Marshall DSL100H in @shroprockdoctor after the own…

Marshall DSL100H in @shroprockdoctor after the owner had changed the valves, I assume he meant power output valves that failed to solve the problem.
My investigation began at the front end checking voltages on tubes but all appeared good. Splitting the amp by outputting from the fx loop send proved the issue lay in the front end & not the power section.
This DSL100H has a digital reverb which I removed to eliminate it from the issue.
Following on by checking the dual rail +/- 12v supply it became apparent there was no minus 12v output from mc7912ct regulator. Turns out it was missing its supply from a pair of 1n4007 diodes D13 & D15. Replaced both & she is good as new.