identical to the original rubber insulation – photo, right). None

of the coil wiring is close to the chassis or other components and


16

therefore does not pose a shorting risk, so the remaining wiring to the coils

should last for decades, and, even if (when) the insulation fails, it will not

cause a problem;

- Re-installed all the coil screening cans - a really fiddly job. I had to make a

special tool to install some of the nuts under the chassis – photo, left;

- Replaced the four tube grid connection leads (all perished green rubber

insulation) using scuffed green PVC insulated wiring to mimic the

appearance of the original wire;

- Inspected the remaining rubber-insulated wiring on the RF sub-chassis and

ended up replacing it all - I used cloth-covered wiring to match the most

prevalent insulation type. I also replaced the cloth-covered antenna connection wire above the chassis;

- Replaced the (5) tubular paper capacitors with reproduction ones;

- Cleaned the band change switch with Deoxit and Q-tips.

This left the resistors to be checked on the RF sub-chassis and replaced if needed before installing the

sub-chassis onto the main chassis for a preliminary function test of the complete chassis.

Main Chassis – Part 2

I then moved back to the main chassis and the Candohm. I had a reply to a post on my Facebook ‘Radio

Restorations’ page suggesting I open up and re-build the old one. At first I thought the person replying

meant rewinding it with suitable Eureka wire, but he then explained that he meant removing the guts

and installing modern power resistors, as he had done on a Zenith restoration. I agreed this was an

interesting idea, but initially thought this would be difficult as power resistors are quite ‘fat’ and the

Zenith had a different style of Candohm – broader and with an open side. But then I had the idea of

calculating the dissipation in each section of the Candohm based on voltage readings I took on the very

similar ACR-111 main chassis under varying signal conditions (the voltages vary due to the effect of the

AGC action on tube biasing). I then developed a spreadsheet (see Appendix) that calculated the

maximum dissipation for each section under these conditions. Allowing for an approximate 1.5x factor

of safety (FOS) and ‘rounding’ (up) the wattage thus derived, the calculations showed that the 4Kohm

section needed to be 6W, the 3.6Kohm section 4W, the 1.2Kohm section 2W and the 200 ohm section

4W. I surmised that these values may be achieved using strings of 2W resistors enclosed in a 'fishpaper'

or similar heatproof insulating sleeve inside the old Candohm metal case. The metal Candohm sleeve

(bolted to the chassis) may act as a heatsink - if so, this would add to the FOS. Before I attempted to do

this I asked the owner if he was ok with

this approach as, if unsuccessful, it would

result in the Candohm being destroyed.

Before I attempted this, I jury-rigged a

network of 2W resistors to make up the

required resistances and wattages for each

section of the Candohm, attaching these to

a plastic chopstick(!) – photo, right, to

https://www.facebook.com/gerrysvintageradio/

https://www.facebook.com/gerrysvintageradio/


17

maintain some physical integrity during testing. The next steps in this process were to:

- Disconnect the Candohm (and the ugly pair of large ceramic power resistors someone replaced the

4Kohm section of the Candohm), and connect the jury-rigged resistor network into the main chassis;

- Power-up the main chassis – it worked just as it did before. I left it running for around 30 minutes and

then made a number of voltage, dissipation and temperature checks on the 2W resistor array. The

checking showed that all the resistors in the array were operating well-within their wattage

specifications per my calculations. The voltage readings were not the same as I recorded for the ACR-

111 – not having the RF sub-chassis installed was likely affecting this, as well as other possible issues

with the chassis not yet identified, but the voltages were all reasonable;

- Check the Candohm resistor string while it was removed from the chassis,: the 200 ohm section

measured 209 ohms, the 1600 ohm section measured 1320 ohms, the 3600 ohms section measured

3540 ohms (but intermittent open-circuit when the connection lugs were moved slightly – not a good

sign), and the 4000 ohms section was open circuit. Given this, and that the connection lugs were all

loose, I didn’t think it was a good idea to re-install the

Candohm in its current state, and the set’s owner agreed to

proceed with the re-stuffing idea. This was done as follows:

- I cut the Candohm metal case open with a Dremel tool

(photo, right), removed the ‘guts’ (photo, below) – its quite

scary to think there is almost 300v across this component

given the insulation they had used (varnish-impregnated

paper/thin cardboard) - and the condition it is now in, ie.

well-baked and crumbling, as well as all the solder lugs

flopping about);

- I installed a strip of thin fibreglass-reinforced silicone

baking sheet on the inside of the metal Candohm

case, holding it in place with a few dabs of epoxy;

- Next, I installed the solder tags: the tags were

modified slightly (one end trimmed off a little), and

the collars of each given two layers of shrink wrap

where they pass through slits made in the silicone

sheet and the corresponding holes in the metal case;

- I then reinforced and spaced the rear of each of the solder lugs with strips of stiff ‘fishpaper’, and

secured these with a little J-B Weld (photo, below);


18

- The 2W resistors were removed from the jury-rigged arrangement on the chopstick and installed in the

prepared Candohm case (photo, below). The resistances between each solder lug was checked and

found to be correct.

The completed re-stuffed

Candohm was then re-

installed in the chassis (photo,

right), and the set powered-

up while monitoring the case

temperature of the Candohm

- after around an hour the

maximum temperature along

the Candohm case was

around 77C.

With the Candohm problem

dealt with, I next set about

removing both the original

and jury-rigged driver transformers from the main chassis, gutting the original one, and re-stuffing it –

thankfully, the NOS Stancor driver transformer that had been working ok jury-rigged under the chassis

just fit inside the original transformer enclosure

(photo, left). I removed the old driver transformer

from the enclosure (it was embedded in tar of course!)

– this needed some heat to be applied and the heat

affected the paint on its enclosure, so I decided to re-

spray it. The transformer enclosure was originally

cadmium-plated, but when I spray painted the chassis,

it was left in place and received the same (‘chrome’)

paint as the chassis. So, as it was now off the chassis, I

decided to respray it with a metallic paint closer to its

original cadmium colour (I used Krylon ‘Satin Nickel’) –

this provides a nice subtle contrast to the chassis

colour. The fixing lugs could not be tightened hard against the underside of the chassis without the use

of a hammer(!) or a large clamp I did not have, so I applied a small dab of J-B Weld under each (between

the lug and the chassis) to fill the remaining small gaps - this secured it tightly, and it was then re-wired

into the circuit.


19

I then measured the resistors on the main chassis – over half were out of (20%) tolerance, though some

are non-critical and could be left in place. The resistors in this chassis are a bit of a ‘mixed bag’ – some

ceramic-bodied ones, some regular cylindrical carbon compo ones, and even a couple of ‘dogbones’ – all

looked original. So, the question was how best to repro them? – my suggestion to the owner was to

replace the ones that needed to be replaced (whatever type) with repro ones all of the same type rather

than make a variety of repro types. I cannot reproduce narrow colour bands accurately (as per the

ceramic bodied ones), so I suggested using either the body-end-spot colour code (per the dogbone), or

the broad colour bands on plain cylindrical types as per the non-ceramic bodied ones. It was agreed to

use the latter method.

Over the years I have tried various methods to repro resistors of a variety of styles – always trying to

improve appearance while expediting/simplifying construction and shortening the time needed to make

the reproductions (as I do with capacitors) – I am not fixated on a particular method and am always

willing to learn from others and from experimentation. This time I tried another fabrication technique

for the resistors:

- Cut a plastic drinking straw7 of the

correct internal diameter to the

correct length to act as a mould for

the resistor;

- Partly fill the straw mould with

two-part quick-setting epoxy

wood-filler (‘Bondo’ or similar

fillers would likely work just as well

– just make sure they don’t

conduct electricity!);

- Coat the new resistor with the

wood filler;

- Insert the new resistor into the straw

mould;

- Top up the straw mould with wood filler

and tamp out any air spaces using a needle;

- Wait for the epoxy filler to set8 (photo,

above);

7 Single-use plastic items such as drinking straws are not available for purchase in several jurisdictions these days, however, paper straws will work just as well for this application 8 Around 15 mins for the wood filler I used, which only has a ‘working’ time of around 3 – 5 mins. I would prefer this to me more like 10 mins to allow larger batches of resistors to be made at a time – I found I could only make 3 or 4 resistors before the filler became unworkable. A slower-setting epoxy, such as ‘Milliput’ modelling epoxy, available from most hobby shops, would likely be much better – almost one hour working time but takes four hours to set hard


20

- Cut the straw mould longitudinally with a craft knife (photo, left), and peel it off from the set filler - any

score mark left by the knife blade in the filler looks like a moulding line on the repro as per the original,

so are ok to leave in place;

- Trim any surplus filler from the

moulded repro resistor and then

roughen the surface with

sandpaper;

- Paint the repro resistors with

the correct colour code, allow to

dry, and then overcoat with IPA-

thinned amber shellac to age and

give a 'satin' finish to the resistor,

similar to the originals (photo,

right). Note - they do not have to

look 'perfect' as the originals

most certainly were not - especially after 80 years!

I fitted all the repro resistors into the main chassis (20 replaced initially), plus 7 more on the RF sub-

chassis – photo below. A photos of the chassis with the repro components installed is shown below

(lower RF sub-chassis cover removed. A photo with the cover installed is on shown on page 33).


21

Troubleshooting

The main chassis was then tested after changing out the resistors – it was working much as it was

before, including an AGC fault that I had noted previously, ie. no AGC action at all.

The AGC circuit uses the 6R7 pentode/dual-diode tube, but changing the original tube for one of the

NOS RCA 6R7 tubes did not fix the problem – so I decided to fit the RF sub-chassis onto the main chassis

and connect it into the circuit to see if that made any difference. After fitting, both chassis seamed to

be functioning ok, though the AGC fault persisted. I decided to troubleshoot the problem:

- Noted that the ‘AGC’ voltage that is derived separately using the 6H6 detector for the eye tube was

working well, with over -15vDC generated on a strong signal on the Broadcast Band;

- The problem therefore seemed to be a biasing problem on the 6R7 tube – this was a bit odd as I had

changed out all the resistors in that section of the circuit: one of the original 1Mohm resistors was open

circuit and I would have put money on that being the problem – but it wasn’t. I double-checked all the

resistor values and the wiring in that section of the chassis and all was ok. A short video of the complete

chassis being tested at this stage can be viewed here.

I finally traced the AGC problem to the first NOS RCA 6R7 tube I had installed in the chassis being ‘dead’

– the heater was working but there was zero emission on both the pentode and the diode sections –

possibly an open circuit cathode as this element is common to all the sections in the tube. If so, this is

an unusual fault. Fitting the second NOS RCA 6R7 completely cured the AGC issue, with well over -

15vDC AGC voltage generated on a strong local signal on the Broadcast Band and no 'blocking’ of the

receiver at all, even on a very strong local signal with the RF gain fully advanced.

Given this issue with the 6R7 tube, I

tested all the remaining NOS RCA

tubes before I installed them in the

chassis (photo, left). All were ok,

except one of the 6C5s which only has

55% emission and a gm of 1450um.

The other had over 80% emission and

a gm of 2100um (2200um is typical for

a good 6C5 tube). I put the weaker

tube in as the push-pull driver for

now, and the supplier agreed to send

a replacement 6C5 tube.

I decided to replace the resistor in the

BFO coil screening can and the two

resistors located in the 3rd IF

transformer can. These were all marginally out of tolerance and non-critical, but I thought I may as well

change them out as they are awkward to get to if there is a problem in the future. In addition, I also

installed three new reproduction resistors in the IF amplifier circuit: a 680 ohm resistor in each of the IF

https://www.youtube.com/watch?v=Ffb4U2qKP-4


22

amplifier cathodes, and a 220 ohm resistor across the bias switch9 - I had noted that these were all

marginally out of tolerance (high), and as these could affect the overall gain of the receiver, decided to

change them out. Doing this increased the AGC voltage on a strong signal by around -3 volts, so worth

doing as this indicates additional gain has been obtained. I also replaced the 1Mohm resistor in the eye

tube holder.

Slow Motion Tuning

I found that the new bushing that had been

machined for the slow motion drive was not

quite correct: the original was made out of

steel and had a slight internal taper, whereas

the replacement was machined from brass

with no taper. However, I thought of a way of

using it together with the old bushing to

prevent the shaft wobbling laterally like it

used to in the old (worn) bushing. The original

bushing actually comprises three parts: the

flange, an outer bushing and a slightly tapered

and stepped insert, all made out of steel. I

suspect the insert is hardened steel as the three spring-loaded ball bearings forming the epicyclic speed

reduction contact this when assembled (I think brass would wear prematurely where it contacted the

hardened steel ball bearings);

The epicyclic slow motion

tuning mechanism (refer

to diagram, right),

comprises an inner shaft

which fits concentrically

into a hollow outer shaft.

Three ball bearings pass

through three holes

spaced at equal intervals

around the outer shaft

that, when assembled,

contact both the inner

shaft and the external

bushing under spring

compression. The inner

shaft has a (fourth) ball bearing attached to its end. Rotating the inner shaft rotates the ball bearings

that carry the outer shaft around with them as they contact and rotate against the bushing. The outer

shaft thus rotates slowly relative to the inner shaft (approximate 3:1 ratio) through this action.

9 The ‘bias switch’ adds an additional resistor (220 ohms) in series with the cathode resistor (680 ohms) of the 1st IF stage to reduce its gain slightly on bands ‘X’ (‘Long Wave’) and ‘A’ (Broadcast Band). Interestingly, the ACR-111 uses lower value IF amplifier cathode resistors (390 ohms), that would result in higher gain in that receiver design


23

To assemble the mechanism, the inner shaft is pressed down against a (captive) spring in the base of the

outer shaft. The end of the inner shaft behind the fourth ball bearing is shaped such as to allow the

three rotating ball bearings that contact the outer shaft to retract slightly within their corresponding

holes in the outer shaft. When positioned in this 'sweet spot', it allows the slightly tapered bushing to

slide over the ball bearings and against the step in the bushing. When released, the compression in the

spring then acts against the shaft and pushes the three ball bearings outwards, which stops the inner

shaft from pulling out of the tapered/stepped bushing and thus maintain contact with the inner shaft,

such as to allow the epicyclic slow motion action between the inner (slow) and outer (fast) tuning shafts.

The outer shaft has the logging scale attached to it only, the tuning knob being attached to the inner

(slow motion) shaft.

I decided to use the new (brass) bushing as a ‘stabilizing’ collar

on the outer shaft (yellow arrow on photo, right), with the old

(steel) bushing located in its original position (green arrow),

contacting the ball bearings. Amazingly, the new bushing was

just the right length to allow this to work, placed behind the

flange where the bushings secure to the RF sub-chassis, and

not interfere with the pinion that engages the quadrant gear

fixed to the tuning gang shaft. Before assembly I lubricated

the inner shaft and bushings with lithium grease. Once

assembled, I added a fillet of J-B Weld around the

bushing/flange function on both the original and new bushings to provide additional stability.

Final Assembly (almost)

With the slow motion drive refitted, I installed the new (repro) dials onto the RF sub-chassis (photo,

below), and re-installed the linkage to the band-change rotary switch. I found that the ‘bias switch’,

which changes the bias on the 1st IF amplifier, and hence its gain, was faulty. The shaft of this switch is

the actual band change switch shaft on the front panel. This is an unusual switch design in that it trips

around half way around its shafts travel. Luckily, a

good squirt of Deoxit and working the switch several

times fixed it, but adjusting it so it switched the bias

between bands ‘X’/’A’ and ‘B’/’C’ correctly/reliably

took a while to sort out. The switch introduces a 220

ohm resistor into the cathode circuit of the 1st IF

amplifier to reduce the gain slightly on bands ‘X’ and

‘A’.

I re-used the original rivets (flipped around as I had

drilled them out) to fix the main dial in place, each

held in place with a dab of epoxy. I did not have any

suitable rivets for the logging dial (the originals were

destroyed during removal), so I used some small

screws instead (photo, right). I also cleaned up all the

dial bulb holders and installed a set of five new #46

dial bulbs.


24

Alignment and Testing

Alignment of the IF was straightforward – simply peaking six slugs on the IF transformers to 460KHz and

zero’ing the BFO. One of the 1st IF transformer slugs was jammed, but I managed to coax it free and all

was then ok.

I then moved on to the RF alignment – all went well, apart from a few of the locknuts on the plunger

type trimmers being stiff and I could not tune the IF trap for a dip when feeding an IF (460KHz) signal

into the antenna terminal.

The specified RCA alignment tool would not turn several of the RF trimmer locknuts, and a socket

wrench would not fit into some of the available spaces. With a bit of effort, I managed to free the

recalcitrant ones sufficiently with some bent-nose pliers to allow adjustment of their plungers ok.

Having a functional IF trap is usually not a significant cause for concern, but can, in some circumstances,

result in unwanted heterodynes and/or unwanted breakthrough of strong low frequency signals. So I

decided to investigate this problem:

- A continuity check on the IF trap coil showed it was open

circuit. I could not see any damage or disconnected wire

with the coil in position, so I removed it from the RF sub-

chassis;

- On turning the IF trap coil upside down, it was obvious the

coil had been zapped by static – black discolouration on the

former and one of the wires had burnt out (photo, right).

Thankfully the burnt-out wire was right at the point where it

entered the coil, so I was able to tease some good wire out,

tin it and attach another piece of wire to it, extending it to

the terminal on the coil former. The coil then measured 23

ohms DC resistance with an inductance of 2.6mH, which

both seemed reasonable values. I also

tested the resonant (series) silver mica

capacitor and it was good/within

tolerance;

- The IF trap was then reinstalled into the

RF sub-chassis (photo, left), and I was

able to obtain a very deep dip in the

receiver's response to 460KHz fed to the

antenna terminal, so that issue was now

resolved.


25

The chassis was now performing very well with good

dial accuracy and sensitivity on all bands. By this

time, the chassis had been operating continuously for

around 5 hours, so I took the opportunity to measure

some temperatures (ambient 25C), eg. power

transformer (50C), output transformer (28C), speaker

field coil (33C), and the chassis side adjacent to the

re-stuffed Candohm (40C – photo, left)) – these all

seemed ok.

Cosmetics

The AVR-11 was in poor cosmetic condition on

arrival: the cabinet had several dings and the

paintwork was in very poor shape, the dials were

worn and discoloured, the dial escutcheon was

missing, the fingerplates were worn, some knobs did

not match, and the chassis and rear side of the front panel were rusted. The speaker panel was also

badly rusted on its rear side and the speaker frame was also rusted. Clean-up and painting of the

chassis and fitting of a reproduction dial set is described elsewhere in this article. The following

describes work to render the cosmetics of the set into a much more acceptable condition:

- The control fingerplates were removed and the paintwork touched-up on each. I then polished them

with Novus #1 and #2;

- Scuff marks and missing paint on the front panel were touched-up with colour-matched grey acrylic

paint;

- I tried to resurrect

the black felt speaker

cloth, but it was too

far ‘gone’ – I only had

green felt (baize) in

stock, so I fitted that,

then cleaned and

touched-up the

winged RCA emblem

to see how it looked

(photo, right);

- Rust was removed

from the rear of the

speaker panel and it

was given a couple of

coats of satin grey

spray paint;


26

- The rust on the speaker

frame was removed and

the speaker panel

reassembled (photo,

right).

- Cleaned all the knobs

and touched up the paint

on the (metal) tuning

knob. I noted that some

knobs had longer

pointers than the others.

Also, some had jammed

set screws that

stubbornly refused to

cooperate… so I decided

to try to source a matching knob set (the knobs are a fairly common style for communications receivers

and electronic equipment of the 1930s). Luckily a friend had a set of eight matching knobs that were

very similar to the originals, though with larger pointers;

Next, I tried fitting the new dial escutcheon to the front panel - it had been removed from an RCA model

10K-1 domestic receiver, but is an identical design to the one that the AVR-11 was fitted with.

Unfortunately the front panel has oversized holes, so I had to secure nuts in them using J-B Weld. Once

the J-B Weld had set, I removed the rust from the rear of the receiver front panel and gave it two coats

of the same satin grey spray paint as on the rear of the speaker panel;

The front panel was then reinstalled

onto the chassis and the dial

escutcheon fitted. However, I found

that it fouled both the pointer and the

logging scale dial. Perhaps the 10K-1

escutcheon design is slightly different,

eg. it was deeper such as needed to

pass through the wooden panel on a

domestic cabinet. There were two

options to mitigate this problem: 1)

space the front panel away from the

chassis, or 2) install a spacer between

the escutcheon and the front panel. I decided to opt for the latter as the former would have shortened

the effective length of all the control shafts and some were a little on the short side already. I cut a

double-thickness gasket from sheets of neoprene to act as the spacer (photo, above). This worked well

and also provided a ‘cushion’ between the front panel and the escutcheon. I had to use longer screws,

and then found that two of the longer ones fouled the logging scale(!), so I trimmed them all down to

the optimal length. The heads of the screws were painted bronze to match the escutcheon colour. The


27

knobs were then

fitted, along with

the phones jack, eye

tube and standby-

light (photo, right)

before testing the

chassis again.

A Zoom call was

then set up with the

set’s owner to

discuss some

aspects of the work

completed so far,

including:

- The touched-up front panel control fingerplates and (less so) the winged RAC emblem are not ‘perfect’.

I had touched them up using marker pens and then toned them down with Novus #2/#1 polish before

applying Armor All. Although much-improved, I advised the owner that the only way to render these as

‘perfect’ would be to find a firm than could refinish them professionally: they are all ‘reverse etched’ (it

even says that in the brochure), ie. the lettering is embossed – this should allow enamelling, powder

coating or some sort of etching/anodizing by a specialist firm to be undertaken;

- I had noticed occasional minor/intermittent 'hesitation' in the slow motion mechanism. This was likely

due to wear. I had not noticed this until the front panel was re-installed, the large tuning knob fitted

and I started to rotate the dial frequently to test the chassis. The slow motion tuning is certainly

useable, but again, not 'perfect' and could become annoying (and possibly worsen) during frequent use;

and

- I only had green baize (felt) in stock, so I used that for the speaker grill (I placed it over the original

black felt, so could be removed to expose the original if desired). I also have a strip of green baize I

intend install inside the rear of the cabinet where there was a strip of very tatty black felt prior to me

removing it to paint the cabinet. The colour of the baize could be changed using dye if the owner

wanted this.

Finishing Touches

As a result of the Zoom call with the owner of the RCA AVR-11, it was decided to:

- Dye the green baize on the speaker grill black to render it the original colour;

- Remove the slow motion drive and try to eliminate the intermittent slight 'hesitancy';

- Paint the dial escutcheon black (the owner had found a photo of an AVR with the original escutcheon

present and it was finished in black).

On this basis, further work undertaken included:


28

- Removal of the dial escutcheon from the front

panel, removal of the plastic dial cover from the

escutcheon, then roughening the surface of the

escutcheon with wire wool, cleaning it with IPA,

then applying two coats of satin black spray paint

(photo, right);

- Applying a half bottle of ‘Feibings’ black leather

dye to the green speaker baize (photo, below);

- Removal of the knobs, eye tube, phones jack,

front panel, and lower RF sub-chassis screening

cover to allow removal of the slow motion drive.

The following work was then undertaken to

mitigate the occasional hesitation of the pointer

while tuning the receiver:

- Disassembled the slow motion drive, cleaned all

the grease off all bushings and shaft surfaces as

well as the ball bearings, and then degreased all surfaces with naphtha. On close inspection for

any issues that might be the cause of the

problem, or at least may be contributing to

it, I noted that the bearing surface on the

inner shaft that contacts the ball bearings

had uneven wear/flat spots (tip of arrow in

the photo, right). Ideally a new inner shaft

would need to be machined to fix this,

however, I decided to proceed with

reassembly to see if any improvement

could be made by careful assembly and adjustment;

- Applied a light coating of lithium grease to the insides of the bushings, and a tiny spot on the

end (thrust) ball bearing on the inner shaft, taking care that no grease contacted the three

epicyclic drive ball bearings or contact surfaces to maximise friction at these points and hence

limit any slippage. Reassembled the slow motion drive, lightly greased the pinion, and re-

installed it into the chassis;

- Adjusted the position of the quadrant gear on the tuning gang shaft slightly to avoid any

possible fouling with the end of the brass bushing;

- Adjusted the pointer clearance slightly to avoid any possibility of contact with the heads of the

rivets securing the dial; and


29

- Adjusted the position of the slow motion drive relative to the quadrant gear to minimize

friction.

I then tested the slow motion drive for any slippage – it was still smooth and wobble free,

however, over multiple rotations I detected a very minor hesitation a couple of times. It was

therefore agreed with the owner that the drive be taken to a local machine shop to see if they

could machine a new bushing and inner shaft. Unfortunately, the machine shop was not 100%

confident they undertake the work successfully and turned the job down. Luckily, I recalled one

of my radio friends saying he knew a good machinist, so I had

a word with him – a couple of weeks later the re-machined

slow motion drive was in my hands: a new inner shaft and a

new bushing (both in steel) had been made – photo, right -

and were a perfect fit. The drive now felt very smooth, but I

felt it needed some lubricant – I found that a thin machine oil

worked better than a light grease for the roller-bearings.

After fitting the drive to the chassis, only some minor

adjustments were needed to render the tuning action

‘smooth as silk’ and there was no sign of any hesitation that

bugged the worn-out original.

- Finally, I had obtained a set of matching knobs from a friend, however, these had large pointers

than the ones originally fitted to the set and the owner thought these did not look quite right. I

had five of the original knobs with the smaller pointers, so I trimmed the pointers on three of

the set of knobs I obtained from a friend to match these. I then painted all the pointers an off-

white colour so they matched and looked slightly ‘aged’.

Cabinet

The cabinet was powder blasted (silica and walnut shell powder), and then checked for dings – finding

several, plus some severe rust pitting, it was decided to

fill the dings and pitted areas before refinishing. Also,

the owner had noted that one of the side strips where

the receiver and speaker fix to was bent outwards,

such that the mounting screws for both the receiver

and speaker unit were difficult to insert. I measured

the width I found it was a quarter inch ‘out’ at the

centre point. I also found a couple of small drill holes

in the top of the case. I filled the holes in with JB-

Weld as a first step, then sanded any remining minor

areas of rust (there was a little remaining after the

sandblasting). I then de-greased the cabinet with IPA

before applying an initial two coats of grey etching

primer to prevent formation of any surface rust as

further work on the cabinet would be delayed by a few

weeks due to other commitments.

On returning to the cabinet work a few weeks later, I:


30

- Filled a deep ding on the top front

corner and pitting where the

previously badly-corroded patches on

the top of the case were with two-

part epoxy filler (photo, right);

- Sanded the filled areas flush with the

surrounding paintwork and feathered

the edges;

- Straightened the bent front

radio/speaker mounting strip back

into place using a large clamp (photo,

below). Following this, the pairs of

mounting holes were now all

18.25” apart, which matched the

receiver and speaker mounting

hole distances;

- Applied three additional coats

of high-build primer/filler to the

top and top edges of the

cabinet;

- Applied two additional coats of

regular grey primer to the

remainder of the case;

- Wet sanded the high-build

primer with 800 grit wet and dry

paper;

- Filled in a small ding on the

upper right corner and some

minor pock-marks on the top of

the cabinet with epoxy filler;

- Wet sanded the

newly-filled areas with

120 then 800 grit

paper;

- Applied three more

coats of high-build

primer to the top of

the cabinet;

- Wet sanded again with

800 grit paper;

- Dried the cabinet with

a hairdryer and

cleaned it with IPA

(photo, right);


31

- Applied three coats of satin

black spray paint to the bottom,

inside, outer side panels and the top

and left it for two days, then

- Lightly sanded the cabinet and

gave it a final coat of satin black

spray paint;

- Installed a strip of felt in the

rear of the cabinet at the receiver

rear apron height – photo, below

(the cabinet had a similar strip of

felt installed that had to be removed

for the refinishing work).

Finishing-up

The chassis and speaker panel

were then installed in the cabinet

– I realized that the screws that

had been holding thes in place

were of different types, so a set of new screws were obtained and the heads painted with satin finish

grey enamel to blend in with the colour of the front panel (photos on pages 34 and 35). The owner has

a cover for the ‘Magic Brain’ RF chassis and this will be cleaned-up and fitted at a later date.

Closure

The RCA AVR-11 is a somewhat a compromise design, tailored to suit the specific needs of the niche

market segment it was aimed at. Maybe not quite as ‘leading edge’ as the contemporaneous ACR-111,

but an impressive receiver nevertheless, and it must have been a revelation to those used to many

earlier receiver designs. Most impressive in my book though is its imposing ‘presence’ in its dual-height

desktop cabinet – it simply looks very, very ‘cool’… (and it works like a charm too!!).


32

Above-chassis views of the AVR-11 after cleaning, painting, fitting reproduction tubular paper

capacitors and resistors on the tuning gang (RF sub-chassis), and re-stuffing the can electrolytics

on the main chassis. All the tubes fitted are NOS, manufactured by RCA, and selected for

combined good performance and cosmetics. A cover will be fitted over the RF sub-chassis


33

Under-chassis view of the RCA AVR-11 after fitting reproduction tubular paper and box electrolytic

capacitors, reproduction resistors, and re-stuffing the can electrolytics and Candohm resistor


34

The restored AVR-11 chassis and speaker installed in the re-finished dual height desktop cabinet


35

Oblique view of the restored AVR-11 chassis and speaker installed in the re-finished dual height

desktop cabinet – a very imposing radio to have on your desk!


36

Appendix

- Comparison of RCA Chassis (ACR-111, AVR-11 and 15K-1) – Main Chassis Top/Bottom*

- Comparison of RCA Chassis (ACR-111, AVR-11 and 15K-1) – RF Sub-Chassis Top/Bottom*

- Candohm Resistor Calculation Spreadsheet

- AVR-11 Schematic

- AVR-11 Brochure

*Photos of AVR-11 chassis before restoration work undertaken


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restoring an rca avr-11 ‘airport receiver’ – gerry o’hara

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