Smoking Resistor?

65blkbkgt · 2nd January 2009, 02:08 AM

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 · 2nd January 2009, 02:32 AM

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

65blkbkgt · 2nd January 2009, 03:01 AM

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 · 2nd January 2009, 03:44 AM

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 · 2nd January 2009, 04:01 AM

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 · 2nd January 2009, 04:03 AM

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 · 2nd January 2009, 04:07 AM

[QUOTE]Originally posted by DigitalJunkie
[B]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 · 2nd January 2009, 09:38 AM

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 · 2nd January 2009, 12:20 PM

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 · 2nd January 2009, 03:25 PM

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?

2nd January 2009, 02:32 AM	#2
I Am An Idiot diyAudio Member Join Date: Mar 2007	A shorted C1 or a short to ground on the wire labeled 15. __________________ If it ain't broke Don't fix it

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2nd January 2009, 03:44 AM	#4
DigitalJunkie diyAudio Member Join Date: Apr 2003 Location: Portland,Oregon Blog Entries: 4	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.

2nd January 2009, 04:01 AM	#5
65blkbkgt diyAudio Member Join Date: Jun 2007	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?

2nd January 2009, 04:03 AM	#6
DigitalJunkie diyAudio Member Join Date: Apr 2003 Location: Portland,Oregon Blog Entries: 4	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.

2nd January 2009, 04:07 AM	#7
65blkbkgt diyAudio Member Join Date: Jun 2007	[QUOTE]Originally posted by DigitalJunkie [B]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.

2nd January 2009, 09:38 AM	#8
djQUAN diyAudio Member Join Date: May 2004 Location: manila	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.

2nd January 2009, 12:20 PM	#9
65blkbkgt diyAudio Member Join Date: Jun 2007	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.

2nd January 2009, 03:25 PM	#10
65blkbkgt diyAudio Member Join Date: Jun 2007	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?