Amp now working but loud Hum.

grabread

Active Member
I have been working on a solid state amp, it's now in working condition but there is a loud hum (AC)?
It is always present on both channels and is not altered with the volume control.
there is about 180mv of ac on each channel dc is only - 1.8 mv .
I have attached a couple of photos of the scope connected to the speaker teminal . one of just the AC ? wave and the second is with a 1khz tone on top of it.

I am thinking it is a capacitor problem any idea's?
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many thanks.
 
just trying to eliminate ground hum .. if you short main amps in to ground it might make a difference . or might not . see what happens ,
it is probably a fault in the power supply but easy things first .
p.s you never said what you have done already and what amp is it ?
 
It's a Hitachi HA -6 . Bought as non working. When opened up it was obvious it had been worked on and given up on. 14 transistors missing and about a dozen fuse resistors needed replacing as they had gone way off spec. I have replaced all those components and had a good check around and it now produces sound.
I hope it's not the main power supply caps as they are huge and likely to be expensive.
 
The original caps are almost the size of coke cans, I would like something that is almost the right size if poss. But is it the main caps at fault or something else?


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I have had a probe around the power supply today with the scope. the main supply looks clean but there seems to be a supplementary supply to the amp board. the yellow wire has both dc and half wave? ac on it ( around 21 v) the yellow wire seems to be supplied by a diode and a cap. Diode checks ok on the meter and cap is not shorted. I have added some more pics.
Any Ideas?
 

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Got a schematic? That would help ID that wire and figure out what it should have on it. Taking a guess, its probably not supposed to have a vast amount of ripple but without knowing exactly what you're measuring or what its supposed to do, its hard to say if its wrong.
 
It's partly stripped at the moment so not able to apply power to try. I think I am in the correct area with the yellow lead that I identified with ac on it.
Anyone understand this schematic of the power supply and if the yellow wire attached to d901 is likely to have ac on it

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yes it will be noisy as no filtering apparent ..where does the yellow wire go to ? p.s that looks like underside of the board .
 
Thanks Pete, the service manual is not that good (glad tp have it though). I'm trying to see where it goes to, if I am reading this correctly it goes to Q801 and zd801


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