Hum in Hafler DH-500

[Bogie]

Any ideas on pinpointing/repair?

I've got just a _little_ hum in my #2 DH-500 (diagnostics: I pulled all the cables, switched all the cables to my other DH-500, etc., etc... so I'm pretty sure we're not talking ground loop). This one is running my rear speakers, and it really isn't noticeable unless you're standing right underneath one of 'em.

Bought #1 from AudioClassics, but this one got ebayed from a music store. I'm guessing it was used for gigging. It's got quarter inchers instead of RCAs, and the protection fuses aren't there anymore.

Sometime I'm thinking of tearing into it, and doing some fixups/replacements... back to RCAs, new caps, etc., etc... But for now, any ideas?

 

[djk]

Use an ohm meter and check for continuity between the grounds f the 1/4" jacks and the speaker grounds. Early DH500 had a resistor that would open up.

Replace the ground strap between the main filter caps with a copper strap.

The caps could be drying up too.

 

[freddymac406]

If your amp has pc-10 driver boards r-30 that connects the input and output grounds together may be open. They ranged in resistance from the factory. It will be either .5 ohm or 2.2 ohm.
Fred

 

[djk]

I told him how to check that in my post.

 

[Slymcfly001]

If your amp has pc-10 driver boards r-30 that connects the input and output grounds together may be open. They ranged in resistance from the factory. It will be either .5 ohm or 2.2 ohm.
Fred

Could this result post a complete cap replacement on the left channel PC-10 board? I got everything hooked back up, everything turns on, hear the click and the moment I plug into the speaker outputs I hear a semi loud buzz/hum. If I jump the input from the RCA common or ground to the center top bar on the two top caps it quiets the hum and music is audible but very low.. Would replacing these resistors and/or failure of these cause this noise? Mos Fets are getting warm on both sides.