6100 30th anniversary. Crunch Channel Mode A->B knob broken not working ! Help.

[Metallsau]

Hi Guys,

i just received a 6100 30th anniversary Amp.
It seems everything works like a charm.

Exept for the 2 Switches / Buttons in
The Crunch (orange) Channel.

Crunch without any Mode engaged works.

If Mode A-B is pressed the amp is going completely Silent.

Only if i crank the channel and master volume you can hear a quiet buzzing noise from the strings you pick.

Mode B-C is dead as well when A-B is pressed in.

With A-B disengaged B-C not silencing the amp but no change in sound.

So my assumption is a broken a-B knob or something related to that specific voice / gainstage

Any ideas?

My Troubleshooting attempt:
i switched every Pre-amp tube one after another
With a 100% fine 12ax7 borrowed from my jcm2000.

Did not work.

As the amp sounds fine with absolutely no humm, buzz / static whatsoever i‘m pretty sure the powertubes don‘t cause that problem.

I could have changed all tubes at once with a working set from my jcm2000 but as everything else seems good i did save me some time.

I only change tubes and never touch anything lethal so i guess there‘s no further way of troubleshooting.

Or is it save to unplug everything and lets say wait 3-4 days before i unscrew the chassis from the headshell to have a look. Maybe take some pictures or do steps you 6100 pro‘s will suggest me to do.

Thanks mates

Short video showing the faulty part

 

[fitz]

to the forum.
Those button functions are typical Marshall confusing labeling.

Crunch without any Mode engaged works.

That's "A" mode.

With A-B disengaged B-C not silencing the amp but no change in sound.

"B-C" just selects "B" or "C" if you press the "A-B" button in.
So, it does nothing with the "A-B" out.

Mode B-C is dead as well when A-B is pressed in.

Focus on this A-B button.
You're not getting B or C when pressed in to change from the A mode.
Could be a dirty contact, cold solder joint, or a bad switch.
But it could also be something that's not so simple.

Those amps are insanely complex inside the chassis.
Just getting at stuff to have a look at it may require extensive disassembly.
If you're not familiar with basic circuit maintenance, the 6100 may not be the best thing to experiment with.
Perhaps a trip to a Marshall certified tech is the best course of action.

 

[anitoli]

There could be multiple things at play here. When in mode "A" it bypasses a huge chunk of circuitry around V2B. When switched to mode "B/C" it engages this circuitry and now you have no output. You need to be able to look at the wave form going through this part of the circuit to see where it stops and to know what voltages are required to make this part function. Where are you located?

 

[Metallsau]

Thanks for the answers.

I know that B-C is only working of A-B is pressed in. But thanks for clarification .

Yes i thought i‘ll ask because maybe a bad Knob is a common issue and so i may be able to change the knob only. I‘d definitely could see myself managing that with some guidance.

The rest of the circuitry is absolutely none of my business. I‘d take it to a tech.
Although i‘m. Pissed because it was sold to me as in working condition

Located in bavaria, germany.