Hum left channel on Threshold SA3 (I know its not a pass labs but...) - diyAudio
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Old 30th May 2008, 05:30 AM   #1
Tino is offline Tino  Canada
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Default Hum left channel on Threshold SA3 (I know its not a pass labs but...)

Hi guys, maybe one of you might be able to shed some light on my situation....

I've got a mild amount of 60Hz hum which is audible when i'm a foot or two away from the woofer ... but only on one channel (left one). From a normal listening distance I cannot hear it but I'd rather try and figure out whats causing it.

Even with nothing but the amp connected to the speakers (no inputs) it hums so its not a ground loop type issue I'd think.

I opened the amp and maybe I'm wrong but it looks like each channel has its own large cap. Would it be likely that this hum is from the large input cap(s)

Any suggestions welcome
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Old 30th May 2008, 06:39 AM   #2
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Default Re: Hum left channel on Threshold SA3 (I know its not a pass labs but...)

Quote:
Originally posted by Tino

I opened the amp and maybe I'm wrong but it looks like each channel has its own large cap. Would it be likely that this hum is from the large input cap(s)

Any suggestions welcome

Suggestion, based on your observations: put the cover back on and get someone qualified to look at it. I'd start with checking small electrolytics on the board
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Old 30th May 2008, 10:40 AM   #3
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Quote:
Originally posted by analog_sa
start with checking small electrolytics on the board
By placing a similar value in parallel with the potential faulty one.
You would have to take the complete channel + heatsink of the chassis if you favor fondling this one yourself.
Sure it's 60Hz ?

(In contrast with the other mid '80s SA/ models, the SA/3 has a single powersupply (700VA toroidal and 2 big Mallory cans) for both channels. I hope you do not mean the blue bottles with "own cap", but the little red riding hood ones on the piggy-back boards ? )
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Old 30th May 2008, 02:53 PM   #4
Tino is offline Tino  Canada
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Well I cant be 100% sure its 60Hz but definitly its not a multiple of that so its not 120Hz and it doesnt sound very low so my conclusion is that its around 60Hz.

As for putting the stuff in parallel, I was thinking of seeing if there is any way I could reach into the circuit board for some of the caps and use tiny alligator leads to gain access...so this way I'm not taking anything apart.

Now as far as having to take the heatsink and stuff apart, it doesnt look like I'd need to do this. I dont see any caps on the main board which holds the power transistors. There are some caps (like the larger red ones) on the signal boards which appear to be mounted on posts...only thing is some of the wires are fairly short. Might have to desolder them to remove the board.....only looked for a couple minutes though. I'll look more carefully tonight.
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