Music Man HD150 hum

[jvm]

[]Music Man HD150 with a loud hum present at screen resistors. It has been recapped but I went ahead and replaced both 100uF/450v caps to eliminate them as potential problems. When I got the amp D12(1N4748) was shorted and one power tube was dead. I replaced both and also replaced D7 but have the same hum that was present before. Hum is present with tubes removed. Any suggestions?ATTACH]29337[/ATTACH

[52 Bill]

Did you test the driver stage transistors, etc. (Q5 and Q6)? Are all of the supply voltages about where they should be?

[jvm]

I attached the correct schematic. Yes, I tested both drivers and they test ok. Also pulled driver IC to eliminate it. Low voltage supple is fine at 16v/-16v.

[jvm]

I measured 370 volts at all 470 ohm screen resistors, 23 volts at D12, 16v/-16v at D13 and 14. I can't measure the 700V supply without a better meter. Mine maxes at 600. I'll dig through all my old test equipment and see what I can find.

[52 Bill]

I measured 370 volts at all 470 ohm screen resistors, 23 volts at D12, 16v/-16v at D13 and 14. I can't measure the 700V supply without a better meter. Mine maxes at 600. I'll dig through all my old test equipment and see what I can find.

Try measuring across C29 and across C30. Each one will have half the B+ supply and then add them together.

Do you have continuity on all three wires of the primary winding of the output transformer?

[Enzo]

Hum is present with tubes removed.

THAT is the elephant in the room.

Power tubes gone and it still hums? If that is the case, then the only way for hum to come out the speaker is for current to flow through the transformer.

One way for that would be power transformer magnetic field coupling into the output transformer, but I want to rule that out because it would mean ALL of the similar amps would do it.

And that leaves electrical current. With no power tubes, we then have left a leaky pin 3 on a socket, or one of the flyback diodes D15,16 or caps C39,40 is leaky, or the worst case, there is a winding shorted to frame or winding to winding.

[jvm]

Sorry for not clarifying, Enzo. I am using a signal tracer to verify that the hum is still at the grids with tubes removed. I just wanted to remove the output tubes from the equation. Sorry for not including that in my original post.

[jvm]

B+ is 374 per side, or 748v. All power supply voltages are correct. I also have continuity on all three windings of OT primary.

[52 Bill]

Okay, I'm confused here now. You are signal tracing a hum to the input grids of the power tubes? I thought that you were talking about a speaker hum in the output.

Trace back see if you can find where it starts.

[jvm]

Sorry, 52 Bill. It hums through the speaker with tubes in stalled. I have traced it back to the power supply. I unhooked the orange PT secondary wires to shut off the 22 and 16v supplies. Still has hum at grids. I have unhooked D12 from from the tubes and still have the hum at D12. So I have the loud hum at the 470's and 220's at the output tubes and at D12. I have subbed a new cap for C31 with no affect on the hum. I'm thinking definitely power supply but can't seem to find anything amiss.

[52 Bill]

I have unhooked D12 from from the tubes and still have the hum at D12. So I have the loud hum at the 470's and 220's at the output tubes and at D12. I have subbed a new cap for C31 with no affect on the hum. I'm thinking definitely power supply but can't seem to find anything amiss.

First, earlier I misstated input grid, as these amps are cathode driven.

So you are saying that you are using an signal tracer on D12 and you are hearing a loud hum? Have you checked C32 and C33? Have you measured the ac voltage on the +22 volt bus?

[g1]

And you mentioned hum at the screens/470's, how about C30, (and even C29)?

[jvm]

I have replaced C32,C30 and C29 with known new caps. No change in hum. I reinstalled the original parts. This amp does not have C33. How specifically do I check the ac voltage at the +22 volt supply?

[Jazz P Bass]

How specifically do I check the ac voltage at the +22 volt supply?

Set your meter to read Volts ac?

[jvm]

85 VAC at D7 and 50 VAC at D12. Seems a little excessive, no?