first time poster, long time tech. having experience with marshall amps I am familiar with issues stemming from problematic circuit boards (i.e.-DSL/TSL etc ) however my experience regarding this model is limited. can't say I have ever been a fan of DSP. for me ,as a player/tech DSP is a deal breaker. like the dreaded nintendo game format red circle of death, once the DSP bites it you're done. I just seek a simple answer for my customer who bought one second hand,never re-tubed. amp lost power.sounds like a badly blown speaker (speakers test good). replaced output tubes and driver tube (original outputs tested badly). preamp tubes test good. supply voltages appear to be normal.overdrive channel LED switches from red to amber to slightly different amber but no green. clean channel button switches normal. red-orange-green.master volume button doesn't come on.sometimes if you perform factory reset all buttons function but sound is still terrible. nice clean signal going thru to pin 7 of V7. in between V7 and phase inverter something is haywire. my money is on the DSP board but not having a flow chart ??? tried to get a signal straight into the power amp but got nothing out. any input appreciated guys. one thing more...i get the 2 separate heater voltages. V6 is 12vdc but the other for the remaining tubes originating at the tranny, is that standard 6.3 ? didn't see it on the schematic.
Solved; what threw me was the poor schematic conn labels. useless. after
a fresh pot of coffee and burning the middle oil I realized if I followed the signal path noted as "a" or "b" it would lead me straight to the problem on the main board. an open R79 100K cathode resistor. at first I couldn't figure out how the signal path got from V8 to driver tube V5. couldn't tell because it has to go thru the master volume circuit which is on a different page with mislabeled conns. but by following the b and a designation it became clear. I had good signal at pin 7 of V8. I had good signal to speakers if I inserted a source at the amp in which is after 8 but before 9 then goes to relay before going to master volume circuit on pin 1 of conn. it then returns to V5 driver input from pin 3 of conn. since there is nothing else in between V8 and V9 but resistors R77-78-79 it had to be one of these. and they are easy to check without pulling the board and desoldering. if you pull V8 there is nothing on either end of R78 and R79 but ground on one end and and dead end pin 8 on the socket at the other end. if you check pin 8 to ground you should get the sum of R78-R79, 1.5k + 100k or if one is open- infinity. this resistor was failing slowly which created the intermittent up and down channel volume.
