Marshall MG100HDFX Overdrive channel no sound

[altazepsta]

Started new thread for topic: per Enzo's suggestion.

Bringing it up to speed, Clean side works fine, with DFX and Reverb. Master control section working great as well. Tested the pedal jack by using a chord and shorting it. Works too.

Overdrive is coming on, gets LED and Clipping LED's are working when playing, but no sound output from either speaker cab or headphone.

I have thus far tested every resistor in the Overdrive section, per the schematic and all are good resistance. Replaced tr1 , mosfet wasn't gating. no effect on problem so far. Also replaced a couple zener diodes (one a switching d) on the front right corner near the master volume pot. The farthest out resistor there as well. Had complete sound loss for a bit when I fiddled around and knocked these out. Got clean back as before.

I am now testing what i can of the Channel Select section ( maybe I should have scrolled down on the schematic first) Thinks are becoming clearer... maybe

Anyways, I know this a much beloved amp, so if there are any more technical ideas for me, please chime in.

[diydidi]

Started new thread for topic: per Enzo's suggestion.

Bringing it up to speed, Clean side works fine, with DFX and Reverb. Master control section working great as well. Tested the pedal jack by using a chord and shorting it. Works too.

Overdrive is coming on, gets LED and Clipping LED's are working when playing, but no sound output from either speaker cab or headphone.

I have thus far tested every resistor in the Overdrive section, per the schematic and all are good resistance. Replaced tr1 , mosfet wasn't gating. no effect on problem so far. Also replaced a couple zener diodes (one a switching d) on the front right corner near the master volume pot. The farthest out resistor there as well. Had complete sound loss for a bit when I fiddled around and knocked these out. Got clean back as before.

I am now testing what i can of the Channel Select section ( maybe I should have scrolled down on the schematic first) Thinks are becoming clearer... maybe

Anyways, I know this a much beloved amp, so if there are any more technical ideas for me, please chime in.

Do you have a voltmeter? I would check the opamps in that section. Maybe there is one faulty.
Use the schematic and test on pins 1&7 on all 8pin opamps. The should be at 0V wrt ground if they are there to pass signal only.
Also check your supply rails on them pins 8&4.

[Enzo]

IC2a and IC2D are teh channel selectors. They toggle one or the other on by SW_A and SW_B. Do those control signals toggle high and low with channel switching? DO you have signal at pin 3 of IC2a? Pin 1 od IC7? Pin7 of IC6?

[Jazz P Bass]

Schematic:
On page 2, far right, is IC 2A.
See if you have signal at pin 3.
If so, check Pin 2.
If not, then that ic is bad.

[Enzo]

Unless the control signal is not reaching that section.

[altazepsta]

Thanks to all of you for the help.

Taking notes and getting more education as I go. Will test these and get back in the morning... Just to show my green... I never really noted ic's being divided up like this before on a schematic. Maybe I was just not looking. Now if they would bother to label the board for all of them. luckily, ic6 and ic7 are labeled, and it is easy enough to spot ic2.

A side note, I was removing the wire connector to the back panel to test them for continuity (the one to the foot switch and fx loop) and now i am trapped in OD. Probably pulled on a wire too hard so i will piggy back them one at a time or something to see... or knocked out ic2 ... arrgh. but on we go.

[altazepsta]

Ok, here is where I got to and how I got there.

I connected a stereo into the input to get a signal. then set my dmm on ac voltage and checked the pins.
All tests done with OD Channel up

for IC2 Voltages DC v-= -15, v+ = 14.9;
IC2A: 1= 8.2, 2=0, 3=0;
IC2D: 6=1.0, 7=0, 8=.5;
IC2C: 9=.5, 10=1.6, 11=0;
IC2B: 14=0, 15=0, 16=8.1;

for IC6; Pin7 = .5 od , in clean 0

for IC7; Pin1 = 30.5 od, in clean 30.6

[altazepsta]

:SOLVED:

Just when you are ready to give up, give it one more try.

I expected IC2 to be fried, but.. I decided to start looking for a signal on AC at the pot connections. I was getting some weirdness at the Volume and Contour pots. Then (and I have no idea if I ever did this before) I turned the Volume down. as soon as it hit probably 3 on the clock I got dc to the speaker. Well enough of that. I shut it off and let it rest a few. of course wondering what I had shorted out messing around. Finally getting my ambition back and not having any of your addresses to mail this thing to, I plugged in again and hit it with my meter.

I switched over to DC and started looking at the connections at the base of pots and sure enough, -vdc13.9 at the two right legs of the Vol pot (OD side) The same on contour pot, but on adjoining legs. I traced it to IC1 first, but lost it at the last cap or resistor before the IC. I checked it anyways and -vdc was right where it was supposed to be, and not on the same trace as my rogue voltage. Went back and traced the other way to IC7.

IC7 (4558)
1=14v 8=0
2=-v13.9 7=0
3=-v13.9 6=0
4=-v13.9 5=-v13.9

Not good. Luckily I had a pair of spares in my box. I swapped it out mostly expecting a meltdown immediately.

But no.... It is one happy amp. That makes me Happy too. It actually sounds half decent even running a crap speaker.
there is still the really weird volume difference in Clean channel between clean and crunch, but I my swap in a lower resistor to see if that balances it out later. for now, I think I will just play. And another note to the next guy, the traces on this board are kind of crap. Be careful. they lift easily.

Thanks for the help you guys. Hope this helps someone else down the road.