RM20 Noise

[Clel Miller]

A friend of mine owns a high school. Somebody donated one of these RM20's for his music program. I wish I could find a schematic.....
Not sure how many versions of these were made. It is 2 x EL84. This has a removable tone stack, DC heaters on the tone stack and the pres on the PCB, AC heat on the PI and power tubes. The amp is real noisy. Sounds like a bad pre tube, dirty tube socket, cold solder type of sound. Kind of a popping, crackling, hissing, noise.
I re flowed all the solder on the pre and power tubes, including the two horizontals in the module. I substituted for every tube a few different times, chop sticked the lead dress, cleaned and jumper-ed the loop, unplugged the fan. Nothing I tried made a difference, not better or worse. Maybe it has a bad plate resistor? I guess I should be able to locate those with a DMM, but a schematic sure would help.
Is there anything else I should try?
Thank You

 

[Clel Miller]

Nobody wants to take a stab at this eh?
Have the slide ins been a problem? Any history with noise, bad solder joints, etc. with those things? Its possible that I may have missed a bad pre tube, maybe I should replace them again; one at a time.
Thanks

 

[Julia]

I have no clue.

Try the one at a time pre-tube thing and see if that does anything. I mean it's already not working right so what can go wrong? Don't answer that.

Have you tried slapping it in the sweet spot? You'd have to have grown up with tube televisions to know how to do this properly. I'm talking about all tube circuits. Don't try this at home. :wink: See the first sentence.

 

[Clel Miller]

Finally got hold of a spare module, the JTM is bad, the Clean module is much better. You could never record with this thing, it has the hiss that is so common in modern high gain amps. Is there a common component problem with these modules. The dual AX7's are not the noise source. Do you guys shot gun the op amps or transistors to solve noise issues? This JTM is a noisy, oscillating nightmare. What have you guys replaced on your bad modules?
Thank You