Got A Hum When Reverb Is On

[Darrell Hendrix]

Here's one for an electronics expert. I have a older solid state PA amp made by Woodson. 4 channel and 100 watts. Its actually a real nice unit. Here's the problem. When I turn on the reverb, there is a pretty nasty hum thru the speakers. Sounds like AC. Now here's what I did. I checked and cleaned both cables going to the reverb tank and that had no result. Then I completely removed the tank from the chassis and put it about a foot away from the amp. Guess what. No Hum at all. Worked perfectly. I picked up the tank and slowly moved it closer to the amp. When it was 2 or 3 inches away from the amp the hum started creeping back in again. When the tank is mounted where it belongs, it's right above two pretty big caps and about 4 inches away from the transformer. Any thought on what the problem might be? Everything is original and there have been no modifications made. So there ya' go..Tank out - no hum..Tank in - big hum.. Thanks !

[Paul Osbty]

Interesting. I would check a couple of things.

Do the returns/shields/sleeves of the tank's input and output connectors have good (0 ohms) conductivity?

Do the returns/shields/sleeves of the tank's input and output CABLES have good (0 ohms) conductivity to the chassis?

I am thinking the tank's chassis should be at chassis ground. Does it's mounting interface connect well with chassis ground? (It's acting like it's not at chassis ground when mounted as your out-of-chassis test proved).

I'll keep thinking.

[Jon Light]

Do you have long enough leads to turn the tank around, north-to-south? I was just reading elsewhere that the output jack on the tank has high Z signal that is sensitive to proximity to the power xformer while the input jack is low Z and more forgiving.

[Darrell Hendrix]

bump

[Darrell Hendrix]

up we go ...bump

[Darrell Hendrix]

Bumpo !

[Darrell Hendrix]

Up to the top please!

[Darrell Hendrix]

Bump please............