Boogie - cuts out

[oc disorder]

No I meant the one top left -see attached -10uF 63 volts.

[txstrat]

Oh, yes, I clipped another cap (22uF/100v) in parallel to it. No change. Do you think I better had to get the 10uF off the board?

[oc disorder]

"Oh, yes, I clipped another cap (22uF/100v) in parallel to it. No change. Do you think I better had to get the 10uF off the board? "

No thats not the problem
Recappin'
So it seems to be an oscillating problem on high gain settings with master down
which means its not related to the output stage .
So all the EQ faders are ok ? If they are the same value its easy to measure the parallel
resistance (base Q2 & Q3) and divide by 5 just in case a leg/connection broke off .
The EQ may be faultless but by its extra gain in boost position causes a problem elsewhere
to rear its ugly head !
Its not just an unshielded reverb return cable or bad ground there?..
Just noticed that the return comes in just before the EQ - can't recall if this was covered further back in the forum.
That 11pF looks like its there for a reason too.
Is it near the sea ? Any salty air causing any corrosion ? Still think it may be faulty grounding
somewhere .
Excuse the mental diarrhea.. just also remembered the shield on the top of the cabinet is also important . If you have a piece of sheet metal handy that will safely lie on top of the upturned chassis stick that on there and see what difference that makes.
In some of the cabinets with the built in shock absorbers there is a braid that runs up (more than one) and attaches to a terminal/washer (can't remember exactly)on the top of the rubber bonded to the central metal bolt "standoff" so when the mounting bolt passes through this and threads into the chassis it grounds the foil or whatever on the inner top side of the cabinet.
One more thing I overlooked and perhaps it shows I was looking in the wrong place.
At the begining you did say it only happened in clean mode.
Check that .001 below the lead master "switch" in parallel with the 5.6K and the 5.6k itself.
That also has the 100k reverb pot to ground so guessing should be approx 4k.
Just thought the lead master pot would load that down although with it cranked right up the fault may surface. Something to chew on !

[txstrat]

So all the EQ faders are ok ? If they are the same value its easy to measure the parallel resistance (base Q2 & Q3) and divide by 5 just in case a leg/connection broke off.

Faders all work smooth and even. You think I should still measure them?

The EQ may be faultless but by its extra gain in boost position causes a problem elsewhere to rear its ugly head !
Its not just an unshielded reverb return cable or bad ground there?..

The reverb works faultless. Cables are heat glued to the pcb and look OK. Willl check again.

Just noticed that the return comes in just before the EQ - can't recall if this was covered further back in the forum.
That 11pF looks like its there for a reason too.

Could be a cap to prevent oscillations. Gonna check it.

Is it near the sea ? Any salty air causing any corrosion ? Still think it may be faulty grounding somewhere.

No sea for the next 200 mls. But corrosion might be the culprit anyway.

Excuse the mental diarrhea.. just also remembered the shield on the top of the cabinet is also important . If you have a piece of sheet metal handy that will safely lie on top of the upturned chassis stick that on there and see what difference that makes.
In some of the cabinets with the built in shock absorbers there is a braid that runs up (more than one) and attaches to a terminal/washer (can't remember exactly)on the top of the rubber bonded to the central metal bolt "standoff" so when the mounting bolt passes through this and threads into the chassis it grounds the foil or whatever on the inner top side of the cabinet.
One more thing I overlooked and perhaps it shows I was looking in the wrong place.

Will try a sheet metal over the amp too.

At the begining you did say it only happened in clean mode.

No, that was only when the lead master was down. When I turned it up the amp showed the same issue as in the clean mode.

Check that .001 below the lead master "switch" in parallel with the 5.6K and the 5.6k itself.
That also has the 100k reverb pot to ground so guessing should be approx 4k.
Just thought the lead master pot would load that down although with it cranked right up the fault may surface. Something to chew on !

Yep. That's a heap to check for the next hours. Hope the wife'll give me enough time.

[oc disorder]

"Faders all work smooth and even. You think I should still measure them?"
well if its relatively easy I would .
Just measure the 5 together just want aprox figure.
Say if the bottom connection had broken on one that would raise the overall resistance but the control could still appear to work.
Don't bother rechecking the reverb if it works fine.
There can't be much left on the missing list.
Of course if it is the EQ I'm sure Boogie could sell a replacement populated board which is the quick way out.
Another cheats way would be to try fitting 15pf caps in a few places to see if you can tame it down.
But that shouldn't be necessary.
How old is the amp and how much work does it do a week?
I just wonder if a recap in the pre area may calm it down.
I have seen some fail early despite the price but they tended to be the main two (220uF)and the owner wasn't using the standby switch much.

[txstrat]

I have seen some fail early despite the price but they tended to be the main two (220uF)and the owner wasn't using the standby switch much.

OK, I'll measure the faders next thing.
The 220uF caps indeed do not look very good. Tend to change them anyway if it's the failure or not.
5.6 K parallel to the lead master measures fine. Haven't nudged it while the amp is on, yet, as well as the .001 parallel to it.

[txstrat]

I thought I just post some pics to give an overview.

[txstrat]

I measured the resistance of the faders and the DMM showed 9.6K. Five parallel faders each having 50K would end up at 10K calculated, so I think that's close enough.

Also I clipped some cables into several stages to rule them out for being bad. I found, that the preamp of the boogie works absolutely fine (clean as well as lead mode) when I clip a cable right from R2 (reverb output) into the power stage of another amp.
When I use the other amps preamp out and go right into the effects return of the boogie everything is fine too.

This leads me to the conclusion that is HAS to be the EQ itself being bad. I've changed the transistors with european equivalents to no avail.
Resistors and caps all look good.

edit
Next thing I'm gonna do is bridge the vactrol (with a clip cable) that switches the EQ on and off ground and see what happens.
edit
No change in between vactrol bridged or not.
But when I measured the faders again the DMM showed 27K. That's a value far off the precalculated number.
Think I'm gonna bridge the faders for a test (that would be a bridge from base of Q2 to base of Q3 - or isn't that a good idea?).

[oc disorder]

"that would be a bridge from base of Q2 to base of Q3 - or isn't that a good idea?"
No, don't think that will prove much The first measurement sounded fine
expecting about 10K ..the 27K reading suggests two things
1. 3 faders are not connected at one end or the other sometimes?
2. The meter is giving you a false reading as the circuit still has some
charge in the capacitors and that is throwing your meter.

Did you measure the 9.6k reading after the amp had been off for a while ?
Thus the 27k reading was taken soon after the amp was powered up ?
If not can you fasten the meter across the measuring point base Q2 & Q3
and try wriggling and twisting the board and see if it jumps between 10k and 25K (9.6 -27K) 5 faders in parallel or 2 faders in parallel.
On the board there should be two pcb tracks as in the schematic that run
across behind the 5 faders "paralleling" them up.
If a fader had some impact to the front perhaps it pushed the connecting pin through severing the track but allowing it to touch ..

I'm afraid I'm clutching at straws now is the board doublesided?
You could probably buy a drop in replacement from Boogie for a price and of course when you have to remove the board completely the problem may be more obvious...!
Tried a new 12AX7 in position 3 ?

[txstrat]

No, don't think that will prove much The first measurement sounded fine
expecting about 10K ..the 27K reading suggests two things
1. 3 faders are not connected at one end or the other sometimes?
2. The meter is giving you a false reading as the circuit still has some
charge in the capacitors and that is throwing your meter.

You're probably right. I blamed the battery of my meter, since the third value was 5K after the amp was off for a while and I set a new battery in.
Now 5K is the value - half than what I would expect.
I'm not sure it's the faders, since when I turn one of the first three faders down to middle position - thus less boosting effect of the EQ - the amp plays fine.

and try wriggling and twisting the board and see if it jumps between 10k and 25K (9.6 -27K) 5 faders in parallel or 2 faders in parallel.
On the board there should be two pcb tracks as in the schematic that run
across behind the 5 faders "paralleling" them up.
If a fader had some impact to the front perhaps it pushed the connecting pin through severing the track but allowing it to touch ..

I'll check that.

I'm afraid I'm clutching at straws now is the board doublesided?

yes, doublesided

You could probably buy a drop in replacement from Boogie for a price and of course when you have to remove the board completely the problem may be more obvious...!

I had a look but I don't see any fixing points to release the EQ board. Might be the faders have clips themselves that click into the chassis?

Tried a new 12AX7 in position 3 ?

Yeah, first thing. Will try another one again. But as I said, with the EQ out of the signal way, the amp has no issues at all.

[oc disorder]

A photo of the board may help..I thought it was held by those plastic clips
(standoffs) that secure through an 1/8 inch hole.
You squeeze them in.
Maybe its just doublesided tape?
Can you see the inductors ? one may have come adrift.
Didn't realize you had already switched tubes.
I'm still thinking it could be
a. A minor fault somewhere else that the EQ exaggerates or
b. A fault in the EQ itself.

If you can get it to fault by feeding in a cut down signal from a CD player or MP3 player, try getting abusive with the board and wriggle it twist it and push on the faders from the front etc and see if it "clears".

Is there any other clue from the user? like what happened did the audience throw beer (or margarita's) ...or did the drum case land on the front of the amp in the truck etc ?

[txstrat]

A photo of the board may help..

Pics are attached. Not quite easy to take them from 2 inch away.

I thought it was held by those plastic clips
(standoffs) that secure through an 1/8 inch hole.
You squeeze them in.

Those are there but just for the main board.

Maybe its just doublesided tape?

I don't see any.

Can you see the inductors ? one may have come adrift.

That comes to my mind too.

If you can get it to fault by feeding in a cut down signal from a CD player or MP3 player, try getting abusive with the board and wriggle it twist it and push on the faders from the front etc and see if it "clears".

That's a good idea, but that has to wait until tomorrow, since it is already 10:20 PM and the kids are sleeping.

Is there any other clue from the user? like what happened did the audience throw beer (or margarita's) ...or did the drum case land on the front of the amp in the truck etc ?

He said it just started like that by cutting out for a couple seconds. But I'll ask him again.

And thanks for your interest and patience.

[oc disorder]

The only other thing I can think of is to phone Boogie.
I have from Aust, doesn't cost that much these days and I have found them to be extremely helpful.
You may have to phone back to get someone clued up in that area.
As they have been using this EQ design for some time I guess they have seen
lots of faults.
You could find out if a replacement board is available while you are there.

ps Think it's doublesided tape on the top and bottom of the faders.
Maybe some heat on the front panel may help without melting the plastic !

[txstrat]

There's a Mesa Boogie customer service in Germany. I'll phone them up tomorrow and tell them the issue. They might tell me how to release the EQ board too.

[txstrat]

Had a talk to a Mesa technician here in Germany this morning.
He said he's never encountered this sort of issue, but he also doesn't rule out the possibility, that the EQ - when all faders are full up - causes the amp to oscillate.
Even in the manual they write about the possibility of oscillation when all pots are cranked.

As you can see in the threads name, the amp cut out when played hard.
This changed to the better after I replaced the transistors.
Next thing would be to hold an oscilloscope to the stages to see where the signal clips.
Maybe my buddy just has to be careful with the faders settings.