Help me understand this fan circuit please

[Randall]

Ok, red herring. Pins 1/2 and 3/4 are indeed connected to a thermostat on each of the two heat sinks. I had to pull the power amps to see them. There is also a second thermostat on each heat sink, as well as one on the chassis floor, for a total of 5. Shorting the ones in question at J5 does turn the fan on, as does hitting the heat sinks with a heat gun, and it is turning in the correct direction. So I stopped thinking this was the actual problem, considering what I know about the equipment savvy, and troubleshooting level of this customer.

I plugged a jack into one of the return jacks in the back, and when I removed it, no signal. I cleaned those jacks, and now it doesn't lose signal. I believe this was the problem all along, and am not convinced that the fan was not working, especially since the amp lives in a road case.

Anyway, thanks for the help guys!

[nickb]

You did the right thing in verifying the fan operation

[Chuck H]

I plugged a jack into one of the return jacks in the back, and when I removed it, no signal. I cleaned those jacks, and now it doesn't lose signal. I believe this was the problem all along, and am not convinced that the fan was not working, especially since the amp lives in a road case.

If the jacks were faulty/dirty to the point of no signal there would be no significant current in the power amp. No current could mean not enough heat to activate the fan circuit making it seem like the fan not working was another problem.

[nevetslab]

I think those Elmwood Sensor (or Equiv) Thermal Switches (mounted below the heat sink PCB) are 55 Deg C COR, while the ones in series with the output from the module are around 80-85 Deg C. I normally put the amp in Bridge Mono, and run it into a 4 ohm load with pink noise and wait until the fans come on....either heat sink will start up the fan, as the switches are in parallel, if memory serves.