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?
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?