Repair and Restoration of Antique

Radio Equipment

byGregory L. Charvat, MIT Lincoln Laboratory

forMIT Haystack Observatory Open Lunch

October 21, 2009

1Wednesday, October 21, 2009

Antique Radio Equipment

Looks cool

Unique sound quality

1930-40’s jazz sounds better

Functional history

Learn how things were made

2Wednesday, October 21, 2009

Outline

Step-by-step process

Case study: Hallicrafters S-76 (1951-54)

Case study: Colin B. Kennedy Model 20B Console radio (1929)

Summary

Hallicrafters S-76

Colin B. Kennedy Model 20B

3Wednesday, October 21, 2009

Step-by-step process1. Do not power up your radio!

• yes, it does not work

• more harm than good

2. Find service manual

3. Replace all electrolytic and paper capacitors

4. Replace anything that looks damaged (burned or exploded)

5. Try radio (wear safety glasses)

6. Signal trace through circuit, replacing resistors where needed

7. Radio will work

‘exploded’ view

wax-papercapacitor

4Wednesday, October 21, 2009

Hallicrafters S-76

•Amateur Radio receiver

•1951-1954

•2 conversion heterodyne architecture

•11 tubes

•538 Kc - 32 Mc

•CW, AM, SSB (with steady hand)

•41 lbs

•Design based on SX-28 (widely utilized during WW2)

Front

Top inside

Bottom inside5Wednesday, October 21, 2009

Hallicrafters S-76: capacitor spewing waxPowered up S-76 without replacing electrolytic capacitors

one electrolytic smoked and spewed wax

cathode capacitor on AF power output tube was shorted

Replaced 2 paper capacitors that looked ‘rough’

No other caps replaced, keeping radio original

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6Wednesday, October 21, 2009

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signal tracing and bias measurements tube bias chart

Signal generator connected to RF input

Signal traced using 10x probe

Signal stopped at front-end mixer tube V2

had to check the voltages on the tube pins

voltage chart included in all service manuals

requires proper impedance volt meter, 20K/volt Simpson 260

If the voltage is outside of +-5% then replace the resistor(s) soldered to that pin

old carbon resistors INCREASE in value with age

R17 and R18 had to be replaced (and others)

7Wednesday, October 21, 2009

Radio started to work

Performed alignment

straight-forward procedure in manual

Tested radio to full sensitivity specification

Success! Long wire antenna; listening to amateur radio stations and many short-wave stations .epEu

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Hallicrafters S-76: alignment

8Wednesday, October 21, 2009

Colin B. Kennedy

Model 20B•AM broadcast band receiver

•1929

•TRF architecture

•7 tubes

•phono input

•unrecognizable parts

•frail cloth wires

•Globe tubes

Front

Top insideBottom inside

Rear

9Wednesday, October 21, 2009

Model 20B: Re-cappingReplaced all electrolytic and paper capacitors

Resistors and some capacitors are nearly unrecognizable

Paper capsSprague ‘Midget’ cap, with nasty

bubbles coming out of it

Power supply caps

Bottom

Top, left-side Unrecognizableresistors

10Wednesday, October 21, 2009

had to replace cloth cord

infamous for electrocuting antique collectors

utilized 3-wire computer power cord

grounded chassis for safety

added 3A in-line fuse for fire protection

old cloth and organic rubber wires crumble easily when strained

protection against possible future shorts

Model 20B: safety

bottom, after restoration

added: 3A fuseadded: grounded computer cord

11Wednesday, October 21, 2009

Model 20B: poor documentation

Courtesy Nostalgia Air

my radio has a single-ended audio output stange with one pentode, model 247 rather than a pair of push-pull triodes as shown here

cathode bias resistors in the RF section on my radio are 1.5K, not 3K, 3K, and 5K as shown here

Difficult restoration: only remaining schematic does not match what is in the radio

Regardless of this, it was a success. Listened to the Red Sox game, first time in decades that this radio has tuned in a ball game.

12Wednesday, October 21, 2009

Problem: very little music on AM band

Feature: phono-input tied directly to grid of detector triode

IPOD wired directly in

1920-40’s jazz playlist

podcasts; talk radio that you want to listen to

Model 20B: IPOD doc

Courtesy Nostalgia Air

my radio has a single-ended audio output stange with one pentode, model 247 rather than a pair of push-pull triodes as shown here

cathode bias resistors in the RF section on my radio are 1.5K, not 3K, 3K, and 5K as shown here

13Wednesday, October 21, 2009

SummaryInstant conversation piece

“Real radios glow in the dark,” anonymous, generally accepted philosophy in antique radio community

Unique sound quality for your vintage jazz collection

Potential for upgrade; IPOD doc

celebrating ’29 CBK’s 100’th birthday soon

Resources:

expert advice: www.antiqueradios.com

tubes and parts: www.tubesandmore.com

more information about these radios: www.mit.edu/~gr20603 click on Restoration of.... S-76 or 20B

e-mail me with your questions: gregory.charvat@ll.mit.edu

14Wednesday, October 21, 2009