Smoking Resistor?

[65blkbkgt]

Hey Guys,

I'm stumped and hope that someone might be able to lend some knowledge here. I just finished restoring a 1964 Ford Truck transistor radio (model 4TBT) manufactured by Bendix.

I removed the output transistor to clean the aluminum heat sink and repaint the chassis. When I put it back together and went to test the radio the wire-wound resistor (R32) shown in the schematic began smoking. What could be the cause of this?

I would be grateful for any help.

Thanks!

 

[I Am An Idiot]

A shorted C1 or a short to ground on the wire labeled 15.

 

[65blkbkgt]

Thanks for the help. I appreciate it.

I checked the #15 wire and that's good. I suppose I'll need to replace the electrolytics in the can.

Is there any chance it could be wire 13 pictured below? I had to remove that one (to the base of the output) to remove the cover.

 

[DigitalJunkie]

Wire 13 looks like the NFB loop,Unlikely that it caused the problem.

I'd probably start with replacing the filter caps,they are due for replacement anyways.

 

[65blkbkgt]

Thanks DJ and true, they should be replaced. What I don't understand is why that resistor was fine before I removed the output then once I replaced it the resistor smoked. Is it possible that I did something to cause a short?

 

[DigitalJunkie]

Yea,that's kind of odd.
Was there a mica insulator on the transistor,that got forgotten when it was re-assembled? (Don't worry,we've all done it at least once!)

I Dunno. 😕

 

[65blkbkgt]

Yea,that's kind of odd.
Was there a mica insulator on the transistor,that got forgotten when it was re-assembled? (Don't worry,we've all done it at least once!)

Nope. The transistor is a PNP germanium transistor. I made sure I wired them correctly too when I replaced the emitter and base wires. I checked for any grounding and all was good.

 

[djQUAN]

maybe you could have just shaken the dried up electros and the metal flakes (which used to be the electrodes) inside the cap and connected both terminals together internally? 😕 could be a possibility. 😀

 

[65blkbkgt]

That makes sense since the caps are almost 45 years old.

I'm going to pull the can today and replace the caps. That will narrow it down.

 

[65blkbkgt]

I know it's not C1 because I just replaced the caps with the same result (smoke). I checked the connections and replaced the output transistor and it's still smoking. Any ideas of what should I try next?

 

[djQUAN]

have you checked if the transistor is shorted from emitter to collector? if not, check the resistance of the 70ohm thermistor or if it is installed.

 

[65blkbkgt]

The thermistor is installed. I get resistance at 103 from the thermistor to the emitter?

 

[65blkbkgt]

That's a pretty high reading. Could this be the culprit?

 

[I Am An Idiot]

The thermistor is 70 Ohms cold. I have no idea what it's value should be at room temp, but too high of a resistance reading will not heat up the resistor, Have you pulled the transistor out of circuit to see if it is leaking? I know they asked about the insulator between the heat sink and the transistor, what about the insulators on the mounting screws?

 

[65blkbkgt]

I've tried several different transistors with the same result. The mounting screws for this particular model don't have insulators for the screws. What I think might have happened is that I'm using a "stand in" Weller soldering iron until my new station arrives. I'm thinking that I may have heated the connector to the emitter too much to desolder the connector and the wire may have separated from the connector.