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Bass Guitar Amp troubleshooting help needed

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
 
R

Rheilly Phoull

Jan 1, 1970
0
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.

Check the electrolytic caps in the PSU since they are a more likely problem.
 
R

Ross Herbert

Jan 1, 1970
0
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.

Snip


Peter, perhaps the first thing you should do is tell us the amp make
and model. That way your description can be related directly to the
schematic which I'm sure someone will have.
 
P

Peter

Jan 1, 1970
0
Rheilly Phoull said:
Check the electrolytic caps in the PSU since they are a more likely problem.

At the moment the bridge is definitely a problem, as it blows the fuse
when connected in circuit, but if that is gone I guess there is a good
chance the PSU caps are gone too. And those things are expesive too
:-(

Peter
 
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