Urei 6500 power amp fan not working

[stereomonostereo]

It's definitely the thermistor. I just swapped them and the problem follows the one that's suspect

[g1]

Excellent. Fairly simple fix then.
Those hand drawn changes on the schem. made me think it may be a mod to correct a circuit design problem. I was thinking back to a similar circuit that got re-designed many times then scrapped. In that model, changing the thermistor often corrected the problem for awhile, then it would come back. That was some scr or triac circuit though.

[stereomonostereo]

We're starting at 50k on the functioning one, and then dropping to 10k when in boiling water. So i believe i'm looking for a 50k with a 10k B parameter then?

[nickb]

We're starting at 50k on the functioning one, and then dropping to 10k when in boiling water. So i believe i'm looking for a 50k with a 10k B parameter then?

Not quite so easy

B= ln(R/R0)/(1/T -1/T0); R= 10k, T=100C (=373K), R0=50K, T0=25C (=298) plugging the numbers in gives B= 2385K. This a an unusually low number, something like 4000K is more common. It could be really be that but my guess is that thermistor is starting to fail too.

So looking at Digikey this could be a candidate B57861S503F40 EPCOS (TDK) | Sensors, Transducers | DigiKey. The B is much higher this means the fan will kick in early which is better than the other way around. Keep looking and maybe you will find one cheaper and better.

[stereomonostereo]

Not quite so easy

I find it never is

I think i'm going to go with that as a replacement and do the other one while i'm at it. Thanks for all the help!