Boogie - cuts out

[txstrat]

Hi all,

I've got a Mesa Boogie Caliber 50+ of a friend on my desk that develops some weird behavior only when played in the clean mode.
If I turn the preamp on 9 or 10 (under these numbers it's not that obvious) and hit the strings hard, the signal cuts out intermittently and distorts in an unpleasant way. I thought it might be a coupling cap of stage one gone bad and letting DC through. If I pull the volume knob to get into the lead mode, the amp plays fine.
The drive stage is switched in BEFORE the first stage (see schem). If it was a coupling cap I think the amp should do the same when driven hard in the lead mode.
What am I missing?

Matt

[acorkos]

does it cut out at lower preamp settings in clean mode if you whack the amp...maybe it's vibration related? i would check for a loose component, maybe the 180pF or 1.5M at the input? poke around with a wooden probe with the amp running....se if you can locate a loose connection

[txstrat]

I've already poket around and no connection seems to be loose. I think it's not vibration related, since the amp does the same when the master is on bedroom level.
Found that it gets worse, when the EQ is engaged. All sliders on middle position and the amp cuts out a little more than with no EQ. Moving the sliders fully up lets the amp cut out even while playing soft.
Any ideas?

[txstrat]

While I poked around a little more I found that the 330K resistor in between grid of V2A and LDR 2 causes the same noise like I described before when I nudge it with a chopstick (nudging the PCB in any other place, even close to the 330k resistor don't cause any noise).
Don't have a 330K at hand and will change it when I bought one tomorrow.
Could this be the culprit?

[txstrat]

Quite single sided.
The noise is sort of motorboating when the EQ is engaged (and full up) and just a cable (no guitar) plugged in. It stops when I switch the EQ off.
The plates of V1 measure 194V both. Plates of V2 measure 220V both.
V3A = 135V
V3B = 188V
V4A = 25V
V4B = 265V.
When I touch the plate of V3A with my probe, the motorboating gets worse, when I touch V3B it stops immediately (no noise anymore).
The resistors of the preamp stages measure in spec (in circuit).
Any idea what to check next?

[acorkos]

i would first resolder all the connections around that 330k and see what happens

[txstrat]

I somehow think it's got something to do with V3A or with the GEQ.
When I mentioned before that the lead mode don't cause this kind of noise I was wrong. There's no noise only when the lead master is turned down (i.e. signal shunted to ground). With the lead master full up (and therefore strong signal level feeding the GEQ and V3A) the oscillations occur as well.
Tried a patch cord in the effects loop to no avail.

[acorkos]

try a new tube in V3...easy stuff first

and make sure your reverb tank cables are making solid contact at the tank and at the chassis...your signal passes by those connections

[txstrat]

Yeah sure - have tried other (known to be good) tubes at first. No change.
The reverb pot doesn't change anything (had thought of that too). Do you think it could still be the reverb circuit? Will check all connections.
And thanks for your interest.

[acorkos]

The reverb pot doesn't change anything (had thought of that too). Do you think it could still be the reverb circuit? Will check all connections.
And thanks for your interest.

rule out the reverb cable connections...after that i would check the GEQ circuit: resistor values, check transistors for shorts, and voltages against the schematic. are all the GEQ faders doing what they're supposed to be doing?

[txstrat]

Reverb cables are OK. Soldered to the board with the pan being inside the chassis.
Will check EQ resistors. Don't know how to check transistors for shorts.
Faders do what they're supposed to.

[txstrat]

Have tried to measure the Transistors but can't make head or tail of the readings. Alright, my DMM isn't that advanced. Can't even measure continuity with it (only by measuring resistance).
I'm quite sure it's the EQ circuit that leads to the oscillating and cutting out, since when I turn the EQ off, the amp can be played with no issues.
I don't feel like digging deeper into it and change every part in a way of try and error.
Thanks anyway.

[Robert M. Martinelli]

Hi Matt!

Does you DMM have at least a "diode checker" function?

( usually the "diode checker" function also gives a reading of the tested diode's forward voltage, and it can indeed be used to check BJTs' junctions as well ( one at a time, of course ).

Hope this helps

Best regards

Bob

[acorkos]

have you measured the voltages in the EQ circuit and compared to those in the schematic? there are 8 places on that EQ circuit drawing that show voltages...to me, that's a good way to narrow down a possible problem without shotgun replacing a bunch of components.

(don't give up yet...this is how troubleshooting works. you have narrowed down the possible problems quite a bit. look at your first post...you thought it was a coupling cap in stage one, then thought it was a clean channel only problem. now it looks like the EQ circuit is the problem...you're getting closer)

[txstrat]

Alright fellas, you've convinced me. (I really know why I love this forum)
I'm gonna measure the voltages first thing tomorrow morning and get right back to you.
And yes, Bob, my DMM has a diode checker - I'm quite embarrassed that I just don't know what to use it for.