Problem with RM20 combo - loud hum/unstable on certain notes

[UltraLow]

Possible solution --

Last night I opened up the RM20 & found that many of the wire termination disconnects were loose. The inside of the RM20 is full of these! I tightened up the loosest ones.

I also did the "chopstick" test, poking around the amp with a plastic tool while it was turned on to see if I could get any noises out of the amp from any components. I found two 1M resistors that seemed to be a bit "microphonic" producting a slight thud through the speaker when tapped with the tool. At first, I thought it was because it was mechanically coupled to a preamp tube, but I could not reproduce with any nearby components, or the tube itself. I don't know if this is normal, since I don't have a schematic & do not know the purpose of these resistors.

Poking one resistor in particular intermittently produced a very small "tick" or crackle, but I could not visually see any poor solder joints.

I didn't replace any components, but put the amp back together to see of tightening the disconnects did anything & the problem seems to have been reduced.

The problem seemed to have gone away at first, but I eventually got it to happen, but it is now bit more difficult to reproduce.

I am contemplating soldering all the disconnects permanently in place, but of course that will make the amp virtually unserviceable I think...

Bruce, if you're still out there, any advice on this and the 1M resistors is greatly appreciated.

Thanks,
Jim

 

[Chad]

Wow, you've sure tried a bunch of stuff. Kudos to your patience!

I assume it's too late to return the amp to whoever you bought it from?

If not, is it under warranty? At this point, I'd think strongly about taking or shipping the amp to a qualified tech.

 

[axecellent]

Jim,

Darn good trouble-shooting there.

Are either one of the 1M resistors labled that you can see? (i.e. R23, R38, R??). The resistor(s) could be bad. I'm wondering if those resistors are part of a filter. The bad resistor could be acting as a dead short at certain frequencies which might be overloading the rest of the circuit.

I ran into a bad resistor once building a band-pass filter in college. The ohms measured w/in tolerence, but it totally hosed the circuit. The o-scope would freak out at certain points while doing frequency sweeps. I don't remember exactly what was wrong with the resistor. I do remember smashing it with a hammer though (after removing it of course).

Thomas

 

[UltraLow]

Hi Thomas,

Soory, had to open it up and check again. I really noticed it on R32, and a little on R22 and/or R25.

I am probably going to have to ship it to Randall & have them fix it... The design (while creative) is not very conducive to user maintenance like a good ol' point to point wired tube amp ;-)

Thanks,
Jim