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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
Hallicrafters S-76: signal tracing
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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
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