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acoustic 370 buzz/noise problem

Started by ilyaa, January 17, 2014, 09:31:15 PM

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ilyaa

anyone used these amps before? great sounding and a really great build inside EXCEPT its really hard to take it apart if you want to work on it. things are connected all over the place with straight up leads so you have to desolder stuff to take the boards out. lame. anyway, heres my problem:

works great, sounds great, BUT makes a loud (sometimes very loud) buzzing sound at different times:

1) when turned on
2) when turned off
3) when jostled, even a little bit, like if i knock it or the cab its on
4) when its on but idle itll sometimes randomly buzz, perhaps a result of outside vibration

opened up, nothing looks bad. the weird thing is, when i poke around in there, no matter where i poke it makes the sound, so its hard to isolate where the problem is (im assuming its a bad joint somehwere). also, the noise is loud at first poke, then gets quieter as i poke more until it goes away completely. if i wait a bit and poke again it comes back, sometimes very loud.is it probably a bad joint? its going to be such a pain in my ass to remove the boards and resolder.....

any ideas?

Roly

Quote from: ilyaalame.

Damn right.   Hate that.  :grr   :loco

Quote from: ilyaathe noise is loud at first poke, then gets quieter as i poke more until it goes away completely. if i wait a bit and poke again it comes back, sometimes very loud.is it probably a bad joint?

Certainly could be, and given that this has the characteristic of a cap charging/discharging ('tho I can't quite see how ATM) I'd start by checking the joints on any larger caps since their mass will tend to give their joints a harder time.

You could try to isolate it to an area by working over the board tapping lightly and as consitantly as possible with a chopstick or similar.

Good luck, and let us know how you get on.
If you say theory and practice don't agree you haven't applied enough theory.

g1

 Try a toothbrush gently on the solder side of the boards.  That way it should only trigger the problem when you are right on the bad joint.

Roly

Quote from: g1 on January 18, 2014, 12:21:33 PM
Try a toothbrush gently on the solder side of the boards.  That way it should only trigger the problem when you are right on the bad joint.

Neat trick, thanks!   :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

well, that's the weird thing about this problem. ive tried what you all are kindly suggesting, but i cannot seem to find any one spot that is more sensitive than another. i can literally tape the amp ANYWHERE and it'll buzz. i can even lightly tap the outside of the chassis with my finger and get more or less the same buzz.....which makes me think there is something else going on altogether....particularly because today the buzz totally went away for a while when i was playing it, only to come back again.

an acquaintance who came over today suggested another possible explanation id like to run by you all: he said that in older boards (and this is an old amp) the dielectric material of the board itself can start to go bad and break down, creating either conductive paths or diode-like connections between points. he said its particularly likely since i live by the ocean and the wonderful crisp air yadda yadda. anyone ready to corroborate that possibility? in which case im really up the creek, im not about to etch new boards....

the only reason i am inclined to believe him is that i really CANNOT find a particular spot that is more sensitive to probing than any other. that makes me think that the whole kit and kaboodle is going bad.....thoughts?

Roly

Quote from: ilyaathe dielectric material of the board itself can start to go bad and break down

Not really.  Marshall appears to have had a (so far unconfirmed) problem like this, but apart from boards getting charred by overheating components this isn't an issue.

Quote from: ilyaasince i live by the ocean

Surface contamination can certainly be a problem and seaside electronics is well known to suffer from salty air.  You could try giving the board(s) a good wash and scrub with (fresh, dry) metho or alcohol absolute if you can get it, both sides, and the undersides of the transistors where the leads come out.  Let them dry right out before applying power.

HTH
If you say theory and practice don't agree you haven't applied enough theory.

g1

What does the output look like on the scope when this is happening?
Also scope the + and - supplies when it happens to see if there is an increase in ripple.  It could just be a bad solder connection at the filter caps or one of the filter caps could be intermittent.

J M Fahey

^^^^ fully agree and add: you have many connections *outside* the PCB itself, from jacks to switches to pots to bolted/riveted ground connections to .... you name it.
In fact what you describe starts so suggest a *chassis* related problem.
Instead of tapping with your trusty chopstick, try pressing together joints such as ground lugs, pot bodies against chassis, etc.
I once repaired an Ampeg SVT400 amp which had been subject to salty mist for years: each and every zinc plated chassis part spot soldered to another one was full of red spongy rust.
The emergency repair (they brought it on Friday and had a huge show on Saturday  :duh :loco) was to drive a small nail between any 2 such surfaces, which scratched rust and made contact ... worth a few days only, but the ONLY way to properly repair it was to pull its guts completely, leaving the bare chassis, and send it to my zinc plating suppliers, where they boiled it in diluted clorhidric acid, then in a cyanide bath (all stuff you'd love sprinkled over your burger  xP ) until all rust/corrosion/old zinc/etc. was removed and only bare metal remained.
Then it was replated, first with copper (not for conductivity but to seal even minuscule pores)  and then properly replated.
Very expensive if you ask me, but they were very fond of that particular amp.

ilyaa

the schematic doesnt tell me what voltage im looking for off the filter cap, but i imagine its larger than the 20V p-p my o-scope can handle...

do i need to get a 10:1 probe or something like that in order to scope it effectively? is that what that's for?

Enzo

You really should have, and in fact should use most of the time, a X10 scope probe.


If you mean the main DC filter cap, if you are looking for ripple, set your scope vertical coupling to AC volts instead of DC.  That way it will ignore the DC part and just show the ripple.


If you mean C403, the big can, it ought to be roughly 90v.  It is a 100v cap, so it wouldn;t be more than that.  Plus look at the output of the power amp, it says +45v there, next to the speaker cap C404.  Since this is a single supply power amp, the output lives at half the power rail.  SO if they expect 45v there, we can infer roughly 90v on the rail.

ilyaa

i know i know there are lots of tools i need....i'm getting there!

anyway, as soon as i asked about using the scope on the caps i remembered that im just looking at ripple, not DC voltage, so i got it.

the ripple looks normal. if i have the scope on the filter cap and i play, the ripple gets wavy, but that's about all. the buzz/hum/noise problem does not seem to really affect the ripple at all. the slope/curve stays perfectly steady - it jumps around a little bit, but less than it does  while i am playing.

with my scope on the output, the buzz/hum/noise goes from a little extra waviness/wobliness on the waveform when the buzz/hum/noise is quiet to being a full on angular/triangular and quite large distortion when the buzz/hum/noise is loud.

so in other words: the scope on the output shows a big angular nasty looking thing when the noise happens. the scope on the filter cap shows nothing significant but a little waver/wobble when the noise happens.

ilyaa

should the ripple/filter cap voltage be fluctuating when im playing? im assuming yes

Roly

Quote from: ilyaa on January 22, 2014, 02:15:16 PM
the ripple looks normal. if i have the scope on the filter cap and i play, the ripple gets wavy, but that's about all. the buzz/hum/noise problem does not seem to really affect the ripple at all. the slope/curve stays perfectly steady - it jumps around a little bit, but less than it does  while i am playing.

with my scope on the output, the buzz/hum/noise goes from a little extra waviness/wobliness on the waveform when the buzz/hum/noise is quiet to being a full on angular/triangular and quite large distortion when the buzz/hum/noise is loud.

so in other words: the scope on the output shows a big angular nasty looking thing when the noise happens. the scope on the filter cap shows nothing significant but a little waver/wobble when the noise happens.

Acquiring tools and instruments is endless - like guitars and amps, you can never have too many.  :tu:

Yes it should jump up and down when you are playing, but the fact that it doesn't with the fault is another indication against a parasitic, these normally cause a full output load current.

What 'scope are you using?  Most have a little warning symbol on the front next to the inputs with "500Vmax" or similar.

I'll repeat my caution above; CRO's can be very sensitive instruments and show you low level gunk that looks frightening, but when taken in proportion is quite normal.  For example preamp hum, hiss, and general noise can look terrible on a fully cranked CRO, but you may not be able to actually hear it in the speaker, and may be only -96dB referred to full output, next to nothing.

In your case "a big angular nasty looking thing" may be an indication that a cap in the power supply is failing open or low capacity.   It could also come from an intermittently opening signal ground path.  (?)

Have you washed the PCB yet?

If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

its a phillips PM 3265. in the manual i think it says it can take 400v max or something

i have not given it a bath yet - whats the best way of doing that? cotton swab with regular isopropyl? or contact cleaner? or something else?

Roly

That's a very worthy CRO that shouldn't be worried by anything inside this amp, even using a x1 probe.  My own bench jobbie ATM is a PM3055.

Cleaning see;
http://www.ssguitar.com/index.php?topic=3365.msg24859#msg24859

Isopropyl is fine (but metho is cheaper).  Pour it on and give it a really good scrub with a toothbrush (but outside, eh?  And no smoking.  :o ).
If you say theory and practice don't agree you haven't applied enough theory.