The fuse of this guitar amplifier blows.
Changing the output transistors had the amp working for a while, but then after a while the fuse blew again.
The output transistors that were changed are new. The configuration based on the schematic here seems to be a Darlington style push pull B class amp output (correct me if that is wrong) and that all seems to be okay.
From what I understand this problem with the fuse blowing could also be the zener diodes, is that correct? If so while I am at it, is it recommended to change the filter capacitors as well?
One other thing. For draining the filter capacitors, is the cement type 2k ohm, 10 watt resistor on the terminals necessary, or is that needed for only tube guitar amplifiers?
Best Answer
That is a typical class AB amplifier. When the transistors were replaced maybe they were not a matched pair or the CR15 temperature sensor is not close to them like it should be.
Those outputs are wired as Darlington's to get the current gain. CR15 must physically touch or be mounted to the same heatsink as the output transistors, or they will overheat and burn up within minutes, short out and blow the fuse(s).
It is a push-pull amplifier with +/- 38 volt supply rails, so ground is the common return for input and speakers. The signal is symmetrical above and below the ground line. CR15 is a temperature sensor that regulates the bias current to the output transistors. The hotter they get, the less bias current CR15 will let them have.