New (to me) MT200 came in today

[MarshallJMP]

You can try but be very carefull there are some lethal voltages inside. But I don't think that's the problem but you can always try to rule it out. When you poke around get a plastic or wooden stick.

[Lyates1987]

You can try but be very carefull there are some lethal voltages inside. But I don't think that's the problem but you can always try to rule it out. When you poke around get a plastic or wooden stick.

So I'm thinking it's the mono input jack. Not seeing any issues with the bridged circuit. Funny thing, I accidentally left the speaker cable in the bridged jack after moving my input to channel A, turned it on and started playing and it actually sounded pretty good, actually slightly warmer than just going with both channel A jacks. I noticed what I had done about 5 minutes into playing. Played a little bit more. It seemed like it would get a little louder too. Thought that was odd but decided to go back to both A channel jacks.

[MarshallJMP]

Just had a look at the schematic and from what I can see when nothing is plugged in all the pins of the mono input should be grounded.