Moderators MrKnobs Posted June 23, 2013 Moderators Share Posted June 23, 2013 See how long it takes you to diagnose it from these steps I took. Probably less time than I took as I'm not used to working on powered monitors.The problem presented as a loud, strictly volume dependent crackling noise in the monitor during use. It happened consistently, and no amount of gain staging fixed it. The monitor seemed to be operating at a reasonable level where it should have run clean. There is no overload LED on this Peavey monitor.First I tried taking the speaker grille off and pushing the cone to see if it was rubbing which would mean a blown speaker. Nope, very smooth. Then I hooked up a sine wave generator to the input, put in some 80Hz and sure enough there was the crackling sound just as we heard it in rehearsal. So far so good. Too many (temporary) miracles happen when you get a box on the bench. So I disconnected the speaker leads to see if the little piezo tweeter was making the noise and it was just fine. I figured strangely blown speaker but I plugged the aux output into one of my own passive monitor speakers and got the same messed up noise meaning it was the electronics instead. Damn. Then I noticed that MY speaker only crackled when HER speaker was hooked up. To be very sure I was hearing both speakers crackling I unhooked HER speaker entirely and took it out of the case, set it aside. Now my speaker sounded clean, no crackle. I hooked HER speaker back up again, leaving it out of the case and that was also fine. Putting it back in the case caused the problem to reappear. So, vibration intermittent, I figured. Time to pull the electronics board out (that was fun figuring out, turns out the outside corner brace has to come out). The power transistors look to have been replaced fairly recently and crudely, so I feared a bad repair job, maybe even wrong parts - BUT - when I took all the connectors loose from the circuit board, cleaned the contacts and exercised them the problem went away. So my guess is that vibration from the 12" speaker was shaking the circuit board and making a poor connection cut in and out. I hope the connections I cleaned and exercised were the ones causing the problem, but now the problem is gone so I can't really check the rest of it unless and until it happens again. I did try lots of pencil tapping. I didn't see any cold solder joints. Putting my hand on the electronics board while the speaker was putting out about 50W of 60Hz seemed to be beating up the circuit board pretty fiercely. This, and heat, are one of the tradeoffs you get with powered speakers, I guess. Terry D. Link to comment Share on other sites More sharing options...
Members Pro Sound Guy Posted June 23, 2013 Members Share Posted June 23, 2013 Bad connection/oxidation That was the fix. Next time you will know that is the FIRST thing you do is check all connections after nothing shows with a visual inspection. Link to comment Share on other sites More sharing options...
Moderators MrKnobs Posted June 23, 2013 Author Moderators Share Posted June 23, 2013 Pro Sound Guy wrote: Bad connection/oxidation That was the fix. Next time you will know that is the FIRST thing you do is check all connections after nothing shows with a visual inspection. On this powered wedge the grille, the speaker and the top right corner brace all have to come off first in order to remove the amplifier section. Not too bad with an electric screwdriver, though. Terry D. Link to comment Share on other sites More sharing options...
Members Audiopile Posted June 23, 2013 Members Share Posted June 23, 2013 MrKnobs wrote: See how long it takes you to diagnose it from these steps I took. I hope the connections I cleaned and exercised were the ones causing the problem, but now the problem is gone so I can't really check the rest of it unless and until it happens again. I "fix" a lot of things by simply taking them apart and putting them back together. I call these sorts of "repairs" as "laying of hands" or "faith healing". The device has a problem... I take it apart and don't see anything wrong, shrug my shoulders and put it back together and the problem's gone. I dunno why. Don thinks I'm nuts when I suggest that inanimate objects might have "feelings"... sometimes an item just needs somebody to act like they care. I can't come up with a better explanation. Link to comment Share on other sites More sharing options...
Members Miko Man Posted June 25, 2013 Members Share Posted June 25, 2013 Mark, my speculation on the "laying on of hands" phenomenon is that a good mechanic's systematic and careful disassembly and reassembly of a device has the effect of re-establishing the normal settings and conditions of the device that are needed for proper operations. I think that because everything gets examined and put back together, whatever little thing (or multitude of things) that needed attention get the attention needed. That's as opposed to a "let's try this, let's try that" approach that misses something, even if one fault gets "fixed" and the "repair" work stops there. That, and the machine or device knows that if it doesn't work, I have a bigger hammer available for the second round.... Mark C. Link to comment Share on other sites More sharing options...
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