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60 cycle hum

ValleyJim

Apr 18, 2012
6
Joined
Apr 18, 2012
Messages
6
Hi all

I have a guitar amp that is brand new out of the box and I have a hell of a hum. So I called the manufacturer and told them and they sent me another unit, did not even want the original one back. <???>

Anyway, I wanted to see if I could fix this one. If I am not mistaken a 60 cycle hum comes from bad caps or am I wrong there? Been a while since I have been in electronics class. :)

I pulled the back and saw about 6 caps and am thinking of replacing all of them.

How can I test a cap to see if it is working properly?

Any suggestions?
Jim
 

davenn

Moderator
Sep 5, 2009
14,254
Joined
Sep 5, 2009
Messages
14,254
hey Jim

if its brand new then its unlikely to be the caps unless its been sitting on the stock shelf for 10 - 20 + years between manufacturing and sale.
( OK there is a remote chance that there were a batch of bad caps??)

More likely its an earthloop or poor earth hum .... firstly I would be checking ALL earth connections right from the input plugs to the output. checking any PSU earth, chassis earth connections.

Assuming the replacement they sent you is an identical unit and works OK .... no hum ... you could do some comparisons of wiring etc

cheers
Dave
 

shrtrnd

Jan 15, 2010
3,876
Joined
Jan 15, 2010
Messages
3,876
I'm with davenn.
It's probably an earth ground problem (no connection or bad connection).
And yeah, a few years ago there was a big problem with a large quantity of
electrolyte the Chinese whipped-up. So bad caps (new) sometimes exist.
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
8,393
Joined
Nov 28, 2011
Messages
8,393
If it's Cheap and Chinese, there's also the possibility that it wasn't assembled or soldered correctly. Look for missing components (by comparing it with the other unit) or solder missing or shorting where it shouldn't.

If it's nothing obvious, take the chassis out and post a few photos here. That will give us some idea of what we're dealing with, and we should be able to make suggestions from there.

What service equipment do you have (or have access to)? A multimeter? An oscilloscope?
 

Jagtech

Feb 22, 2014
43
Joined
Feb 22, 2014
Messages
43
Does it hum all of the time, or just with volume turned up? Constant hum with volume off indicated power supply faults. Bad caps will produce a 120 cycle hum. Check first for bridged solder joints, or a joint that didn't get soldered at all. A shorted bridge rectifier in the power supply section would also be suspected.
Its really difficult to adequately test a capacitor without the proper equipment. Better to just replace it with same value and voltage rating, or simply bridge another one across it in circuit.
If it hums only with volume turned up, you have a bad ground.
 
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