Marshall JCM800 2205 Reverb hum problem

[Fred G.]

I'm working on a Marshall JCM800 2205 with a hum problem. I plugged a guitar into the effects return jack - nice, quiet, sweet and clean tone. There is a loud hum in the reverb circuit - I'm pretty sure it's in the recovery side. I've swapped out preamp tubes - no difference. Turning down the Reverb control kills the hum.
Unplugging the tank makes no difference. I'm hearing two types of hum here - one a little higher pitched and slightly "buzzier", another lower-pitched and much quieter. Grounding the grid pins on the recovery tube socket makes no difference; however, grounding the "output" side of C29 does remove a great deal of the hum, and wipes out the higher-pitched "buzzy" hum. I'm also measuring about 9 mV of DC on the output side of C29. C29 is also somewhat microphonic, giving a noticeable clunk when I chopstick it. Before I yank out the circuit board and start shotgun replacement of components in the reverb circuit, I'm asking if there are any other tests I can make beforehand, comments, suggestions, etc.? Thanks in advance!

[Helmholtz]

Schematic?

[Fred G.]

https://www.thetubestore.com/lib/the...-Schematic.pdf

[dubulup]

I'm working on a Marshall JCM800 2205 with a hum problem. I plugged a guitar into the effects return jack - nice, quiet, sweet and clean tone. There is a loud hum in the reverb circuit - I'm pretty sure it's in the recovery side. I've swapped out preamp tubes - no difference. Turning down the Reverb control kills the hum.
Unplugging the tank makes no difference. I'm hearing two types of hum here - one a little higher pitched and slightly "buzzier", another lower-pitched and much quieter. Grounding the grid pins on the recovery tube socket makes no difference; however, grounding the "output" side of C29 does remove a great deal of the hum, and wipes out the higher-pitched "buzzy" hum. I'm also measuring about 9 mV of DC on the output side of C29. C29 is also somewhat microphonic, giving a noticeable clunk when I chopstick it. Before I yank out the circuit board and start shotgun replacement of components in the reverb circuit, I'm asking if there are any other tests I can make beforehand, comments, suggestions, etc.? Thanks in advance!

I could have written this myself...I believe it's the exact amp (markings have faded), and I'd describe the issue the exact same way (not measuring any DC on C29). Everything seems to check out with the controls...but the reverb knob adds quite a bit of buzz even with every other knob at 0 position.

Wondering what Fred G. found out or if the shotgun replacement approach solved the issue. If not, I'll post some finding...

[dubulup]

I don't understand much about reverb tanks...but this doesn't look factory. Are these just mechanical stops that have been hot glued back in place?