I hear a “hum” coming from the transformer of my MOOG System 55 PSU (930). After recapping it and tightening the bolts keeping the transformer pack together little has changed. The low freq noise comes definitely from the transformer, like a mechanical vibration. Moreover, being the modular located over my Hammond B3 (EMO docet 
when I power it up a lot of interference / hum is heard through the organ output.
I think that the problems are independent
Any help is appreciated!
FERNANDO
I don’t know much about original Moog modulars but the power supplies used by synth.com style (whether MOTM, COTK, etc) use linear transformer based types and they exhibit a hum as you described; (and I am assuming it’s a vibration, not anything present in the audio path).
I bought a harvested heavy duty industrial power supply off eBay, and it’s the same type that synth.com use on their big system. Same problem.
I simply took four L-brackets, attached to the aluminum body of the power supply, then put rubber grommets into the part that attaches to the inside of my oak cabinet. Problem solved.
Certain things (like the pickups in the tonewheel) are going to be more sensitive to being in the proximity of the power supply. I wish I could elaborate further.
Eric
You could check the output voltages with an oscilloscope. If they are dirty / have higher frequency oscillations on the output rails this could be a hint for a defect, but not totally dead component like a power transistor. These oscillations can be heard as a hum or noise in the transformer or can result in a mechanical vibration.
I put my Memorymoog on top of my Hammond tonewheel organ and got instant hum from the organ (things that sound dirty but aren’t…). The pickups on the tonewheels in the organ are extremely sensitive in that the signals induced from tonewheel to pickups are very miniscule, thus by consequence Hammond tonewheels are very sensitive to EMI such as large power transformers. In fact the pickups are so sensitive that crosstalk was a problem - Hammond had to put RLC tank circuits on most of the pickups to filter out neighboring tonewheels and arrange the wheels so that crosstalk was minimized.
Can’t remember what keyboard I had on my L-100, but one of them would create that line noise from the transformer. I placed an aluminum plate between the keyboard and the Hammond. This did not completly eliminate all noise, but greatly reduced it. Could experiment with a metal shield like to one I made to see if that helps. Worked for me?
Aluminum can help, especially if grounded.
For magnetic induction tho, MuMetal is probably best, but is expensive.
(see: http://en.wikipedia.org/wiki/Mu-metal)
Otherwise, iron or steel would be the cheapest and for this application perhaps better than (also expensive) copper.
If possible, relocating the Moog power supply would help not only cut the induced hum, but would also remove the largest source of heat in the Moog.
So while it’s not a trivial job, doing this would eliminate the hum and keep the synth better in tune.
Great advices!
Removing the problem relocating the PSU can be an interesting operation.
THANKS
I shall let you know the advances
Fernando