Thermistor: 1K TD5-C210/Adcom GFA-555

[Mayank]

Hi:

I need to replace a broken thermistor on the Adcom GFA-555. The service manual specifies this to be TDS-C210 1K thermistor. Unfortunately, I have not been able to find its specifications, datasheet or references to substitutes.

Would it be OK to replace it with this one from EPCOS/TDK:

B57164K0102K000 EPCOS / TDK | Mouser

Thanks in advance,
Mayank

 

[Andersonix]

Where in the GFA-555 is this thermistor located? If it's for inrush, then 1k is not right...

 

[Mark Johnson]

It's connected between the base and the emitter of Q621, which is the NPN "driver" transistor that drives the bases of the four parallel NPN output transistors on the high side. Interestingly there is NOT a thermistor from base to emitter of the PNP driver transistor that drives the bases of the four parallel PNP output transistors on the low side.

See post #32 dated 14 October 2011, of the thread "Adcom gfa 555 help", for schematic.

 

[Mayank]

The thermistor is (was) glued on top of the driver 2SD525 and connected to its emitter and its base. I'm replacing the drivers with MJE15032/33 but need a new equivalent thermistor.

See the highlighted section on the schematic (attempting to attach)

Rgds
Mayank

 

[Kay Pirinha]

I think that this NTC isn't critical at all wrt the drivers, despite it's position in the schematics. Electrically, it relates to the bias spreaders Q10 and Q9, reducing bias voltage with increasing heat sink temperature. So you'll just have to look for a new 1k thermistor.
Best regards!

 

[Phloodpants]

Confirmed!

I have just installed and tested these in a GFA-555 MK1. I used the 3% tolerance part instead of 5%, as that's what Mouser had in stock.

Part number
B57164K0102J000
in stock at Mouser and Digikey.

Bias level versus heat behaves pretty similar to the stock thermistor. If I connect the amp to a dummy load, and heat it up to 80C, I get about 80mV in bias current. Remove the signal and it slowly drops back to 16mV.

 

[dbxdx5]

Confirmed!

I have just installed and tested these in a GFA-555 MK1. I used the 3% tolerance part instead of 5%, as that's what Mouser had in stock.

Part number
B57164K0102J000
in stock at Mouser and Digikey.

Bias level versus heat behaves pretty similar to the stock thermistor. If I connect the amp to a dummy load, and heat it up to 80C, I get about 80mV in bias current. Remove the signal and it slowly drops back to 16mV.

@Phloodpants What do you use in place of the original factory adhesive to get a secure thermally conductive bond between the thermistors and drivers?

 

[Phloodpants]

I think silicone adhesive is best, but epoxy or super glue is fine if you don't mind that it's permanent and not serviceable. I use GE Silicone 2 for lots of things because it's non-acidic.