I received this in bad shape as one channel finals were missing and the other channel didn’t sound great. I was able to recap the unit, repopulate the outputs and replace most of the driver transistors and it sounds fabulous but I still get a random pop from time to time. I traced it down to the protection relay activating but it doesn’t throw an overload lamp but the circuit looks to operate off of offset as well. Placing my scope probes on the feedback to the diff amps (both brand new) and waiting for the pop, one side jumps to 9v, flat lines, and activates the relay. The 9v immediately declines, audio returns, relay pulls back in and all is well for another 20 mins.. strange. Any thoughts? The VAS transistor is still original, one channel finals are original as well as all the current limiter transistors. Resistors have all been checked. I’m kind of at a loss on this one.

Driver board

Relay protection

Looks like I found it. Broken pad on the constant current source was barely making contact.