Preamp, white noise, spitting

[Colas LeGrippa]

Hi evrrybody, I still have this Mesa Boogie preamp on my bench. What would cause a preamp to generate white noise with spitting noise more or less frequently ? I tested the 8 × 12ax7 of this quad preamp and all test good.
Filter caps gone ?
This unit is from the 80's, so it wouldn't surprise me that all the caps would have to replace.


Thanks


Colas

[shooter]

pick one so I can do better than spit back


https://el34world.com/charts/Schematics/files/Mesa_boogie/Mesa_boogie_Schematics.htm

[Colas LeGrippa]

https://el34world.com/charts/Schematics/files/Mesa_boogie/Boogie_quadpreamp.pdf


Here it is !

[shooter]

 


now you got yourself a right complex box of crayons Son!


NO PS schematic though, so all the same "rules" apply for the simple boxes;


Volts DC for all the plates n cathodes look good?
LOW volts DC +/- 8vdc, 40ish VDC, 5VDC


anything "physically" stand out like 2 leads sticking up with charred stuff between them?
caps swelled up like a well fed Mid-western farm girl?
 switches that operating like a 60's Dodges gearbox, grinding the last of the gears?


best WAG; the Footswitch ain't playing and the "work-arounds" for testing are gonna require some top-notch creative engineering with jumper wires, scopes, sig-gens


a full Check of DC volts will narrow it a bit though.

[acheld]

I think Shooter's got it right.

I don't have a magic answer, but I have seen cold solder joints around the tubes on Mesa's.   Just a thought.

Those things are a real slog to go through.

[Colas LeGrippa]

Hi buddies, I couldn't find no schem of the PS either.
At Mesa, they build amps thinking they'll never fail...
Removing and putting back the PC board only would take hours.
And why 2 reverb tanks ? like if one was not enough...
Ok, I tested the 8 tubes in another amp, all voltages are good inside the Mesa. The reverb is noisy, anyway the amp is noisy with the reverb volume down. I ll get Mesa sending me the PS schem, if I get it, I'll share it here.
Thanks


Colas

[shooter]

anyway the amp is noisy with the reverb volume down.

as in guitar sounds with noise, or just random noise n no guitar sounds??

[Colas LeGrippa]

The preamp works fine, footswitch, controls , volumes, but the reverb. The noise appears randomly. Sometimes for 5-10 seconds, the tone is clear , solid like we'd expect from a good shape amp. Then, it starts to spit, not enough loud to kill the guitar sound though, but  just annoying.
While writing my reply, I just got an idea. I 'll try to find and remove the reverb 12ax7 to put it out of the main circuit. Even with the volume totally down, the reverb circuit could have an effect on the overall tone.
Colas

[shooter]

there's 2 E-caps in the verb circuit, make them new


look for, as mentioned, cracked solder joints around the verb circuit, tank cables need checking n testing, you came a long way from "Preamp spitting" though, almost there 

[Colas LeGrippa]

Thanks buddy

[acheld]

At Mesa, they build amps thinking they'll never fail...

Truer words were never spoken!