thermistors and capacitors---what the???

[randomair]

i am having trouble indentifying thermistors next to capacitors. i have never worked with thermistors and i need to replace some in my amp. can someone explain the labeling process on thermistors? the ones i need are 100 ohms and need to handle at least 75 volts. thanks.

[Enzo]

What amp? What model? What part numbers?

[randomair]

acoustic 150b...part#: GE 2D304 (both are the same)
the numbers on the board are r316 and r317
i know they are thermistors but my problem was trying to find the appropriate replacement to order. thermistors look so similar to capacitors and i have alot of spare parts here in my shop. i was hoping by having an identification number or reference point i may have something in my shop to replace them without having to order them. thanks.

[Enzo]

Those are 100 ohm (cold) thermistors. And would have a NTC characteristic

You have a photo?

Are they open? Check them out of circuit. With that 6.8 ohm in parallel with each, they shouldn't be under much stress.

I don't know what they look like, so I can't help there. I will assume they are mounted on the heat sink?

[randomair]

i pulled them from the circuit and one reads 100 ohms cold and the other reads 5 ohms cold. i know the one is bad. they look like miniature wheels of sorts and they have a lead soldered to each side. i was able to download the acoustic 150 service manual and read that a cause of low volume distortion is that if one or both of these read below 100 ohms. sure enough one does, 95 ohms below!!! the attachment picture below is pretty much what it looks like except the lead mounts are on the outside of the body and it appears to be just a solid round brick of carbon with no markings.

i did replace a couple of 10 uf capacitors on the preamp boards and that seemed to help with the distortion but i think changing these thermistors would really be wise.

[Enzo]

The 100 ohm one is likely OK. Look through Mouser for an NTC 100 ohm thermistor. See what pops up. SLap a 100 ohm resistor in there just to test the rest of the amp, but watch current closely, because it is there to keep the transostor next to it from runnign away thermally.

I know the style part you mean - always looked like a disc cap to me.

CHeck Mouser:
527-1004-100
527-2003-100 (this one is physically larger, might be the better choice)
527-3006-100 (or even this one)
334-NTC101-RC

[randomair]

thanks enzo! as always, you are a great help here. what is your opinion on this causing the distortion? have you had experience with this problem yourself? thanks again!

[Enzo]

I don't recall what repairs I have done to these over the years. it is a typical push pull output with transformer drive. that resistor is part of setting the bias for the transistor next to it - next to it on the schematic, I don't know if they are adjacent on the board itself. If one transistor is not biased up right, the amp will distort, sure.