Need help troubleshooting CD100 amp

[jdemarti]

I am having trouble with a CD100 amplifier.  I know it is in the amp as I tried another one and it works fine.  Here is the issue.  When I plug up the amp and turn on the jukebox I get a very loud buzzing sound coming out of both speakers. Here is the troubleshooting I've done so far.

1) I've disconnected the volume control wires and the audio input wires. Nothing is plugged into the aux input or mic.  Still has buzzing sound.
2) I tried disconnecting the left and right channel pre amp control board separately to see if one of of the channels could be causing the issue. Still buzzes out of both sides.
3) I changed the Darlington transistors.
4) No appearance of blown resistors or transistors. Caps look good (no bulging or anything).
5) getting the correct voltage out of the transformer, and the two large blue caps. getting the correct +40v and -40V,

Any ideas on what to do next would be greatly appreciated.

Thanks



 

[jennifer]

    That's a common thing with caps Id start there,  and then getting into checking the transistors, The resistors are a bit trickier, You are going to have to remove one leg to take a resistance reading.... luck have it its probably a relatively easy fix however since it doesn't appear to be a thermal problem from what your saying.. 

[ami-man]

Hello jdemarti,

Do you have access to a dual beam scope and a signal generator?

I would remove the pre amp board and put a signal into each of the drver boards to determin if the fault is on the driver boards or the pre amp. The two wire wound resistors on the pre amp can be an issue, these tend to overheat and if this is the case you will see the signs on the board.

Please let us know how you get on.

Regards
Alan

Alan Hood
ami-man
UK

[jdemarti]

No access to a scope.  Wouldn't know how to use it anyway.  I checked the wire wound resistors and there are no burn marks.  I'm doubting it is the driver boards as I get buzzing out of each of them separately, as I would be surprised if both went bad as the same time. Not impossible though.  I'm really thinking it's got to be in the preamp somewhere.  I have a couple extra boards, I may do some swapping to determine which board is faulty.    Thanks