This Marshall belongs to a fantastic local Metal band. The amp sounds great, until we put a pedal board through the loopback circuit. Then it sounds like an old shortwave radio (see video). We took it all apart. One cap was loose , but when checked, the ESR (see recourses) was fine. Put it all back together, biased the power tubes at 96mv (a tad high, but hey, it’s metal), and gave back to customer. We may just spend a weekend recreating the issue and scoping the circuit till we find the issue (I’m open for ideas). He paid over our charge out of extreme niceness.

Status: returned to sender.