I need a new circuit board for my Marshall 20/40 combo amp. I turn the amp on and it will play for a little while and then all sound ceases until I turn it back on. I've change out tubes to no avail and I broke out my old amp mod equipment and tested circuits. That's the only piece I am unfamiliar with so I'm thinking that's the issue. My vendor tells me they can't order it because the amp is discontinued. Where can I order one so I can fix my amp?