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Thermistor replacement: Carver M500

Raccoon1400

Raccoon1400

Super Member
I've been working on this carver amp and it has been fighting back every step. It has blown a couple sets of outputs now. The last time it pre full wall power testing, then it sat with stable bias on full wall power for 5 minutes then bias spiked and it blew. Bktheking said to suspect the thermistor. I tested them with an ohm meter, the good channel one measured 610 ohm, the bad channel one 720. So it appears they are going wonky. The part number is SDT-65, and according to this thread, that should be a 650 ohm thermistor. http://www.audiokarma.org/forums/index.php?threads/help-me-id-these-parts.676454/#post-9063835

The closest value digikey or mouser has is 680 ohm. Should that be sufficient?
 
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Did you measure while applying a little heat to the thermistor? Pinching it with your fingers should warm it up enough.
 
I don't know much about tolerances with thermistors but that range doesn't sound like they are too far out. How about bias - is it manually adjustable with trimpots on this amp and how do you think bias was reading before it blew? You should build a Dim Bulb Tester and only powerup the amp using that, then you have a safety net and no blown outputs.....
 
slimecity said:
I don't know much about tolerances with thermistors but that range doesn't sound like they are too far out. How about bias - is it manually adjustable with trimpots on this amp and how do you think bias was reading before it blew? You should build a Dim Bulb Tester and only powerup the amp using that, then you have a safety net and no blown outputs.....
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Bias is adjustable. I used a dim bulb tester first and it passed. Bias was good before it blew.

Both thermistor values start to drop when I apply some heat.
 
These amps can be plagued with cold solder joints. Look real close at the bias components. The bias transistor is subject to thermal movement (heat sink to pcb tug of war).
 
dennismiller said:
These amps can be plagued with cold solder joints. Look real close at the bias components. The bias transistor is subject to thermal movement (heat sink to pcb tug of war).
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I've been over the board many times, no bad solder left.

Can anyone tell me if a 680 ohm thermistor will be close enough?
 
I have a parts amp. I pulled both thermistors. They both measure around 650/700 when first connected to the DVM. If I let them rest/stabilize with the DVM connected for 5 minutes they measure around 800 ohms. If I pinch them the ohms drops rapidly. I think yours are OK. I don't think the bias would be/could be adjustable if these where at fault.
 
dennismiller said:
I have a parts amp. I pulled both thermistors. They both measure around 650/700 when first connected to the DVM. If I let them rest/stabilize with the DVM connected for 5 minutes they measure around 800 ohms. If I pinch them the ohms drops rapidly. I think yours are OK. I don't think the bias would be/could be adjustable if these where at fault.
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Thanks for measuring. I got new thermistors but haven't tried them yet.

What makes your amp a parts amp?
 
I bought a Carver M-500 amp a couple of years ago that someone gave up on for really cheap. He really butchered the foil runs and solder pads attempting a re-cap. The required work to restore was well beyond what I wanted to do in repairs. I have worked on a dozen or so of these amps. I needed a faceplate and a meter to make two other amps whole again.
 
So what else could be the issue if not the thermistors? Haven't tried the new ones but I don't want to blow any more parts.

I'd put the onsemi MJE15032/33 parts in as outputs just to try to get a couple watts through it before getting better parts. So it only blew $10 worth of parts this time. And it will cost at least $10 if I try another theory and it blows.
 
These amps are a bit tricky to troubleshoot. There are some 10 ohm resistors (R175/177/178/174) and 22 ohm (R179/181/180/176) that frequently fail when heat sink devices fail. If you send me an email (available on my profile) I will send you a trouble shooting guide in pdf.
 
dennismiller said:
These amps are a bit tricky to troubleshoot. There are some 10 ohm resistors (R175/177/178/174) and 22 ohm (R179/181/180/176) that frequently fail when heat sink devices fail. If you send me an email (available on my profile) I will send you a trouble shooting guide in pdf.
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Came across this post that states you have a PDF of a troubleshooting guide for the Carver M500. I have a dead one on my bench and have little to no experience with these amps. I'd appreciate a copy of the PDF.
Thanx.
 
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