Humming Mystery Amp (Univox U-1235 Bass amp)

Humming Mystery Amp
This amp came in with a lot of hum. The owner thought it might be a Silvertone, but we eventually determined that it was a Univox U-1235 Bass amp, produced in the 1968-1971 time frame (thanks to ebay, confirmed with info from http://www.univox.org/amps/uniamp.html). The owner wasn't sure he wanted to put much money into it, so I agreed to try some basic hum reduction techniques. All the electrolytics needed replacing, including the bias cap, which was wired in backwards! (That also explained the low power output.) One diode was bad, so I replaced both B+ diodes. I also added a DC bias to the heater wiring. Finally, I added a three wire, grounded power cord. These steps reduced the hum to about 80% or so of its former level, and brought the power output up to where it belonged. Getting rid of the rest of the hum would require at least redoing much of the rat's nest of wiring, and possibly replacing the circuit board. The owner didn't want to put that much into the amp. I probably wouldn't, either.
Click on a photo to see a larger version. Larger photos are anywhere from 50KB to 1/4MB JPEGs, and may be larger than most monitors can display.

Front view. Power cord is laying on top. Inputs: Bass, High, Low. Switches: Deep, Sharp. Controls: Treble, Bass, Volume. Power switch is Off (down), On (middle) or Standby (up).
Rear view. Power tubes are Audio Glassic Blues Tubes (Chinese 6L6s). The preamp tubes are Mullard 12AU7s. The 6L6s are beautiful, and sound OK, but the Mullards are worth more than the rest of the amp!

Front top view. This shows how the chassis is assembled. All the passive components are on the PCB except the trannies and the largest power supply cap. Aren't the power tubes pretty? If only they sounded as good as they look!
Top view. The chassis is on an old towel laid across the top of the cabinet to prevent the chassis from scarring the cab. In normal use, the preamp tubes have shields over them.

In this photo the B+ caps have been replaced, but that's it. Those beautiful Atlas oil caps will all prove to be horribly leaky as well. The "buttons", or "pills", at the left of the PCB are the old B+ diodes. Note the lead markings on the power transformer; I wish all trannies were this well marked!
All the caps have been replaced, as well as the B+ diodes. A trace on one of the diodes died during desoldering, so the diode was soldered directly to the wire. This was a portent of things to come; the traces on this board were all ready and eager to secede.

For the curious, that's a Wavetek DVM in the lower left of the bottom right photo. In the same photo, just above the amp is an old, tube-based frequency generator.
Last updated: 26 October 2002
Copyright 2002 Miles O'Neal, Austin, TX. All rights reserved.
Miles O'Neal <meo@XYZZY.rru.com> [remove the "XYZZY." to make things work!] c/o RNN / 1705 Oak Forest Dr / Round Rock, TX / 78681-1514

Front view. Power cord is laying on top. Inputs: Bass, High, Low. Switches: Deep, Sharp. Controls: Treble, Bass, Volume. Power switch is Off (down), On (middle) or Standby (up).	Rear view. Power tubes are Audio Glassic Blues Tubes (Chinese 6L6s). The preamp tubes are Mullard 12AU7s. The 6L6s are beautiful, and sound OK, but the Mullards are worth more than the rest of the amp!
Front top view. This shows how the chassis is assembled. All the passive components are on the PCB except the trannies and the largest power supply cap. Aren't the power tubes pretty? If only they sounded as good as they look!	Top view. The chassis is on an old towel laid across the top of the cabinet to prevent the chassis from scarring the cab. In normal use, the preamp tubes have shields over them.
In this photo the B+ caps have been replaced, but that's it. Those beautiful Atlas oil caps will all prove to be horribly leaky as well. The "buttons", or "pills", at the left of the PCB are the old B+ diodes. Note the lead markings on the power transformer; I wish all trannies were this well marked!	All the caps have been replaced, as well as the B+ diodes. A trace on one of the diodes died during desoldering, so the diode was soldered directly to the wire. This was a portent of things to come; the traces on this board were all ready and eager to secede.