Marshall 8100 Dead od channel.

[mauser]

Have a Marshall 8100 with a dead overdrive channel.  Have plugged into return loop and power amp is ok...so its in preamp.
Have tested the 12ax7. Also tried several known good tubes.

Whats next?   I have whatever gear is needed to test.

[phatt]

Check switching socket , make sure it switches on and off and search around TR1 and TR2 as they control the switching of IC5a/b.

Also note the input socket has an extra switch which grounds the grid of the valve when you remove the plug, that switch may have broken causing no signal to pass even if the switching is working.
Phil.

[Enzo]

And then look at your schematic.  That channel has a signal path through it.  Apply a signal to the input and follow it along, see where it disappears.  IC1a, IC3b, triode gain stage with cathode follower, IC4, IC5b.

Play something loud through it, do those two LEDs blink?  If so, signal gets that far.

[J M Fahey]

The VS 8100 OD path goes thriugh the tube, so a missing or dead tube will kill it.

There's a more or less common problem there:even if the tube is fine, socket makes contact and heaters light, you may be missing +V (+200/250V DC rail, some 130V on 1st plate or 2nd cathode) because it comes from a kludgy autotransformer, connected to the ~30VAC main secondary winding and providing some 180VAC.

It's wound with hair thin wire, think pickup wire, and breaks easily, so check that.

[mauser]

Im going to go out and work on it here in a little bit. I can inject a signal into and have an oscilloscope and a dummyload. 
You want me signal tracing live?  While testng dcv+ and dvc- ?     Will look over everything suggested first.

[Enzo]

Well, you can't signal trace when the amp is not live, nothing goes through a non-powered amp.