Fixing a 1965 GM Car Radio AM | Troubleshooting Tips

[jim hardy]

I connected 8 ohm speaker where it normally would connect

Where is that ?

 

[John1397]

I was just moving speaker connection around so I drew up three different speaker hookups and voltages around output transistor. 0 volts at collector will never work. I have been thinking I do not know if anybody does this but it seems if unit is drawing to much current start opening up circuit starting at power supply then moving to output transistor and so on to see where those amps are going if power supply heads in two direction you can't till which way short is by measuring volts but with amp meter you could. I am going to change output transistor to a different good one sure transistor tester says it is good but that could be wrong. And transformer is wound that way as two wires coming out of transformer to speaker are soldier together almost way down to the point where they come out of transformer.

 

[jim hardy]

I drew up three different speaker hookups and voltages around output transistor.

Okay when troubleshooting you must resolve things that defy logic one by one until the mysteries are all gone.

You say you've heard sound out of this radio so it must be almost working.
Now what doesn't make sense ?
First thing that jumps out at me is..

Curiously
all three of your recent sketches show emitter hooked straight to +12volts by a wire
yet a much lower voltage reading there of between 0.13 and 1.3 volts.

Is your 12 volt supply really collapsing that severely ?
Or is that wire to emitter broken in the middle ?

Not much chance of anything working with supply less than a volt.
Figure that one out first.

 

[jim hardy]

Is there something missing from your three sketches ?

does that say ".47 fuse" ?

Where'd you get that drawing ?

 

[John1397]

Is there something missing from your three sketches ?

View attachment 231732

does that say ".47 fuse" ?

Where'd you get that drawing ?

I found this online a genuine delco book that drawing is right . I measured 0.8 ohms across this . On one side of this device on the circuit board it says fuse and the other side it says res. It is kindy 3/35" diameter 3/8" long looks like in metal can has cotton sleeve over the outside. I thought it was a thermal type fuse to protect the output transistor.
I never measured voltage just amp draw of unit was .56 amps I just seen this now on the manual diagram says normal amp draw is 1.3 Amp that is more than likely what is happening the fuse is opening up after you apply power. Know I see why in the three drawings in drawing three where speaker is connected to base the voltages go up, applying load increases resistance then voltage drops. You need 12 volts on emitter. This is probably why the output transistor is still good I have heard using radios and amps without speaker connected can burn out transistor I do not know if this radio is one of those.

 

[jim hardy]

Now you're seeing the approach - understand what voltages to expect then check for them with a meter.
To that end
it is vital to have an accurate schematic drawing
and know physically on the board where are key points to measure.

Can you read voltage on both sides of that fuse ?

That'll work better than an Easter egg hunt for a short that might not even be there.
Unless you're extremely lucky.

 

[John1397]

Two diagrams a year apart both circuit a little different problem is burned out predriver to the finale output transistor. In my drawing I have NPN driving the DS-503 and one year earlier they used an PNP to drive the PNP DS-503 output tansistor. They used what appears to be two of the same transistors but no number I think an ECG103 would work or how hard would it be to make a Silicon say ECG158 work?

 

[jim hardy]

What are ECG103 and 158 ? You've already looked them up so save us a search.
What's in the radio ?

I cannot read the numbers on your drawings. Can you clean them up a little and repost?

2N3903, 2N3904, 2N3905 and 2N3906 will replace 90% of all general purpose small signal transistors.

A good circuit is tolerant of variations in transistor specs.
Just make sure your Vce and Ic ratings are adequate
and hfe is same order of magnitude.

 

[John1397]

I seen online some one wrote when changing geranium to silicon double resistor value from base to collector. Mine is 22k so if I change to 44k should work?

 

[jim hardy]

I seen online some one wrote when changing geranium to silicon double resistor value from base to collector. Mine is 22k so if I change to 44k should work?

sounds like gibberish to me. And since i can't make out numbers on the schematic i don't know what the 22k does.

You SHOULD adjust the two resistors that set base voltage to account for higher Emitter- Base voltage of 0.6 instead of 0.3.

I can't read the values of those two resistors on your drawings.
And i don't believe your hand drawn schematic because it shows two bases tied together and there's noplace for collector current to go..
You haven't yet tried to understand transistor biasing.

Try first just swapping in a silicon. If base is biased for mid-supply it'll most likely work.
If not try decreasing your 22K (base pulldown resistor?) by 10% and see what happens.
Easiest way is by tacking a 220K or 240K in parallel with it.

 

[John1397]

The drawing is still right just need to reverse b and c on both preamps . When installing new transistor it was ecb I thought ebc when making drawing so not drawn right. I changed 22k to 44k I see some recommend as much as 1 meg, I do not know if there is a formula to figure that out. I don't know if that exactly right. I made a new drawing where every thing is right and have the correct voltages. I finally decided to inject audio signal at volume control to see if amp part works rather than fixing every thing and it did so I then connect signal from radio and it worked.

 

[jim hardy]

I finally decided to inject audio signal at volume control to see if amp part works rather than fixing every thing and it did so I then connect signal from radio and it worked.

Can't argue with success, can you ? Must feel great to hear it playing .
Got a '60's radio station in town ? I think "Wooly Bullly" was #1 that year...

old jim

 

[John1397]

Found the on off problem anyone fixing these watch for tab punched out of top cover so it doesn't touch circuit board and short out signal. The tab bent down on cover about 1/4" should go on part of circuit board with no foil.

 

[jim hardy]

Found the on off problem anyone fixing these watch for tab punched out of top cover so it doesn't touch circuit board and short out signal. The tab bent down on cover about 1/4" should go on part of circuit board with no foil.

Got a Picture ?

 

[John1397]

 

[jim hardy]

View attachment 232088

Good to know about, Thanks !

glad you got it going.

old jim