Hello! I am repairing an Adcom GFA5800 amp which has a thermistor mounted to each heat sink to drive the fan. The problem is that one of the leads broke off flush with the part. This side of the amp calls for a 40C part. How would I go about finding a sub for this part? I looked on DIGI-KEY but didn’t see a spec for 40C. Any help would be much appreciated!
Man, I've been doing some searching, and finding a PTC thermistor at 40C with around 140 ohms at 25C is very hard to find, not to mention mounting options. What would be the best approach for a substitute? Could the thermistor be replaced with a normally closed thermo switch? Attached is the schematic the part in question is TH1.
You might be able to adapt a different value thermistor. The fan circuit is an LM317 regulator that is using the thermistor as part of the voltage divider that determines its output voltage. That voltage divider could go up or down in total resistance, as long as the regulator is happy. (Check the LM317 datasheet on minimum impedance values for the divider.)
The math is easy, but here's a calculator to do it.
https://circuitdigest.com/calculators/lm317-resistor-voltage-calculator
You'll have noticed that one channel as a 40C thermistor and the other is 60C. They're doing this to keep the noise down. Pulling air through the one channel's heatsink tunnel gets the air moving and actually does pull a little air through the other channel. If it gets even hotter, the second channel's fan kicks in, and they will both start moving faster if they need to.
It's not an awesome arrangement, one channel gets hotter than the other and may die sooner because of it.
I don't remember who posted it, but someone either here on AK did a fan replacement and found a nice, quiet fan that worked well with the 5800. I would be inclined to change both regulators to 40C and put in the quiet fans, and they will just run slowly most of the time. They're nearly inaudible.
The math is easy, but here's a calculator to do it.
https://circuitdigest.com/calculators/lm317-resistor-voltage-calculator
You'll have noticed that one channel as a 40C thermistor and the other is 60C. They're doing this to keep the noise down. Pulling air through the one channel's heatsink tunnel gets the air moving and actually does pull a little air through the other channel. If it gets even hotter, the second channel's fan kicks in, and they will both start moving faster if they need to.
It's not an awesome arrangement, one channel gets hotter than the other and may die sooner because of it.
I don't remember who posted it, but someone either here on AK did a fan replacement and found a nice, quiet fan that worked well with the 5800. I would be inclined to change both regulators to 40C and put in the quiet fans, and they will just run slowly most of the time. They're nearly inaudible.
Thank you, that is very helpful, I learned something! My biggest issue is trying to find a 40C thermistor. Either I am searching and using the wrong parameters or they are really hard to find. The lowest temp I’m finding is 60C.