Old Mesa Studio .22+. I believe this amp came out in the mid 80s and was supposed to be a stripped down version of the Mark series. It has two channels but the EQ is shared between both and it has less switching options. Stripped down for a Boogie, but it still has a graphic EQ and some push/pull pots. It’s a 1×12 combo that runs two EL84s in the output section for around 20W of power.
Not too much to see since all the components are on the other side of the board. This would not be a fun one to troubleshoot with component side inaccessible. The PCB is held in place by plastic standoffs that mount to the chassis. If they are anything like the plastic standoffs in the Sunn Concert series amplifiers (they seem to be exactly the same) they will most likely dry out and become brittle over the decades and when you need to unclip them to work on the circuit board they may snap off without too much trouble. Maybe I’m wrong hopefully. This one just had a power tube that had a thermal runaway condition. It would eventually red plate if you left the amp on for a while. You can switch the power tubes and see if the problem follows the tube or stays with the socket. The tube red plated in the other tube socket too so it was definitely the tube. Here’s 3 pages worth of Mesa Boogie Studio .22+ schematic .