Hello EG. One of the possible suspects you've correctly identified is the OPT, but there are a few others. Before taking a multimeter out, I'd like you to know that all-valve amps have certain places that are nasty to touch, in a "death" sort of way. First off, do a visual inspection (and bear in mind I'm not familiar with the newer Marshalls).

The speakers in the combo will be connected in parallel and it's unlikely that both have blown, so (as long as the wiring from the amp to the 1st driver and back from the 2nd looks decently connected) they're probably not the source.

While unplugged, remove the back panel so that you can see the guts as best as. If possible, undo the chassis from the cabinet so that you can have a look at the underside, where all the non-valve, non-transformer gubbins are. Avoid touching things; wear insulating gloves if nervous; keep your weak (left?) hand in your back pocket, etc. All of these things are good precautions, and the filter capacitors (big metal cylinders) are the enemy to avoid. If you don't know how to discharge them using a 100k resistor, at least make sure that the amp's been unplugged and stays unplugged for a day.

So, if you can get a look here, visually scan the filter capacitors for bulging, bubbling or white powder around the seams. Unlikely cause, but worth a look. Most of the electronics will be on a PCB but have a look for any obviously burnt resistors or scorch marks. OK, we want to take a look at the upside of the chassis now. Make sure the chassis's screwed in place (if you did take it out) but keep the back panel off. Connect a guitar (you know the guitar and lead works, right?) into the input and plug the amp into the mains. Oh, a really obvious Q; the Marshall is set up for 120V, 60Hz I hope?

Anyhoo, with fairly low lighting, switch things on (to standby; I'm assuming this combo has a mains switch and a standby switch). Have a look at the bottom ends of every single valve. The heater filaments should be glowing orange in both the power valves (the two big ones, EL34s) and the smaller ones (ECC83s or 12AX7s). If they all do, then the PT is fine and the valves are likely to be fine. If none do, then you have a prob with the PT or something in between (more later). If one or more doesn't, then you have a valve/socket problem.

It's very late here, so for now I'll just direct you to this very useful page:
but please come back here with any updates and I'll see if I can help further.


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