any marshall service guys? this one is kind of strange vs102

[mattball826]

vs102r

came in doa. bad 64 and 65 op drivers. replace drivers and op's and check dc offset. all good. rework main board and amp board inspecting for bad connections (alwasy on valvestates)

plays fine, then switch clean ch to drive ok. switch back to clean and amp makes a loud popping noise. no measurable dc change in output when this happens, but it is definitely audible.

swap out those 2120d switching op amps. still doing this. so far i am not seeing much on the scope signal line contributing heavy to this. occasional jump of a few mv but nothing that obvious to be as loud as this popping sound is.

some have suggested this is normal? i've seen these before, not too many lately and i know they can be noisy, but nothing like this popping.

ideas?

[Jazz P Bass]

Try removing the OD tube to see if that is part of the problem.

[mattball826]

did that, even swapped the tube, tightened the sockets (usual maintenance). i was hoping one of those would solve this. really sounds like cap discharge when you turn off an on amps but louder. not usable imo. as soon as switch, its fine. its just if you needed to switch dirty back to clean, then it pops pretty bad. just doesn't seem normal although i have read many thinking that it is normal to do this.

[Jazz P Bass]

Is the Pop on the Preamp Out jack?

[jeff brisson]

yea it could be the preamp out jack is showing its age even if you don;t use it,change it out, or you could have some bounce from the switch itself?

[Jazz P Bass]

yea it could be the preamp out jack is showing its age even if you don;t use it,change it out, or you could have some bounce from the switch itself?

My intent was to see if there is a voltage spike on the preamp out.

[vavlehead]

was it doing the popping before you got it in??
I know with Haze 40 amp this is an issue and Marshall says there is nothing to do about it. Dont know if its the same design.

[mattball826]

changed jack

no dc spike on preamp out jack.. there is a slight spike on the scope (1khz tone applied to input) and it seems about .1v to .2 v when the pop occurs. i know it's getting amplified and is visible in the op when scoped. no dc offset change though.

[Jazz P Bass]

Now I am confused.
So, with a signal applied, at the preamp out jack, there is a spike riding on it.
Correct?
Just trying to narrow down where this thing is coming from.
If true, then we work our way back.
"Divide To Conquer"

[mattball826]

yes, this noise signal was trace back to ic2 which feeds tube. noise present (ic2) with tube out but no longer on preamp output.

ok, i am seeing the glitch noise on scope. scope is showing this level change (glitch noise can go up or down scale on scope, so its not something related to single dc line i dont think. +/- lv rails are clean too.

when the clean> od switch (sw1) is pressed glitch signal can be seen at op (pin5 of ic2). i replaced all those op amps since they seem to be common problem for strange behavior in these amps.

i don't see any outstanding dc on the inv or non inv inputs of ic2 (replaced), but there is significant signal change at pin 5 (o-put) and 6 of this ic. i can see smaller similar signal on pin 2 and 3 of ic2. seem to be amplifying that noise. i think ive narrowed down to this area since that feeds the tube. noise is there with tube out. all other components check good. i also did resistance measurement to ground on all pins just to see if something out of ordinary.

its thermal maybe since after about 30 min, the noise level is greatly reduce. i still hear the pop when changing channels, but not near as loud when unit is cold or on for fist 20 min. i checked all surrounding elect caps with esr meter and they seem ok. no shorts/opens.

on d1 which is part of switching circuit, level remains at near 12v. i though that was suppose to change with sw1 being active. idk, some times you just seem to chase yoru tail and it turns out to be simple solution.

do you need copy of schem? or should i post here? not the best schematic imo for troubleshooting.