HELP, Millet Amp is buzzing
Mar 1, 2006 at 3:33 AM Thread Starter Post #1 of 18

brschmid

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not sure what the heck is wrong. turned it on tonite and was going to enjoy some tunes (duh) anyway, there was an annoying buzzing sound. Been out of this stuff for far too long to try and diagnose it myself, so here is what happens.

1. tried different headphones, no help.
2. unplugged source, got worse
3. touched the amp, got louder (worse)
4. opened it up, looked ok to me, noticed one cap had a dent in it, not sure what that means, looks like it has been there a while (built so long ago i can't remember)
5. changed tubes, no change
6. change power outlets, no change

what else do i need to look into?
 
Mar 1, 2006 at 3:34 AM Post #2 of 18

fewtch

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Bee's nest inside it?
eggosmile.gif


Edit -- sounds like a grounding problem, but not sure what to suggest...
 
Mar 1, 2006 at 3:51 AM Post #3 of 18

ericj

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Quote:

Originally Posted by brschmid
4. opened it up, looked ok to me, noticed one cap had a dent in it, not sure what that means, looks like it has been there a while (built so long ago i can't remember)


Dent in or bump out?

A dented capacitor can't be healthy long-term. That capacitor could be leaky now, even if it was working fine before. Best to replace it.

Bumped out? Capacitor has failed. Replace it. If you have a capacitance meter, check other capacitors of same type in the circuit - they may be on their way out. keep in mind they can only be tested out-of-circuit and should be completely drained before testing.

Also, check the power supply voltages before and after regulation. If you have an RMS meter, check for ripple by switching it to AC and testing the DC output.
 
Mar 1, 2006 at 3:59 AM Post #5 of 18

brschmid

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i was thinking ground problem, will try to find my meter now. unless it was a bad solder joint, i can't see how anything failed from moving as the amp has not changed position in 2 years.

btw: cap is dented inward, doens't look as if anything has been leaking
 
Mar 1, 2006 at 4:03 AM Post #6 of 18

ericj

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Quote:

Originally Posted by brschmid
btw: cap is dented inward, doens't look as if anything has been leaking


I meant electrically leaky. It could be passing some current that it shouldn't.
 
Mar 1, 2006 at 4:24 AM Post #7 of 18

brschmid

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not sure on the cap yet.

another observation i just noticed. if i hold the headphone plug and then touch the case, the buzzing stops. that means it is for sure a ground problem? If so i have no idea where
frown.gif
not even sure where my instructions article went too

i was probing around with my meter and i am not even sure what i am looking for
frown.gif
the voltage at the plug is 23.83v (24v so that sounds correct) and the voltage at the connection points on the amp was the same. after regulation i didn't check, because i can't remember where the heck those read points are at
 
Mar 1, 2006 at 4:40 AM Post #8 of 18

ericj

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That does sound a bit like a grounding problem.

Volume pot body grounded to the board properly?

Since this is a DC coupled design, all ground points should be tied together.

(That excludes earth-ground if your power supply is in the same case)
 
Mar 1, 2006 at 4:49 AM Post #9 of 18

brschmid

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volume pot is not grounded at all, is it supposed to be? i built this amp 2 years ago, and never had this problem until tonite.

power supply is a wall wort as that was easiest for me at the time i built it.
 
Mar 1, 2006 at 6:15 AM Post #11 of 18

ericj

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Quote:

Originally Posted by brschmid
volume pot is not grounded at all, is it supposed to be? i built this amp 2 years ago, and never had this problem until tonite.

power supply is a wall wort as that was easiest for me at the time i built it.



Sometimes you get some noise if the frame of the pot isn't grounded. The usual complaint is with the 'blue velvet' alps. You have to unscrew one of the screws on the back part way, wrap a wire around it, tighten it back down, and then ground the wire.

If your enclosure is metal, and grounded, the nut on the pot's stem will ground the pot just fine - unless it gets lose.

Anyway, just throwing stuff out there.

If the wall wart has severely overheated, the capacitor in there could be all dried out. But since you have way of muting the noise, it's probably not power ripple.
 
Mar 1, 2006 at 6:33 PM Post #12 of 18

brschmid

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the case is metal, and the nut on my Alps Blue Velvet has not loosened up at all, so i think we can rule that out (grounding of it anyway)
 
Mar 1, 2006 at 8:15 PM Post #13 of 18

ericj

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Quote:

Originally Posted by brschmid
the case is metal, and the nut on my Alps Blue Velvet has not loosened up at all, so i think we can rule that out (grounding of it anyway)



Still might be worth it to set your DMM to ohms and touch one probe to the front panel and the other to ground on the board, just to be sure.
 
Mar 1, 2006 at 8:30 PM Post #14 of 18

brschmid

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any ground or like the ground where the power comes in? still can't find my article/schematics...
 
Mar 1, 2006 at 9:19 PM Post #15 of 18

ericj

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That's what i was saying earlier - since this is a DC coupled design, all grounds are the same ground, and you should read 0 ohms between all 'ground' points and all other 'ground' points.
 

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