Marty, I haven't scanned the entire schematic yet but i have located a possible source for problems. Your master volume is part of the bias circuit, it is a 220k dual pot that takes the place of the standard 220k resistors found in most Marshall bias circuits. Check to see if the pot operates normally. If the carbon strips or wipers have been damaged , that could cause you all kinds of problems. most important is what it measures across the pot since that is the part of the pot that is in the bias circuit. Check that and i'll look some more.

Hello: You can send me private email and I think they give you more characters for that. Any how, here is a copy of a 401 schematic off of Dr. Tubes website. You will need Adobe Reader to open it.

http://www.drtube.com/schematics/mar...60-02-iss9.pdf

Just copy and paste it to your browser. Hope it helps. Marty

Hey Marty, I'm not too familiar with a lot of the newer Marshall's, most of the work I do is on vintage equipment , but without looking at a diagram (which I will do later if I can find one) I would say you've got A bad resistor in the bias circuit or a shorted winding on the output transformer. The transformer is easy to check, just measure resistance from each end to the center tap, it wil be a very small reading but should not be "0", If it is, you have a shorted primary . If that is not the problem, then look at the bias supply. After the adjustment pot the circuit will branch into two identical circuits, follow these branches up to the tube sockets. Check each resistor to see if any have fallen out of tolerance or shorted completely. You can check the voltage with the tubes out to see if it is the same for both circuits. Sockets could be arched, but not likely. If that doesn't help, let me know.
Brian