Broken AOR combo, Gut pics inside.

Hey guys, I bought this AOR 3012 combo a few months ago and I haven't played it much because I think my GH smokes in in the tone department. Anyway, It's a friday night and I had nothing to do so I dusted off the AOR and gave it a cranking, I was running it at about 3/4ths of the master and it started to drop in volume for 5-7 seconds at a time before coming back again for a few minutes.
On the 5th or 4th volume drop I look around the back to check the valves and smoke starts pouring out of the front. I unplugged and opened her up to find this...


I'm not completely knowledgeable of the components within the amp so i was just wondering if any of you could tell me what part it is that has failed, why it could have failed and how easy is it to get replaced/fixed.

It's not worrying me too much because I have the GH but it's nice to have a backup.

Thanks in advance, Baz.

Check those four small diodes to the left.