I need a bit of help with a bass guitar amplifier, or at least a few questions answered. Bear with me while I tell you what has happened up to now with it.
It blew a while ago. I had made a compressor to go between the bass and the amp. It was constructed from a kit. The chip used is an Analog devices chip that is supposed to be a mic preamp with a compressor built in. The kit is sold as a compressor for guitar. Anyway, there was a huge amount of gain through this thing. It probably would have been OK for a guitar, but my bass has an active preamp on board that already boosts the signal quite a bit. I had a bit of trouble controlling the gain (I had the gain control on the compressor turned right down, the volume on the bass turned right down, playing very softly, and it would still distort a bit). Anyway, I gave up on it that day as I had to go out to gig. Figured I would sort it out later and left the copressor at home. There was a bit of a cooked plastic smell, but I ignored it as the amp was still working fine (in hindsight that was probably a mistake). Anyway, during the gig that night the amp stopped working, with a much stronger smell of burned plastic.
I opened it up, and the fuse holder was melted. Not quite sure why the fuse got so hot that it melted the holder, but it had. I replaced the fuse holder and fuse, and on power up the fuse popped instantly. This amp has three PCB's - Preamp, Graphic EQ, and Power amp. When I disconnected the EQ board the amp would power up without popping the fuse. I plugged the EQ board into a bench supply and two zener diodes across the -15-0-+15 rails glowed red and smoked. I replaced them. The board then powered up without blowing the fuse, but no signal got through it - with the EQ switched on there was no sound, with the EQ switched off the amp worked and sound got through to the speaker. I took a bit of a guess and replaced all the op amps on the EQ board. All was then OK - it worked as it was supposed to, and was quiet and hum free to boot. Put it all back together and enjoyed haing my friend restored to me. That was a few weeks or maybe even months ago, but I haven't needed to use it much during that time (maybe once or twice).
The last time I powered on the amp, the fuse popped again. It kept popping. This time it seems to be in the power supply. I disconnected everything except the switch and fuse and everything was fine. I connected up just the power transformer, and everything was fine (That was a great relief - I imagine a 240V to 80-0-80V toroid trannie is expensive). I soldered in the bridge rectifier by itself - and pop. I assume the bridge is faulty, and am looking to get a replacement. Anyway, I am a bit puzzled, and have a few questions. Firstly, given the history I've described, is it plausible for the brisge to die a little bit after everything is fixed? Could I have another fault that made everything seem to work (and sound) fine but overstress the bridge? (it is rated at 400V and 35A and is rectifying -80-0-+80V and about 3-4A I would guess). Also, I tested the old rectifier with a meter, and couldn't find any shorts, and each individual diode seemed to test OK with the diode check. That puzzled me a bit - is it possible that there could be a breakdown that only happens at higher voltage?
Thank you
Peter