Fixing a 1965 GM Car Radio AM | Troubleshooting Tips

[John1397]

The radio would work sometimes if it didn't pushing on the tuning buttons hard or hitting the dash underneath would make it work if it didn't work, this has been that way for 30 years, now it does not come on at all. I always thought it must be broken connection some where. I had applied voltage from a battery and sure run down voltage on battery in a short time. I tested with ohm meter between ground and positive had real low reading so I took out circuit board. With board laying on bench it tests 206 and 215 ohms when reversing leads between ground and the point where 12 volts goes to circuit board. I never worked on these type of radios does this not seem kind of low for ohms between positive and negative? It seems to only have three transistors and the big audio output transistor.

 

[CWatters]

I would flip it over and inspect all the solder joints carefully with a magnifying glass.

 

[Tom.G]

As @CWatters said. Also power it on and flex the circuit board and other components a little. If you get a response, try to narrow down to the area or component that require the least movement for a result. Then a close visual inspection.

The 200 ohm readings you got are reasonable for the low voltage that the meter uses to make the measurement.

There seems to be manuals and schematics available online for these older car radios. There is probably a sticker or marking on it somewhere that states the brand, it could be either Delco or Motorola.

 

[Baluncore]

I never worked on these type of radios does this not seem kind of low for ohms between positive and negative?

That is about right for a single transistor, class A audio output stage, that runs on 12Vdc.
The problem is oxidation of a contact, or the fracture of a solder joint. Find it by gentle mechanical provocation.
Time will take it's toll on the electrolytic capacitors and on the leakage of germanium transistors and diodes.
Carbon resistors fail open circuit but can become microphonic or temperature sensitive.
Find the solid fault first, then check and replace the components that are showing their age.

 

[jim hardy]

I would flip it over and inspect all the solder joints carefully with a magnifying glass.

if it didn't pushing on the tuning buttons hard or hitting the dash underneath would make it work

@CWatters is right on.

What I've seen is the joints crack around their bottom right where they join the board.
Like the ones on right, here

Left one looks like a botched repair ? Two distinct layers of solder...

you'll need to use a good magnifying glass and take it out in the bright sunlight. I prefer a jeweler's loupe, myself .

'65 GM you say ?Small world - my daughter has her Great Aunt's 1965 Buick and i expect to be working on that radio soon enough. It stopped working about twenty years ago, before she inherited it

old jim

 

[Bystander]

Delco or Motorola.

Philco-Ford equals Motorola (think "Edsel" McNamara); i.e., not a GM product.

 

[Baluncore]

Remember how expensive transistors were back then. So many passive R & C components, yet only three transistors, with a couple of diodes for the AM detector. That suggests a reflex receiver circuit with variable inductance tuning.

If those are IF transformers, then it is a superhet and there needs to be a local oscillator, an IF/audio reflex amplifier, and an audio output stage. That will be an interesting circuit when we find out what it is.

 

[anorlunda]

Carbon resistors fail open circuit but can become microphonic

I never heard that term before. What does it mean? Does the resistance vary with sound?

 

[Baluncore]

I never heard that term before. What does it mean? Does the resistance vary with sound?

Yes. Just like a carbon microphone, only not such a good quality sound, more like clipping.
The same happens with some coaxial cables where the braid or conductor wires can rub together when moved.

 

[davenn]

I never heard that term before. What does it mean? Does the resistance vary with sound?

what ? microphonic ? … a VERY old term in electronics

it means they generate noise due to touching / vibration etc

this can be a major problems particularly in RF oscillators where microphonics cause FM'ing of the osc. signal

That is why you often see oscillator circuits with wax all around them even in older transistor radios.

 

[John1397]

I think I will soon find it as it was drawing to much current must be short in tuning coil or audio output transistor I applied power to board only, draws only small amount of current all transistor caps diodes test good. Tuning coil has 8 ohms do not know without diagram if this is right, but will put every thing together and unhook one item at a time until I find short. If it still draws excessive current should be able to feel a warm component some where. I do not think a broken connection can cause excessive current draw can it?

 

[jim hardy]

If it's the "Sonomatic", ie no 'Wonderbar' motor driven tuning, it's probably a Delco 728934
and from a search, an antique radio forum said Sam's Photofacts AR30 repair manual covers it. Ebay has a few listed. But you have to be careful for misinformation abounds.
Probably there's a Delco manual out there someplace.
Blogs like this one are sometimes helpful - let Google or Bing work for you.
http://antiqueradios.com/forums/viewtopic.php?f=6&t=308550

In my days i'd have expected five transistors
an RF amplifier near the tuning coil
a local oscillator near the other tuning coil
an IF amplifier , doubtless that metal one between the IF cans
an audio amplifier probably that black one just left of IF cans
and the output power transistor probably mounted to the sheet metal with an insulator .

If you can inject signal from an audio source like a cassette or cd player headphone jack (through about a microfarad to block DC) to wiper of volume control you'll maybe get a clue which direction to chase your trouble.

Good Luck - if you find a schematic, post it ? I'll do likewise.

old jim

 

[jim hardy]

PS might this be a candidate for our DIY thread?

 

[John1397]

It is a 7291063 AR 23 they had many different numbers but I think they are mostly the same board. More than likely it's one of those non electrolytic capacitors is shorting out those are the one's I figure can never fail.

 

[jim hardy]

I applied power to board only, draws only small amount of current all transistor caps diodes test good.

Big output transistor isn't on the board, is it ? Likely suspect..

I do not think a broken connection can cause excessive current draw can it?

If it is in the base biasing circuit of a big transistor with transformer for collector load(hint hint- output transistor) it sure can. It could turn the transistor full 'on'.

Check the dial lamp and its socket too.

 

[jim hardy]

What do you mean by 'excessive current' ?
Have you measured it ?
What kind of battery did you use to test the radio ?

do you have a voltmeter ? Ammeter ?

 

[DrClaude]

PS might this be a candidate for our DIY thread?

Yes. I moved the thread.

 

[John1397]

There seems to be a short some where and checking all the capacitors on that board might find problem anything that has winding's I will not be able to find. When apply 12 volts from power supply the initial amp draw is 1.6 amps in about one second it drops to .33 amp remains there and when measuring voltage at to3 audio output at both base and emitter voltage starts out at 4 volts and one second later goes to ten volts. Out of curiosity I removed wire to base of audio output then amp draw is less than tenth of an amp. I did change audio output transistor but it made no difference.

 

[jim hardy]

When apply 12 volts from power supply the initial amp draw is 1.6 amps in about one second it drops to .33 amp remains there

Not unreasonable if it includes the dial light.

both base and emitter voltage starts out at 4 volts and one second later goes to ten volts

That sounds curious.
Is there a resistor from emitter to chassis?
if so check it.
Is there an audio output transfomer ?

Place one voltmeter lead on output transistor base, other on output transistor emitter.
That way you measure its base-emitter drive.
Is it silicon or germanium ? Silicon should be close to 0.6 volts, germanium 0.3.

Out of curiosity I removed wire to base of audio output then amp draw is less than tenth of an amp.

So it draws maybe ¼ amp ? Doesn't sound bad for a several watt class A amp.

Do you understand transistor biasing ?
see https://www.electronics-tutorials.ws/amplifier/amp_5.html

ten volts across R_E is too much for a 12 volt supply..
C₁ who blocks DC from previous stage might be leaky,
Biasing resistors R₂ or R_E which establish quiescent operating current might have drifted high.
R₁ who with R₂ makes a voltage divider to set base voltage could be low, but carbon resistors usually fail high.

You need your schematic.
Or you'll have to trace out your circuit board and draw a schematic of the part under study
so as to find physically where to measure
Draw one and write on it voltages at E B and C .

old jim

 

[jim hardy]

Of course if that output transistor is a PNP

the circuit will be upside down from what i pasted above,
with base and emitter near 12 volts and collector near ground.
You already measured base and emitter near 12 volts - what does collector show ?

Troubleshooting is like working a jigsaw puzzle .
You establish known facts one by one , building a mental picture from them.

Once you've established what is that output transistor, NPN or PNP, germanium or silicon,
you can judge whether your voltages make sense.
So that ought to be your first "fact" to establish.
Voltages at E B and C will be a clue.
Values of the resistors will tell you what the designer had in mind.

Go man, Go !

 

[John1397]

I was checking to see if speaker wire is connected to any thing I took picture and drew every thing paper but I don't see no connection from collector of output transistor to speaker wire. I was always looking at the transformer and I drew it in diagram as it is hooked up with one wire going to chassis ground and another wire going to electrolyte capacitor ground and the the other two wires are going thru a sleeve and get connect to speaker wire/ Maybe I am not understanding the electorate capacitor right I thought the four pins coming out one is 800 mfd another 400 mfd and another 100 mfd and I thought the fourth pin is ground where the transformer get hooked. I took my speaker wire and connected it to collector of output transistor and I got it to play for a little while then it cut out again

 

[jim hardy]

t I don't see no connection from collector of output transistor to speaker wire.

Shouldn't be one. Collector drive primary as in that tutorial.

looking at the transformer and I drew it in diagram as it is hooked up with one wire going to chassis ground and another wire going to electrolyte capacitor ground

Both wires to ground can't work. Might one of those wires go to the collector(case) of the TO3 transistor instead ?

I took my speaker wire and connected it to collector of output transistor and I got it to play for a little while then it cut out again

That probably should have wrecked the output transistor and the speaker.

Take some pictures and post them ?
What is part# of output transistor?
What are voltages at emitter base and collector of output transistor? Measure to 1/10 volt, include polarity with black voltmeter lead to chassis gnd. .

 

[John1397]

Mine is like the above photo both wires from transformer to ground and other two to speaker output only thing is the pin where they made connect to speaker wire is not connected to any think. If I get this to work I am just going to take and place wire from collector to speaker then at leas circuit board output will be connected to speaker. Not sure what it does looked to me more like a filter.Mine is DS-503 instead of DS-501 as in schematic as schematic is two years older.

 

[jim hardy]

your photo doesn't show, try pasting it into Paint and see if anther format will show up.

Your schematic doesn't show a transformer with four wires but only an inductor with two and it's in parallel with speaker
One speaker wire goes to ground
other one goes to junction of collector , inductor and a capacitor.

EDIT (late entry) That the speaker is connected to output transistor collector makes my remark yesterday about wrecking parts , well, just plain WRONG.

It's a PNP so
Collector should be at quite low voltage
emitter and base near ten volts sounds okay.

 

[John1397]

I drew out all wires I disconnected on paper and even took picture when I am all done pin on board where speaker and inductor wire is soldier on goes no where could have been wire underneath that fell off but haven't found any yet. I would think only logical place to connect speaker is to E of Ds-503. I drew up a partial diagram how it is wired the 12 volts goes around capacitor not thru it..

 

[jim hardy]

You're making progress.
Focusing on just the output stage

 

[jim hardy]

On those TO3's it's easy to confuse Emitter and Base when swapping your mind from top view to bottom view
note if you you hold it at the hole nearest the pins it's E B C clockwise .

ps here's a great resource for hobbyists interested in old timey transistor circuits. Page 233 has an output stage not unlike yours.

https://www.americanradiohistory.co...lan-Lytel/Transistor-Circuit-Manual-Lytel.pdf

 

[jim hardy]

I would think only logical place to connect speaker is to E of Ds-503.

E ? perhaps C ?

 

[John1397]

I drew it right c is the case and case is bolted to aluminum heat sink and this is mounted to chassis no plastic spacers around bolts just seems like you need to connect speaker wire to emitter. I have picture before I unsoldiered any thing and picture of board. And after first soldering every thing back the first time I did pickup sound after connecting 8 ohm speaker to emitter or yellow wire.

 

[jim hardy]

Your friend's picture shows plastic spacers

yours doesn't look like the same heatsink

and i think you have already noticed that.

If your "inductor " has four wires it is almost certainly a transformer.


Is there a part number on that output transistor?
Is there a mica washer underneath , between it and the heatsink?