Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: judgeamps on September 16, 2020, 06:50:00 pm

Title: Mesa Maverick Oscillation when Volume turned up and Reverb noise
Post by: judgeamps on September 16, 2020, 06:50:00 pm

Hey all,

I have a Mesa Maverick head that starts oscillating when the volume (first stage, not the master or output) moves up past 9 o'clock and also produces a lot of noise when the reverb is turned up. This occurs with both channels and with or without the guitar plugged in. The pre-amp tubes have all been replaced, reverb tank rca connections/sockets/input jack/everything else have been cleaned with d5. Input jack was triple checked.

Ay ideas of where to look next? Schematic attached

Thanks,

Title: Re: Mesa Maverick Oscillation when Volume turned up and Reverb noise
Post by: pdf64 on September 17, 2020, 04:18:51 am

I suspect inadequate decoupling at the D and possibly C HT nodes, ie a failing cap or connection to the same.

Title: Re: Mesa Maverick Oscillation when Volume turned up and Reverb noise
Post by: judgeamps on September 17, 2020, 02:06:09 pm

Thanks, pdf64. I replaced the C and D filter caps without any luck. The rest of that section tests fine it seems. I do see that only the rhythm channel oscillates. Not sure if it was fixed or it was always like that and I missed it. If I disconnect the reverb tank, the problem goes away, but no reverb obviously.

Title: Re: Mesa Maverick Oscillation when Volume turned up and Reverb noise
Post by: pdf64 on September 17, 2020, 02:50:39 pm

Hmm, the preamp HT supply has 2 further stages, ie a dedicated reservoir, 470ohm, then a second stage.
The reverb driver is fed from the B node.
Maybe one or more of the above 30uF caps are dodgy?
If a connection is bad, good caps still won’t work right; so verify that the returns to 0V common etc are good.
Regarding the lead / rhythm mode differences, note that the HT supply gets more filtering in lead mode.

Title: Re: Mesa Maverick Oscillation when Volume turned up and Reverb noise
Post by: judgeamps on September 17, 2020, 04:33:46 pm

Thanks again. I believe I solved the problem. I cleaned the heck out of the jacks in and out of the reverb tank with a brush and more contact cleaner and I believe that did the trick.