This is for anyone having the speaker cut-out issues as described above.
My AD120VT started to have the described issues. Slowly at first it would cut-out on the right side. I dis-assembled the entire amp and checked the solder on the board areas. Some areas did show some signs that they needed a retouch of solder on them. From vibration and I guess just not very cleanly soldered from factory maybe. Anyway after reassembly of the entire amp to include removal of the tubes and reseat them.
The amp fired up and was working on both sides again. For about 1 minute and then went back to the exact issues again. I had noticed if I reach over the amp and push on the back of it in different areas or wiggled it around with pressure against it on the knobs and input jacks. It would scratch in and out trying to play out the right side again. I did not have the contact spray and I am a Certified Computer Technition.
I was comfortable taking it all apart just to get to know the unit better. SO! Two days later while out, I bought a $14.00 can of Dextol spray contactor cleaner and sprayed every input jack on the amp, and pushed a guitar cord in and out for 15 seconds on each. I Fired the amp up and nope! It still didnt play right side. I reached in and unplugged the leads the left speaker and then started pushing a guitar cord in and out while the amp was powered up and one of my guitars plugged into the low input jack up top.
As I went around strumming with one hand and pluging in an out with a spare cord in the other. I noticed that one of the lineout jacks made the right speaker scratch and even work sometimes. WOW! I spayed a good dose of the spray in it again and wiggled it around and pushed on it a bit and started getting a steady sound out the right side. Scratchy and noisy but got better and better over about five minutes of fooling with it.
I continued to alternate the spray and guitar plug in and out of the other rear jack recepticles and finally went ahead and plugged the left side speaker lead back in! IT worked as described by Voxman. It took some work and patients but it worked. I played for an hour and have not had the first issue again.
The very next day when I got home and went to check it out! It was still working! I sprayed every connector again and pushed aguitar plug in an out for 15 seconds on each input or output on the amp again and My amp is now like the day I got it! Beutiful stereo sounds again!
Moral! The spray does fix this problem and it must be the good stuff! No cheapy spray. It also takes a little patients and some elbow work to let the cleaner do its job. Dont be affraid to spray this stuff either. It cant hurt it. Just dont try to feel the plugs up with cleaner though. These jacks are very sensitive and do require them to be used often or cleaned if you never use them! I never use them and this problem ended up being from just tarnish inside and outside the plugs and mainly the line out plugs.
I have tried to give you a step by step of my entire process here and all in laymens terms. Good Luck!