Ampex 350 60Hz Hum

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Humdrumaudio

Member
Joined
Sep 25, 2018
Messages
9
Hello, I've been working on this Ampex 350 that pegs the meter ever time it's output is plugged into another piece of equipment(any other piece of equipment). To me this seemed like a grounding thing but everything checks out from the schematic to the unit, except that I can't find the output transformer's core pin to verify it's being grounded (I figured this was the issue). There are two unused pins on the output transformer, and after verifying that they aren't part of the windings, I grounded them but it didn't fix the problem.

The "hum" that is pegging the meter is 60Hz and only reveals itself when plugged into another piece of equipment. Also the volume doesn't impact this hum. Any ideas?

Thanks
 
What sort of work on it did you do exactly? Did you change any electrolytic capacitors? Has it been previously modified?

Is there anything plugged into, or did you or anyone else mess with, the monitor out or phones outputs?

Does the line termination switch make a difference?

Unplug everything and plug the power into the same power strip as the destination gear. Does the hum still occur?
 
This unit had been re-capped by someone else but I checked everything over and it looks to be correct. I was going to do a setup of the tape machine but when the meter pegged out I wasn't able to really do any of regular adjustments so I took the unit home.

No mods done to this unit.

I've tried plugging headphones into the phones jack and I've tried hook up equipment to the monitor, nothing seems to effect the hum.

Termination switch seems to attenuate the hum a little bit but not my much.

I have powered the unit under all kinds of conditions (on the same power stripe on different power circuits). This unit also has a two pronged plug so I even wired in a ground pin from the outlet to chassis and nothing as affected the 60Hz hum.

Kevin-
 
Yes the 60Hz hum that is pegging the meter can be heard in the headphones.

I haven't checked the ripple on the high voltage. That's a great idea. Thanks!
 

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