Hiss/Crackle in a Marshall 1959 Super Lead head from 1969

[telecaster]

The guy's wife put a dehumidifier on top of it and it overflowed. Following the visible mineral trail, it messed up two preamp tube sockets. I changed all 3 preamp sockets and cleaned up the grounds on all the PS filter caps - the one under the board by the preamp sockets was slightly corroded. I fired it up, and it still crackled/hissed loudly. The amp had all filter caps replaced previously FWIW. I've attached a clip of the schematic and annotated my attempts to lose the noise. I replaced all the resistors circled in red (V1 and V2 plates plus the 2 power supply dropping resistors). I've isolated the noise to the "B" side of V1. If you ground out the volume pot wiper (circled in blue) there is no noise and you can enjoy the other channel working perfectly (V1 "A" side). If you ground out the junction of the two 68K input resistors, the noise decreases significantly, but is still annoying. I swapped the cathode resistor and cap on a whim, no difference. Also swapped the .0022 blocking cap, no difference. All that's left to try on that stage is the two 68K resistors, and the 1M on the input jacks. Any suggestions on what to try next?

[pdf64]

If not already done so, a known good tube in V1?
Pete

[telecaster]

Thanks pdf64, I should have mentioned in my original post that I did try a new tube in V1 after isolating the problem to that stage. I should also mention I checked and reflowed all solder joints around V1, and I checked all the amp grounds (filter caps, wire on the back of the pots, etc).

[olddawg]

Thanks pdf64, I should have mentioned in my original post that I did try a new tube in V1 after isolating the problem to that stage. I should also mention I checked and reflowed all solder joints around V1, and I checked all the amp grounds (filter caps, wire on the back of the pots, etc).

I wouldn't recommend filling it with rice, but I would recommend letting a blow dryer on low heat circulate air in it for a couple of days, especially if you suspect any board of absorbing moisture. After that..... Break out the scope.

[The Dude]

The guy's wife put a dehumidifier on top of it and it overflowed.

Is she still alive?

[Chuck H]

Is she still alive?

My thoughts exactly! I do understand that the women in our lives don't appreciate "cool guy stuff" and would rather we kept it in garages and man caves, but to put a dehumidifier on that amp is just plain disrespectful. I'm sure hubby has never been subtle about how special that amp is. I'm thinking favorite meals and gratuitous BJ's for a year might be in order!

[olddawg]

My thoughts exactly!.... and gratuitous BJ's for a year might be in order!

Reminds me of the old joke... Question? "What's the difference between your job and your wife?" Answer: "After 5 years your job still sucks!"

[telecaster]

I asked the same question - Yes his wife is still alive.
I'm going to follow olddawg's advice and try heating and scoping.

[Aloha_Mark]

I would swap out the two 68K resistors, and the two jacks that feed the B side. I can't read the pot value that is connected to the 0.0022 uF and the 0.005 uF caps. Did you replace or plan to replace those too?