Possible power supply problem, hum

[stickers]

Items: PSO3 with mu metal power supply shielding

Preamp modules from left to right: 4 A12, 2 N72, 2 J99

Problems noticed:

1. All units have a ground hum when gain is set fairly high. A12 closest to the power supply has the most hum.

2. Both J99 have a little buzz going on in addition to the hum

Both my J99's where serviced by Tim about 8 months as it turned out 2 out of 4 of my 990 op amps were duds. And one of my N72s was checked out by him as well.

I have tried moving modules around, leaving some out, and hum problem is still there. When I put the J99s near the PSU the buzz i mentioned about is worse.

I've been using the pres since the amount of gain to really hear the hum is high but still I know it shouldn't be there.

Any help would be great.

[tpryan]

Remove the chassis from the rack.

For each channel:

Connect only a headphone amp to the output

Connect a 150 ohm resistor across pins 2 and 3 of the input

Does the hum change?

[stickers]

I only have 200ohm resistance on my meter and used that.

Tests done to A12 on far left side.

removed from rack.

Yes it did change the hum. It became louder and some slighly different fundamental frequency, half step down maybe, but more upper harmonics. Sorry, I am having a hard describing it. But it certainly changed.

[tpryan]

I only have 200ohm resistance on my meter and used that.

What?

You need an actual resistor to physically terminate the inputs. Get an XLR plug and solder a 150 ohm resistor between pins 2 and 3. Insert the plug into the input of the channel under test. Any tests or observations are meaningless unless the inputs are properly terminated.

[stickers]

Ok.

I will go to radio shack and buy a 150 ohm resistor and find an xlr plug somewhere and then do the test. I'll report back tomorrow as I am off to work.

[stickers]

Tim,

I injected the 150ohm resistor across pin 2 and 3.

It greatly reduced the hum.

A good discovery:

The additional buzz that was occuring in the j99s was caused by being near the head phone amp. As I was executing the 150ohms test to all the pres, I noticed the buzz was gone. Out of curiousity i just picked up the chasis and as I slid it back in the rack where it was before and the buzz came back. Mounting the unit 2 spaces below resolved the problem.

Anyways moving on. What does it mean knowing that hum was reduced with 150ohm resistor across pin 2 and 3 at the inputs?