Marshall MA100H blows fuse

[themusic]

I'm working on a Marshall MA100. The fuse at FS2 keeps blowing.
Fuse at FS1 no problem. Pull all tubes still blows. Checked tubes ok.
Tested resistorsr87 r88 r89. In fact all resistors. Pulled diodes tested ok. checked caps ok. 15volts at transformer side of FS1.
I'm staring right at it but can't see it !! Thanks

[g1]

Disconnect con6 to make sure it is not a problem with the "valve illumination" (whatever that is ).
Also try lifting one end of R89, then R87, then R88 to see which line is causing the fuse to blow.

[Jazz P Bass]

You need to isolate the problem as there are multiple voltages on that rail.

By lifting R87 & R88 you can limit the circuit to D65- D68, there snubbing caps & C73.
If that holds, install R88, which will bring in C74.
Then install R87 but remove R89.
That is the 15Vdc section.
If that is good reinstall R89.

As it sits, with V1 & V2 removed, what is the resistance to ground at F2?

[themusic]

Thanks , on that tonight....Yes I did disconnect the "valve illumination" "party lights" ...thanks again

[themusic]

Installed R88 held. Installed R87 but with R89 removed...fuse blew... thanks
Should I replace R87...tested ok resistance to ground at F2 open

[g1]

R87 is not the problem, the problem is somewhere fed by R87 and the 15V line.
The fault is either C75 or somewhere on the "switching and mute" page of schematic.

[themusic]

Point me in the right direction on the schem...thanks

[g1]

With tubes removed, you should be measuring a low resistance from R87/R89 junction to ground. Whatever is creating that low resistance is the fault. Once it is disconnected, you won't measure that low resistance anymore.
On the page of the switching/muting page of schematic the line across the top is the 15V line. Those are all the things connected to the R87/R89 junction. Standard method is that semiconductors are more likely to fail than passive devices. So IC3 is a likely suspect.
See if you can disconnect pin 8 and check if you still have low resistance from 15V line to ground.

[themusic]

Thanks, hit it in the A.M. I really appreciate it ...

[themusic]

Disconnected pin 8 . drop in new fuse and fired right up... reading about 16.5 at R87 is that ok...anything else before I replace IC3..thanks

[g1]

That should probably do it.

[themusic]

Thanks again for your help...Just so I can learn what is the function of IC3

[g1]

It's sole function is to trigger the TR45 Fet, which is what kills the reverb signal when you switch the reverb off with the footswitch.
It seems a bit odd to be using an op-amp for this application, most amps use a much simpler circuit, like a single transistor to trigger the Fet.
The dual op-amp here seems like real overkill, but maybe I'm missing something. If anyone knows a good reason for using an op-amp here, I'd like to hear it.

[themusic]

Well this baby's really kick'in my butt...installed new op-amp .. no problem ..everybody fires up...good voltages...but no sound.
Is there a mute transistor ? I see lots of them on the switch and mute schematic page...so many to test...Thanks so much

[Enzo]

OK< I could be wrong, just my first look at the circuit. Stream of consciousness...

TR45 is a reverb mute. I am guessing they had pops when channel switching or some such and this momentary mute solved it. Why the dual op amp? Look closely, the op amps are single sided between +15 and ground, because the TR45 is biased to some + voltage to keep it OFF. Look closer, the same input feeds both op amps, but one is wired inverting and the other non-inverting. I see input coming through C115 or C41. I am guessing it could be either polarity of a pulse. If we fed that straight to TR45, then only negative pulses could turn the JFET on to mute. So with this dual op amp, either polarity of a pulse will be amplified and fed to the gate of TR45, if it passes the threshold set by R52,53,54. Note each op amp gets a different reference voltage to compare the input to.

I guess we could calculate those, there is a 47k, 15k, 47k voltage divider across 15v, so I suppose IC3-3 gets about 8.5v and IC3-6 gets about 6.5v. The inputs are set at 7.5v by high value R50,51. So if that input ever swings more than a volt either way from 7.5, it will trigger one or the other of the op amp comparators to toggle the output to zero and momentarily mute the reverb.

This muting happens when you toggle the boost of channel circuits, relays 1,2,3. TR45 keeps noise out of the reverb, but I see no connection here to reverb switching. Does this amp even HAVE a reverb footswitch control?