Mesa Boogie Road King 2 - Microphonics Squealing everywhere

[christarak]

A Boogie Road Kill 2 came in not working with what was described as a "a power supply issue". I got that sorted, replaced a blown 5U4G and replaced one of the failed rectifier 5U4 indicator LEDs. With a short in one of the 6L6’s, the customer approved me installing a new set of TungSol 6L6’s. The EL34’s tested fine. It’s now working but the microphonics are insane. There was no mention of the microphonics and crackling noise on the customers fault report.

With the Gain on ZERO, the Master Volume on 7 and the Output on FULL the slightest tap will send it oscillating.

It also is making that crackling noise that has me suspecting plate resistors. I have chop-sticked around every component paying particular attention to plate resistors and coupling caps. Needless to say I have replaced V1, V2 and V3 preamp tubes several times over. With the Gain up, V1 is the noisiest, as I'd expect, but this thing will squeal with the Gain totally off.

I pulled out the preamp tubes and re-soldered the tube joints from the top side. Resoldered the V1 and V2 plate resistors and a little 0.003uF (C41) that made a different, deeper, sound when tapped. Because the squeal is happening with the Gain on ZERO I suspect the cause is further in to the amp than V1 or V2.

I just can’t find the culprit components. Every component and wire seems to make the same noise.

I won’t embarrass myself to say how many hours I have spent on this amp, but will I go to hell for saying I hate big Boogies?

May I entreat your invaluable advice once more?

[Jazz P Bass]

I would monitor the preamp bypass capacitors to see if the chassis tapping makes them ring.

There job is to bypass any signal from the power supply.

MesaBoogie_RoadKingII pwr supply.pdf

[J M Fahey]

1) Also *scratch* along each component body with a chopstick.
Tapping shakes the whole board andmay vibrate components a few inches away, scratching one by one is way more focused and specific, you shoiuld hear the scratching noise through the speakers.
Not a magic bullet by any means , but might reduce the search area from 100 components to maybe 10, that helps.

2) go backwards, get a .047uF x 400V at least cap, dolder a wire to one end and ground it, and with the other end touch each ane everu grid tgravelling from then PI backwards to input jack; you will find a series of grounded grids which kill oscillation, another series which do nothing; the one between them is the suspect.
Again, just redu ces the search area, you´ll still have to troubleshoot but with higher chance of success.

[christarak]

Thanks Juan.
1) I suppose "tap" may have been an exaggeration. I've been letting the chopstick fall by its own weight from the height of half an inch. Scratching has the same result. I might try it again tonight with the master volume a bit lower so the whole amp isn't on the verge of squealing.
2) I like the .047 cap suggestion. I'll try it tonight.

[christarak]

I would monitor the preamp bypass capacitors to see if the chassis tapping makes them ring.

There job is to bypass any signal from the power supply.

[ATTACH]48713[/ATTACH]

Sorry, Jazz. I'm having a seniors moment. I'm not clear about what you mean by "preamp bypass capacitors". I don't think you mean plate or cathode bypass caps because you sent me the power supply schematic (thanks for that, mate! I have no schematics for this amp). Are the preamp bypass capacitors the ones after the 12v regulator? Or, the 7x10uF filter caps? Thank you for your help...again. Chris

[Pedro Vecino]

With the Gain on ZERO, the Master Volume on 7 and the Output on FULL the slightest tap will send it oscillating.

You talk about gain but it does not say what channel it refers to.
That amp has four channels. The first two make one block and the last two, another (although they have elements in common).
Select on all channels 2x6L6 to make it easier. Reverb to zero. Bold mode and Diode rectification. Bypass the loop with the rear switch (this removes the loops, the general master and Solo). The fifth 12AX7 is deactivated. In this way, only the gain and volume of each channel remain active, as well as their equalization and presence.
Check now each channel to see if they all oscillate or just one of them. Or none...

[christarak]

Gracias Pedro. I am embarrassed that I did not mention your points. I was only testing Channel One, the lowest gain channel. I was using the setting with 4x6L6 + 2xEL34 with both rectifiers. Reverb set to zero (with no reverb tank connected). Your other suggestions are excellent, and I feel embarrassed that I did not check these possibilities. especially ass I replaced the 6L6's and the 5U4Gs which may have introduced this squealing problem themselves. I am not familiar with this complex animal. I will follow your suggestions tonight and report back in 24 hours .

[Jazz P Bass]

Preamp bypass caps are C3 & C9.

[christarak]

Preamp bypass caps are C3 & C9.

Aha! I will check and report back.

Thanks Jazz.
Chris

[christarak]

I did what I should have done in the beginning, and check every exhausting combination of tube combinations and rectifier combinations for each of the 4 channels. The unacceptable level of noise was common to all settings. I also tried removing the 2 new 5U4Gs and replacing them with solid state yellow jackets. Same. Same. Same. Then I turned a knob marked Send Level, and the noise went from insane to acceptable. I traced the loop send back to V5. Changed that tube and cleaned the tube socket. Cleaned all of the loop in and out sockets and the Send Level pot. With the Send level set to zero, the microphony and noise of a high gain behemoth like the Road Kill 2 seems acceptable.
Thank you Pedro, Juan and Jazz P Bass for your invaluable support. I'm saving this file for my problem-solving skills.
Chris

[g1]

Pedro had mentioned a switch that bypassed V5. I hope that helped you find out V5 was the problem.

[christarak]

It did G1. I wasted a lot more time than I could have without schematics, but as usual, it was a learning opportunity from my more experienced colleagues. I thank you G1 for your support on most of my queries in the past (and hopefully in the future).
Chris