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Messages - ilyaa

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316
Amplifier Discussion / Re: acoustic 370 buzz/noise problem
« on: January 23, 2014, 02:51:16 PM »
is 70% okay or should i use the 99%?

317
Amplifier Discussion / Re: acoustic 370 buzz/noise problem
« on: January 23, 2014, 11:00:44 AM »
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?

318
Amplifier Discussion / Re: acoustic 370 buzz/noise problem
« on: January 22, 2014, 03:07:05 PM »
should the ripple/filter cap voltage be fluctuating when im playing? im assuming yes

319
Amplifier Discussion / Re: acoustic 370 buzz/noise problem
« on: January 22, 2014, 02:15:16 PM »
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.

320
Amplifier Discussion / Re: acoustic 370 buzz/noise problem
« on: January 21, 2014, 05:03:26 PM »
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?

321
Amplifier Discussion / Re: ampeg ss150 supersonic/parasitic oscillation
« on: January 20, 2014, 10:17:54 PM »
oh yeah one other thing:

im supposed to be getting +/- 39v on the rails (according to the scheme), but i am getting +38 and -48...is the too large negative voltage perhaps a symptom that i should investigate further?

322
Amplifier Discussion / Re: ampeg ss150 supersonic/parasitic oscillation
« on: January 20, 2014, 09:18:05 PM »
okay i think i need to rewind a bit - afraid im going to get in over my head without the tools to get out.

roly's comment on the amplitude/furriness relationship alerted me to some potential misinformation i might be providing.

im relatively new to electronics stuff and this is my first time using a scope to troubleshoot an amp, so bear with me as i get my bearings. the furriness ive been repeating indeed appears when the scope is cranked, at about 0.4 V p-p some other stuff starts to happen and the overall fur seems less apparent....help me sort it out:

here is my measurement technique -> amp plugged into an 8 ohm speaker -> scope probes across speaker leads; my computer is generating a 1K sine wave that i am putting into the amp. i was keeping the signal kind of quiet because that tone is freaking annoying, but i guess i have to crank it a bit to really see what's going on, huh? i guess ill have to get a dummy load....dont have one. i was looking at a relatively quiet signal before, so maybe the furriness was more due to noisiness inherent in magnifying something teeny on a scope.

what p-p voltage should i ideally be looking for at the output of an amp on a scope? in terms of clearly seeing what i need to to troubleshoot?

and keep in mind the amp works except for an occasional and quite loud THUMP, possibly a supersonic oscillation.

as ive messed around with the amp, changing levels and moving where im scoping, im getting all kinds of crap, from a sine wave with a weird dip in it, to something totally beyond recognition, to a curve that just looks like a cap charging and discharging. but no where am i getting a clean, normal looking 1K sine wave, except right at the signal generator output (before the amp).

in the interest of making this troubleshooting process efficient, anyone care to give me some quick guidelines as to where would be most useful to scope (right before and right after power amp, for example) AND what i should really be looking for? what range of p-p voltage? should the wave start getting all wacky if i boost any of the EQs? should gain and level be cranked as im scoping? in other words, who wants to buy me a new amp?

should this amp be giving me a nice sine wave? or am i looking for something different altogether?

thanks guys.....





323
Amplifier Discussion / Re: acoustic 370 buzz/noise problem
« on: January 19, 2014, 03:57:28 AM »
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?

324
Amplifier Discussion / Re: ampeg ss150 supersonic/parasitic oscillation
« on: January 19, 2014, 03:51:49 AM »
okay i made a little progress with the scope, i think.

put in a 1K sine wave and got a 1K furry sine wave. this is my first time using an o-scope to test an amp, but i compared it to the same wave into my homemade champ and, although that wave had a little dip in it (probably something i should address at some point, too), it was certainly not furry like the one coming out of the ampeg.

looks a lot like a) below



i guess b) is a parasitic oscillation, so perhaps thats not my issue, after all??

anyway, hope the scope information can guide us in the right direction. furriness is bad, right? i should be getting a nice clean sine wave, particularly since the sound out of the speaker is a totally clear tone (i have the o-scope across the speaker leads). whats a good next step? should i start going backwards across the board with the scope until i find the spot where the wave goes bad? that sounds fun, kind of, is it a good method?

325
Amplifier Discussion / acoustic 370 buzz/noise problem
« on: January 17, 2014, 09:31:15 PM »
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?

326
Amplifier Discussion / Re: ampeg ss150 supersonic/parasitic oscillation
« on: January 17, 2014, 09:15:59 PM »
okay some progress, but we might need to reassess our theories.

i DONT think its an oscillation issue. looking at a larger amplitude waveform from my guitar, there is NO discernible fuzz or distortion other than what the amp naturally provides. it looks pretty clean, in other wo

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