Classic 50 410 sizzle and crackle

[01redcobra]

I've got a Classic 50 that's got me going in circles. There's sizzle crackle noise only on the od channel. I isolated it to V1. replaced the plate resistors and it stopped. Let it set overnight and the noise was there again. Re-tensioned and cleaned the tube sockets. Once again ok until it sat for a while. Using a chopstick, if I drag the chopstick across the board near the trace for the plate current, I hear the scraping of the chopstick or if I tap in that area I can get noise. If I spray electronics cleaner on the grid that's on the board( the one created with the plate and grid traces) if I turn the amp back on,the amp starts to sizzle, just like throwing water in a hot frying pan. As it dries, the noise subsides. I cleaned the board off with alcohol (after it was discharged, of course) and after it dried, fired the amp up again. No noise! I'm going to let it sit overnight and fire it again tomorrow to see if the noise returns. Is it possible that this board is holding moisture? The amp had been stored for a few years in a basement. New tubes in all preamp spots. The only thing I'm seeing are some darker spots on some of the traces, but shining a light through the board doesn't show me any breaks or carbon arcs. Also, all the solder joints having anything to do with V1 have been reflowed. What am I missing?

[01redcobra]

And of course, the noise is back again today. :/

[01redcobra]

Has anyone experienced something like this before? I really feel the board is holding moisture. It's fine after it is worked on, or on for a period of time, but after an overnight, it's noisy again.

[Jazz P Bass]

Do you have a heat gun?
Or a hi wattage hair dryer?
I am thinking 'bake the boards'.
Have you tested any of the voltages?
You may have a duff decoupling cap.(Vac ripple)

[01redcobra]

Jazz, I do have a heat gun. And the DC voltages are as follows V1: (1)-150 (2).002 (3) 1.23 (6) 166 (7) 0 (8) 1.22 and on V2 (1) 149 (2) .01 (3) 1.24 (6) 165 (7) 0 (8) 1.24 Of course it's pretty quiet this AM. Just a little noise when first fired up. It's only on the dirty channel. the clean is quiet. And it is directly affected by how much you crank up the pre pot. I will attach a schematic. I have a scope, but I'm not too good with it. I did see some noise on V2 at pins 6 & 7, even though I can't hear the sizzle it normally makes. Just the hiss from the gain being dimed. Thanks for getting back to me.

[01redcobra]

Forgot the schematic. Old-timers disease I suppose.

[Jazz P Bass]

The voltage readings are all Vdc.
That is fine.
The next step would be to measure V1 at the plates (pin 1 & pin 6) but set your meter to read Vac.
No signal.

[01redcobra]

I'm getting .010 at V1 pins 1 & 6 and V2 pin 1, but I get .16 to .17 vac at V2 pin 6, unless I roll the pre pot down to 0. Then I get the same as all the other pins.

[Jazz P Bass]

C11 may be leaking.
0.047uf
I am assuming that the measurement was made when the amp was quiet.

[01redcobra]

Yes, it's been quiet all day. It gave a little noise this am, but hasn't since. It's the mechanic theory, it only does it when the mechanic isn't there. I was thinking maybe C21 since I don't hear anything on the classic channel.

[The Dude]

I know you said you replaced all of the preamp tubes, but of late it's not uncommon to get new defective tubes. What you are describing sounds like a tube arcing to me. Have you tried subbing V1 or swapping it into a different position?

[01redcobra]

I was second guessing myself, so I swapped tubes in all positions again. I have a stock of about 10 JJ 12ax7's, so I pretty much went thru all of them. The hardest part (or the most time-consuming) is that once the amp warms up, it usually doesn't have the noise again again until it does a long cool down. Even after sitting overnight last night, it only sizzled for a second or so. Gonna try some freeze soon to see if I can induce the noise that way. And yes, it does sound like arcing. I've had the amp on with all the lights out in an attempt to see something. I know when this has run its course and I find the culprit, it's gonna be something stupid that I missed.

[Jazz P Bass]

To rule out C11 you would measure the non plate side of it, Vdc.
Get a 'quiet' reading (zero is ideal)
Then a 'noisy' reading.
If the voltage goes up, the cap is bad.

[01redcobra]

So, I have been letting this one set for a while. The craziness factor was setting in. Fired it up today and all was quiet on the western front. I decided to do a little freeze spray diag to see if that would make it act up. The amp went into fits when I hit the main board where the ribbon cable comes in from V1.Putting the heat gun on it, the noise goes away. Looks like it's time to pull the main board and see what's going on.

[Jazz P Bass]

Cool troubleshooting (pun intended )
'Classic' Classic failure.
Those ribbon cables are great if the amp never ever gets taken apart.
But if it does get worked on, well.......