SF MV Twin Reverb 120 HZ hum in reverb circuit

[booj]

I know it's normal to hear a littlehum in the reverb circuit when the reverb is turned up, but this is an annoyingly loud hum even with the reverb turned down.
The amps been re-capped (electrolytics) and otherwise sounds great.
It has the waxed fiber boards and brown coupling caps. The reverb reads normal dc resistance (about an ohm and 180+ ohms) I've tried several tube swaps. Possible WTF syndrome? The amp was worked on before (clean work), and I had to perform a bit of de-mod work.
This is a several hundred dollar repair, and its a bit dissapointing to have an "unfixable" problem!
Any help greatly appreciated!

[überfuzz]

Try running the reverb with the reverb tank outside the chassi. (Just to narrow the search area a bit.)

[pontiacpete]

you've probably already tried this, but,,, new cables? or a new tanK?

[booj]

I'll try swapping tanks. I already tried running it with the tank out of the cabinet. The hum is there with the cables disconnected from the amp although not as bad. Pulling V4 kills the hum totally. I'm beginning to suspect the mods that were done earlier haven't been "cleaned out" totally. I'll give it a good point to point around that area.
this is 120 HZ power supply hum I'm getting. V1 and V2 are connected to the same supply, but turning down the channel controls has no effect.

[Chuck H]

Check the ground scheme on both the tank and the amp. It it's a replacement tank, and grounded to the tank body on both ends, you probably have a ground loop. If the circuit grounds aren't isolated from each other you may have a ground loop.

Chuck

[R.G.]

120Hz is always power supply noise. AC ground hum is always 60Hz (in 60hz countries at least!). If it's 120Hz, it's coming from picking up either the power supply ripple, or grounding that lets the ground of the reverb be wiggled around by charging pulses in the power supply rectification circuit, or - especially if it's buzzy sounding - it can be picking up the 120Hz repetition rate of either capacitive or RF emissions from the diode turnoff in the power supply.

[trem]

During the Re-Cap and the De-Mod, were any of the ground connections changed?

[jmaf]

The hum is there with the cables disconnected from the amp although not as bad.

Does not sound like a reverb tank problem.

V1 and V2 are connected to the same supply, but turning down the channel controls has no effect.

This means it's not being inserted on those grid circuits.

Pulling V4 kills the hum totally.

This eliminates the inverter and power section.

My 2 cents: Since all tubes before V5 share a common power supply point, it could be on the power supply. First bet: a cap job, especially the three 20uF caps after the choke. Second bet: leaky coupling cap, check for DC on all preamp grids.

[booj]

Problem fixed.


Chuck H, Thanks for your input! The tank is the correct one for the amp. I checked the grounds anyway, and they were correct.
R.G., You da man! I was thinking along those lines. I just couldn't wrap my head around waxed fiberboard leaking that much p. s. signal. But it was.
Trem, I checked the best I could. I didn't do the mods. I did however look under the fiberboard for any stray solder blobs or wires and such.
JMAF, I did a complete re-cap of the amp. I did however, replace the coupling cap from pin 6 of V4. It tested good with a cap checker.
My main problem was going into panic mode. I figure I'm trying to FIX something. Not RE-DESIGN the MF. But, thems the cards we are dealt sometimes.

[jmaf]

I'm glad you solved it. I followed your explanation on the attached image, thanks for sharing.