Peavey Deuce VT low voltage power supply problem.

[jvm]

I have a Peavey Deuce VT with AC hum in the low voltage power supply. All power supply voltages are to spec including the -15/+15 low volt supply. However, the +15 volt node also has 32 VAC, thus the hum. No AC present on the -15 volt side. I found ICR10(1N4744) LV PS, and also CR14(1N4148) at the preamp out, both shorted. Replaced both diodes. No change in the AC . All electrolytic caps have been replaced. The hum was present before and after the cap job. Tubes are out of the amp while testing. I am verifying the hum with signal tracing ( and the 32VAC still being present, of course). I have removed each IC one at a time to eliminate a shorted IC as a potential problem. I have 32 VAC at ICR10-IR11 junction. Any help appreciated.

http://www.thetubestore.com/lib/thet...-Schematic.pdf

[Enzo]

Do me a favor just to test your meter. You measure 32VAC there. Now reverse the leads and see if the reading is still there. Also, remeasuer the other clean 15v supply both ways. Does one way show 32VAC ther too now?

I find it hard to believe 32v AC is actually there, even though the meter ways so, simply because the power supply starts with only 20vAC.

The only other possibility I can think of is if the three orange wires are not plugged in correctly. If the connector is moved over one pin so it hangs off the end one pin, that would put 40v AC on the circuit.

Otherwise, we should scope the power supply for ripple.

Do the +15 and -15 sound the same on your signal tracer?


With the tracer, is the hum present throughout the preamp circuitry? Or only in parts?

[jvm]

You're correct. Reversing the meter leads shows no AC and the other side shows the same results. The three orange wires are soldered to the board on this model, no plug. I signal traced the preamp and the hum is present only on the +/- IC pins except for U11 and U12. The hum is on both pin 6's. Voltage measurements on both IC's are as follows:
U11
pin 1 -15 8 +15
2 -13 7 +15
3 -14 6 -15
4 -15 5 -15

U12
pin 1 -13 8 +15
2 -13 7 +15
3 -14 6 -15
4 -15 5 -15
I subbed new 3094 IC's for both. No change so I replaced the original chips.

[jvm]

I signal traced the entire preamp up to C72(power amp input). I input signal to each input and traced all the way through the preamp. I have a clean signal, all controls and functions work properly including the phaser. I measured 30mV of ripple on both positive and negative sides of the low voltage power supply.

[Enzo]

Wait, exactly what are we fixing ? The 32vAC was an illusion, so we still have hum out the speaker?

If that is the problem, how large is the signal at the speaker? We do testing with nothing at all plugged into the input jacks.

What controls affect the hum in ANY way? Level of it, tone of it, etc? What controls have no effect on the hum?

If not hum, then what?

[jvm]

Yes, I still had hum out the speaker but not the preamp out. I lifted the power supply board and checked my connections at all the new caps, orange AC wires, etc. I decided while I was there to replace the diodes and the .01 cap in the low volts supply. Hum gone. Also had a shorted reverb IC but the hum was gone before I changed it. I appreciate your time in trying to help Enzo.