I'm trying to fix a Crest CA2 poweramp with what seems like a pretty straight forward problem. The two cooling fans are supposed to run at a low idle most of the time but these ones kick in to high speed as soon as the amp is turned on and stay there.
I figured there would be a temperature sensor somewhere that's controlling them, maybe on the heat sink, but I can't find anything. I found a schematic for the closely related CA6/9/12 and located the Darlington transistors that drive the fans, but I can't find anything driving the transistors (on the schematic there's just a tag that says fan_driver but doesn't seem to go anywhere).
Can anyone suggest what might actually be driving the fans?
(There are temperature leads coming from the transformer, but I assumed a transformer overheating would be a more critical failure and would open the protection relays. The fans are nowhere near the transformer. )
I figured there would be a temperature sensor somewhere that's controlling them, maybe on the heat sink, but I can't find anything. I found a schematic for the closely related CA6/9/12 and located the Darlington transistors that drive the fans, but I can't find anything driving the transistors (on the schematic there's just a tag that says fan_driver but doesn't seem to go anywhere).
Can anyone suggest what might actually be driving the fans?
(There are temperature leads coming from the transformer, but I assumed a transformer overheating would be a more critical failure and would open the protection relays. The fans are nowhere near the transformer. )