Boogie Mk IIA ch switching relay noise

[McBarry]

This is killing me....
switching from clean to lead channel, there's a loud thump/pop. Going back is almost nothing. Switching from front panel pull switch - footswitch not even connected, but if used will also replicate the symptom.
I know relays can emit some noise, and this series is renown for it, but this is obtrusively loud, not a little pop in the background. Enough for the rest of the band to look at me as if to say "what was that.."...
It occurs with no signal, and occurs before volume pots ie; volume(s) on zero silence the amp and also the noise.
I've re-soldered all joints from V3B coupling cap forward to ground and all around relay.
There's no DC on the signal or reverb tracks to/from relay.
Coil volts pre/post are a nice 7.99dc.
No stray dc between tracks although I can't get right in under relay.

I need some help/ideas from the tech guys.
Especially to deduce if it's noise from the coil getting into the signal, or ON the signal side, like a cap suddenly charging/discharging..
Eg; if it's the coupling cap suddenly discharging as it goes from being shorted to ground to now connecting to the lead master pot (and is the schematic drawn wrong in that relay connections section?), how to prove and also how to rectify??

thanks for all thoughts..
Dave

[Enzo]

This might be easier to see:
http://www.schematicheaven.com/boogi...oogie_mkii.pdf

relays don't generally "emit" noise in my experience, but they can cause it. The coil runs on DC and doesn't set up much of a field, so there is little to pick up from it. If you get the relay coil ground return mixed in with signal grounds, you might get some ripple current hum.

When the coil is de-energiozed, its inductance can make a voltage kick. That is what the reverse diode across the coil is for in most any well designed relay circuit. I'd like to see one added here.

Most relay pops however are due to unterminated components being switched in and out.

Does it matter if the EQ is in or out?

You say the noise is before the volume controls. ALL of them? Or just the Masters? What effect has the pre volume over there right after the tone stack? How about the lead drive control?

[McBarry]

Thx Enzo.
On my board there is a diode across the coil, but the diode after the relay is omitted.
Mine doesn't have the graphic EQ shown in c/diag.
Thump volume changes as one changes the lead drive/ lead volume and also the volume 1/master 1.

Am I right in suspecting it's something to do with the lead channel being grounded, but then when it's switched into circuit, there a spike from "somewhere"??

Here's the relay. Signal input is the T track up to top of pic, goes to .047coupling cap on V3 and 330k off V2p2. The 220k to ground is down next to relay. haven't re-done it's ground side solder - will do soon. Pretty tight in that area.
Left side relay is reverb side. Rev input is red wire going up to fat grey cable which goes to reverb pot rear panel.

[Enzo]

Yep, looks like a relay alright.

This could be many things. The essence of troubleshooting is to isolate the problem. And that is what you have to do. Pull V1, still pop? SCope each step through the amp, where do you see the pop signal?

[stokes]

It was a common problem in the MKIIA's.Mesa fixed it in the subsequent B's and C's.If you call Mesa and ask to speak to Mke B he may tell you how to fix it.Or try the Boogie Board at forum.grailtone.com and do a search,I am sure its been discussed there.

[McBarry]

Thx guys..
having difficulty seeing the "pops" on cro, but they seem to be isolated to signal side of relay.
Storage CRO would be nice...
Removed V4 - no change. likewise V1-3.
Local Mesa repair centre suggested open rev tank and join both RCA leads shields to give definite grounding continuity, as well as ensure all jacks grounded properly. No change.
I've emailed Mike B - await reply.
some general chat on Grailtone board, but nothing really specific.

Thx for replies, if anything comes to mind I'd be eternally grateful.