acoustic 370 buzz/noise problem

[ilyaa]

is 70% okay or should i use the 99%?

[Enzo]

The difference in cost is a few cents.  I buy a pint of 99% for my shop every few years, it lasts.

The rest of the 70% is water.  No need for water on your board, so use the 99%.  It will evaporate much more quickly.

[Roly]

99%.  Decant what you need and keep the bulk supply tightly sealed, it's very hygroscopic, sucks humidity right out of the air.  Throw away your decant when you are finished with it.

[ilyaa]

okay so i cleaned it and maybe made some progress...maybe

i scrubbed it really well and let it dry out nicely, plugged it back in, and for a second the noise was gone, but, alas, no such luck.

except it's changed a bit now. back when this noise first happened like 6 or 8 months ago, i thought maybe it was a bad joint somewhere on the preamp board (that one seemed to react the strongest when tapped with a drumstick - the badum-tchhh kind, not the chicken kind - im a vegetarian), so i reflowed/resoldered the whole preamp board and it actually seemed to go away for a while. before on this thread i mentioned that it doesnt seem to matter where i tap the amp, it buzzes/crackles anyway. after the cleaning, seems like the power amp and EQ boards are not nearly as sensitive to being jostled as the preamp board. ALSO, the sound has gone from a buzz/hummy/noisy to more of a distinct pop (which is what it has manifested as at its worst in the past). i can most reliably reproduce it by tapping the modular connector that goes to the preamp board.

with my scope on the output, the noise appears as a pretty big spike on an otherwise flat line (while im not playing, just tapping). it spikes up for a second and then flattens out. with my scope on the filter cap, the ripple itself remains about the same when i evoke the noise, but it flashes the screen of the scope, kind of, and the whole ripple waveform shifts around on the vertical plane, moving up and down just for a second.

keep in mind that the noise is usually pretty unpredictable. sometimes the amp will just be sitting there and itll happen. sometimes itll happen very loud when i switch the amp on or off. sometimes it wont happen at all. now that ive cleaned it the noise turned from being a buzzy crackle that lasts for 2-5 seconds to just being a loud pop.

seems like it must be the preamp board, right? these symptoms dont really sound power supply related, do they? keep in mind the amp works perfectly otherwise. guess ill try to isolate the mechanical issue thats probably causing this.

another quite strange development is in the process of cleaning the amp, i turned it on its side to dry and ***a transistor that must have been laying around somewhere in the bowels just fell out!!!**** a 2n4248. there are a few of those on the schematic, one near the input and two in a differential amp in the power amp. i cant see any missing transistors anywhere but i was not able to actually remove the preamp board (jesus acoustic makes it hard to work on these amps) because i did not have an allen key small enough to remove the knobs. wtf. someone did add some components on the underside (its on the top the way its installed) of the preamp board before i bought this amp, so maybe this particular 2n4248 was replaced and left inside to rot. weird, huh?

[Roly]

im a vegetarian

meh, a live one will do.


Mains-powered free-range chook (better than battery).


Well it seems like in cleaning/washing you have at least "touched" the fault, and it's now almost certain that it's physical, not electronic; dry/fractured solder joint or corroded connector.  Does wiggling the input connector have any particular effect?

seems like it must be the preamp board, right? these symptoms dont really sound power supply related, do they?

Seems so, and no, they don't.

Can you disconnect the preamp signal feed from the main amp?

If so try that and see if is stops, then we can be fairly sure that we should be looking at the preamp.

If you can do this you can look at the preamp output in isolation with your CRO and see if you can see it coming from there.

***a transistor that must have been laying around somewhere in the bowels just fell out!!!****
...
weird, huh?

Well that's a first.     I've had all manner of strange things found in/fall out of the backs of amps I've been fixing, the odd valve even, but never a transistor.  Yes.  But it's certainly a clue that somebody has been in there because it's vanishingly unlikely that it's a hangover from construction.  It's possible they were chasing what you're chasing (or that they have caused it).

[ilyaa]

it wont be easy to tap the signal feed - the modular connectors are soldered onto the boards. every precaution has been taken to keep any part of this amp from being removed from any other.

BUT it does have two line outputs coming straight from the preamp. would a CRO on showing the symptom point to the preamp as the culprit? guess i may as well check!

[g1]

someone did add some components on the underside (its on the top the way its installed) of the preamp board before i bought this amp, so maybe this particular 2n4248 was replaced and left inside to rot. weird, huh?

You said the amp is tough to get the boards out of.  Perhaps they desoldered a bad transistor and had to push it through (rather than remove the board).  Then it may have got stuck somewhere and they couldn't shake it out.
  Is there a circuit board that is solder side up with a 2N4248 mounted to the solder side?
  This is how repairs are sometimes done when it is too difficult to remove the boards.

[Roly]

guess i may as well check!

When in doubt (which seem to be most of the time with me) collect as much information as possible.

[ilyaa]

alright!

so i gave it a realllyyy good scrub today. got ALL of that gross, dark orange, kind of amber/sap looking stuff out of the board(s).

preliminary test before going to bed indicates that maybe the noise is gone. i dont want to jinx it, of course, but i was bashing that board with a drumstick and it was silent. and the amp continues to work fine, otherwise.

so maybe it was just a gross, oxidized/corroded board, whatever that orange stuff means! ill do a thorough check tomorrow and report back, but it may have been a strictly mechanical issue all along. i really cleaned the modular connectors, too, as i think they may have been part of the problem. but, we'll see - its an intermittent issue so who knows, might come back with a vengeance tomorrow.

[Roly]

i was bashing that board with a drumstick and it was silent. and the amp continues to work fine, otherwise.

Aaay!  Great!   :dbtu:

"Orange stuff" to me means nick-o-teen residue, but it also could have been downwind of cooking or a fire.  The problem is normally moisture in the contamination.

Given that cleaning has now made such a large difference, even if it comes back to some extent this shows you are on the right track.  If it does happen to come back I'd persist with more cleaning, and as I said, make sure you include the underside of transistors where the leads come out of the packages because that is where any leakage could be at its worst (very close spacing).

[ilyaa]

yeah i think we did it! the real test will be playing it again and just keeping my ears open, but things definitely seem to be better.

thanks, guys!