Suggestions for troubleshooting a damaged amplifier

Hello!

A few weeks ago I picked up a solid-state Fender BXR 200 bass
amplifier. I got it cheap from a friend who accidentally doused it
in Wild Cherry Pepsi. Per suggestions from sci.electronics.basics,
I ended up running the circuit board under warm water in the sink,
and rinsing it off with distilled water afterwards. It cleaned right
up with no hard water deposits. I let it sit for 10 days to dry out
before hooking it all back up and applying power to it.

However, while it seems to work better than before, it's
still got some issues (surprise, i know). It still has a problem
where the volume level will fluctuate upon its own whim. There seems
to be two different symptoms, which could be two different situations:

1) Usually when it is first started up, the volume level is very weak,
and distorted. It almost sounds as if there is a transistor or opamp
misbiased. Then, it will suddenly correct itself. The sound level
comes up to where it is supposed to be, and it plays/sounds fine.
Sometimes it will be alright for half an hour, sometimes it will act
up again in a few minutes.

2) Sometimes the volume level just drops, with no distortion. It
doesn't act like a dirty pot, either, as I can turn the volume up and
down, and it follows it perfectly, it's just as if it is running a
huge powerbrake. In my experience, dirty pots can make the volume
fluctuate, but usually a small turn of the pot will move the wiper off
of whatever piece of crud it is on and it restores to its proper
volume level. This is different.

Looking at the schematic, I don't see any bias adjustment points.
Things are pretty much fixed, and that still wouldn't solve whatever
condition is changing things up. My first thought is then maybe the
power supply could be acting up, but I'm not sure what sort of event
would cause that, or how to measure it (aside from hooking my DVOM up
to +57V and -57V and seeing if it fluctuates from ~114V over time).
My next thought is to probe the pins of all the opamps and transistors
to see if their levels are reasonable, and/or change when it acts up.

If anyone could throw me a bone, that would be appreciated. I won't
expect someone to walk me through testing a live circuit they've never
seen before, but I'd appreciate any help or pointers. I know that
this post qualifies for the canonical "if you have to ask these
questions, you have no business messing around with that circuit", and
on Tuesdays and Thursdays I might agree. However, how else do you
learn?

I can provide a link to the schem if anyone wants it.

Hopefully, I don't sound as crazy as I think I do.

Thanks.

-phaeton

 

Look at the "higher volyage caps" for a start , they tend to dry out
and cause mayhem....( hope it helps )

 

Did it work ok prior to the Pepsi spill?
Was it powered up before the Pepsi spill was cleaned out?

I would be concerned with the misc pots not getting cleaned out properly
or replaced since they can hide contaminates well.

 

Drinks like Pepsi are *extremely* corrosive to electronic circuitry -
particularly if it happens to be on at the time of the spillage. In such
cases - I see probably 3 or 4 a year - my first move is always to examine
both sides of the board under a strong light, with a powerful headband
magnifier. You are looking for corroded leadouts on small resistors and
caps, and the slightest signs of corrosion under and between the pins of the
opamps. Often, a resistor leg will be eaten through right where it joins the
endcap, and pretty much invisible as a problem to the naked eye. These
damaged leadouts can make or not at a whim, which sounds pretty much like
the situation that you have. Meat's suggestion of pots hiding contaminants
is also very valid, as is his question as to whether it worked before the
spillage. Try to get as much info from the original owner as possible. The
internal track end connections can be compromised by corrosion, as can
contacts inside panel switches for channel selection, channel gain, and
effects loop jacks.

You could also try flexing the pcb by pushing on it firmly with the butt end
of a screwdriver or similar, to see if you can find an area that is
mechanically sensitive. Failing all of this turning up a result, it's signal
generator time. If you haven't got a signal generator, use something like a
cheapo MP3 memory stick player, with the output taken from the headphones
socket via at least a polyester cap of 0.1 to 1uF, or preferably, a little
1:1 transformer. Use the player's volume control as a level control, and
start at minimum volume at each point. Start at the input to the power amp,
and work back towards the instrument inputs, one stage at a time.

The key to success with a project like this is patience, and adopting a
logical approach. Occasionally, problems like this brought on by spillage,
defeat the best of us - from a commercially viable point of view that is -
but as you are doing it for self-training and personal satisfaction, then
the time it takes you doesn't matter. If you fix this problem though, it's
not to say that there are not further, related ones, lurking round the
corner. Whenever I get a fix on something like this, I always warn the
customer that this may not be the end of it ...

Arfa

 

well said, Arfa

id like to suggest checking the input jack(s). clean with Q-tip or in severe
cases a 32 cal rifle brush. from the description the problem is localized
to the input section. also spray the 3 pushbuttons next to the input jacks
with de-oxit or similar.

 

Washing the board as you have is fine but you needed to dry it off
quickly, not let it sit for 10 days. That encourages electrolytic
action between dissimilar metals which can cause metal to be etched
away - or other problems such as moisture ingress into resistors or
caps via any small imperfection in the coating. You should have blown
as much water out from under components as possible and then dried it
by applying hot air from a hair dryer or hot air gun for around 10 -
20 minutes, making sure you don't actually cook anything.

Water tends to get trapped beneath components mounted flat on the pcb,
such as IC's and electrolytic caps. Any water left under these
components is BAD...

Without seeing the board it is going to be difficult to offer help.
About all you can do is test elctro's for high esr or simply replace
them if you can't. Other than that it is going to be a matter of trial
and error as far as I can see.