Recapping Power Supply for an Altec 1592A - Nightmare HUM!!

[lassoharp]

I just attempted to recap the power supply in my 1592A and was greeted with a nasty hum in the output. If you know these units you'll note the original caps were 4 Mallory single cans (just like the 1591s). Replacement cans for these values are nearly impossible to find so I decided to use 4 new single caps mounted inside the chassis. Also note that because of the limited space inside the mixer the only place the new caps would fit is in the space above the mic input jacks. I installed mounting brackets and made extended leads for the positives, then tied the grounds to the original tie point just off the diode bridge board on the existing can cap ground tab. I left the original can caps in place with all the original ground points tied together. The original positives were of course disconnected when I tied the leads to the new caps. The unit powered up fine and worked as normal except the nasty 120Hz hum! I must have done something wrong but I can't figure out what it would be. Anybody ever recapped any gear that used cans where you had to remount the new caps elsewhere? Anyone know what might be causing the hum?

[The Scum]

Assuming that you used the same value & rating as the original: doublecheck that those caps are electrically connected to the right places. They're in there to smooth the rectified 120 Hz into DC. The hum is a sign that they're not doing their job.

And no problem with fashioning your own brackets and stuff...very commonplace in old Hammond organs. Just be careful not to short things by using plenty of insulated wire and heatshrink.

[lassoharp]

Values are all spec on and connections were as per the original connections. I am wondering if one of the 'new' caps may be bad. I used two Sprague atoms for the 2000/15s but the two 1000/35s are Multi-Comps which, if price is any indication are nearly always way cheaper than most any other brands. Chinese? Haven't had much experience with these caps but the Spragues are usually very dependable. I'm gonna take it to a local broadcast engineer who's very good at finding strange ground loops and let him look at it before I tear out the caps. Cheers