Rocktron XDC - Intermittent buzz ID?

[stakattack]

Hi All-

I'm about to attempt a repair on a Rocktron XDC rack distortion unit... with no signal running through it makes a pulsating or intermittent buzz sound, kind of like buzz, silence, buzz, silence and I'm wondering if any of you experts would hazzard a guess as to what that might be or where I should start when trouble shooting it?

I have an audio clip but not sure how to post it

Thanks

[stakattack]

Well... I removed it from the rack and the buzz stopped! moved it into the rack and the buzz starts again. I hear a ground hum but the power cable has no ground, just he two prongs

With that in mind any suggestion on how to silence the hum? Or what the issue may be?

[g1]

Is it's input or output connected to other units that are mounted in the rack?
Some ground loops can be solved, but sometimes you need isolation washers to insulate the chassis from the rack.

[Leo_Gnardo]

With that in mind any suggestion on how to silence the hum? Or what the issue may be?

Thought #1:

You need humfrees! Or something like 'em. Plastic tabs that let you rack mount your gear without being electrically attached to the rack. They can help if there's a ground loop made thru the rack rails. If you can park your preamp in the rack without the mounting flanges touching the rails, and standing the chassis on some temporary insulator, a slice of cardboard box or a chunk of wood or plastic, and you get no hum, this is probably the answer. And if not:

Thought B:

Is your preamp located right under, or over, a power amp or something else that might broadcast a large magnetic field? We hope metal chassis are enough to stop that, but sometimes it's not enough, you have to put some space between whatever's sending out a field and whatever's picking it up. Sometimes as little as 2 inches, sometimes a foot.

[The Dude]

Not saying it's so, but most Rocktron pieces I've worked on have a 1 ohm resistor from jack sleeve to ground at the output, specifically for ground loop issues. They do occasionally burn up if there is a problem. You might pop the top and see if you spot a burnt resistor near the output jack.

[stakattack]

No Isolation from rack so I will try that and yes two rack mount power supplies are in the same rack - I will move those

Thanks for the 1ohm resistor tip, i will try to locate that