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?
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.
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.
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:
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?
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
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.
^^^^ 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.
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?
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.
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.
should the ripple/filter cap voltage be fluctuating when im playing? im assuming yes
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?
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?
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 (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 ).
is 70% okay or should i use the 99%?
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.
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.
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?
Quote from: ilyaaim a vegetarian
meh, a live one will do.
(http://www.lifeisajoke.com/Fun/chick3.gif)
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?
Quote from: ilyaaseems 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.
Quote from: ilyaa***a transistor that must have been laying around somewhere in the bowels just fell out!!!****
...
weird, huh?
Well that's a first. :o 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).
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!
Quote from: ilyaa on January 24, 2014, 04:00:53 AM
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.
Quote from: ilyaaguess i may as well check!
When in doubt (which seem to be most of the time with me) collect as much information as possible.
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.
Quote from: ilyaai 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).
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!