Thermistor in PSU

[jstark]

Hello,

I've got a Peavey bass amp that keeps blowing the fuse (Not a glass fuse, but a little resettable fuse on the back of the amp.  Is there a better name for that?)  The speaker makes an awful buzz for a second, and the pops the fuse.  During this time a thermistor on the IEC inlet board gets super hot and starts to smoke. 

I don't have much familiarity with thermistors.  As I understand it, it's in there to limit inrush current.  Is replacing it going to solve my problem, or is this just indicative of something else downstream pulling too much current?


thanks for any insight,
David

 

[PRR]

> Is there a better name for that?

Circuit breaker?

> speaker makes an awful buzz for a second

One side of the push-pull power stage has gone dead-short.

The thermistor smoke is a side-effect. You mostly need to rebuild the power stage.

 

[JohnRoberts]

Hello,

I've got a Peavey bass amp that keeps blowing the fuse (Not a glass fuse, but a little resettable fuse on the back of the amp.  Is there a better name for that?)

Circuit breaker

The speaker makes an awful buzz for a second, and the pops the fuse.  During this time a thermistor on the IEC inlet board gets super hot and starts to smoke. 

I don't have much familiarity with thermistors.  As I understand it, it's in there to limit inrush current.  Is replacing it going to solve my problem, or is this just indicative of something else downstream pulling too much current?


thanks for any insight,
David

The thermistor getting hot is probably a symptom of the problem and not the problem.

Check amp power transistors for a short.

JR

 

[jstark]

Circuit breaker

haha, ok, wasn't sure if that term was reserved for the panel in the basement.

The thermistor getting hot is probably a symptom of the problem and not the problem.

The thermistor smoke is a side-effect. You mostly need to rebuild the power stage.

Ok, that's what I suspected, but I wanted to verify before tearing apart the output section.  After having smoked once or twice does the thermistor need to be replaced?


Thank you both for quick and helpful responses.
David

 

[joaquins]

If it has released the magic smoke it probably does need to be replaced, but you probably will notice that once you have worked out the power section.

JS

 

[CJ]

you can ditch the circuit breakers and thermistors for a simple slo-blo fuse on the mains and a simple speaker fuse inline with the speaker jack,

that way you will have easily replaced parts which will make the amp more serviceable during troubleshooting and future use,

 

[PRR]

> wasn't sure if that term was reserved for the panel in the basement.

I see your point. But that's what I always called it, that's what it is called in the catalog (and probably in the service manual, which you will want), and they do the same thing (blow like a fuse then re-set at a touch).

"Circuit breaker" actually covers a LOT of things. Some don't trip on overload, they are just "switches" for insane power levels. Others trip-out and then re-close by themself. The one on your amplifier is about the simplest kind.

CJ advises ripping-out these safety bits. I've done that. However what you really need at this point may be a Lamp Limiter. (Or a trip to the pawn shop for an un-sick bass amp... high-power amp repair is expensive.)

 

[JohnRoberts]

Circuit breaker

haha, ok, wasn't sure if that term was reserved for the panel in the basement.

The thermistor getting hot is probably a symptom of the problem and not the problem.

The thermistor smoke is a side-effect. You mostly need to rebuild the power stage.

Ok, that's what I suspected, but I wanted to verify before tearing apart the output section.  After having smoked once or twice does the thermistor need to be replaced?


Thank you both for quick and helpful responses.
David

Peavey service is generally pretty helpful with advice and parts.

I would not ASSume that any parts in the original design are not needed or optimal.  Peavey has designed quite a few power amp stages over the decades so they understand the technology and even understand keeping the repairs lower cost.

JR

 

[CJ]

TV sets back in the 70's had no access to  fuses, just a little red button in the back,
there were all kinds of fuses inside, chemical fuses, resistor fuses with a rough sandpaper like coating of some kind,

some shoddy practices were sometimes used to fix a set that kept popping that breaker, as the TV Repair bidness was full of crooks and misfits trying to make a fast buck off the unsuspecting customer, the logic being get the money first, and let the new customers pick up the slack of the problem customer with the breaker wired across with buss wire,  hoping that the house breaker would trip before the place burned down, a rough intro into the cold but real world of capitalism for a high school electronics student, have things changed that much?

well, there is a  lot more verbage on the back of most consumer electronics products, and not many TV shops left, so yes, things have changed, my boss made the mistake of dusting off an old picture tube and charging the State Board of Electronics 150 bucks for a new tube, (sting operation), one small paragraph in the newspaper and the phone quit ringing, which meant  no more checks for the two high school freaks who worked there for a while,

 

[jstark]

> Or a trip to the pawn shop for an un-sick bass amp... high-power amp repair is expensive
> Peavey service is generally pretty helpful with advice and parts.

Yep.  This was a favor for a friend, and I got as far as removing the cooling "ductwork" before giving up.  It's hard to tell what's going on underneath the seemingly-permanently-affixed heatsinks, but it appears the output section is using SMT transistors.  I'll go back to my 5881s, thank you.

I've advised taking it to a Peavey service shop, but as PRR has noted, I imagine it might be cheaper to find something else.

>  a rough intro into the cold but real world of capitalism for a high school electronics student, have things changed that much?
haha, I had a similar (but less life threatening) introduction as a high school IT worker.


thanks y'all!
David