Right channel issue TU8600 anyone have a copy of the manual?

I’ve had this amp for about 5 or 6 years and it’s been mostly good. For the past year or so I get a weird right channel where it sounds quiet but distorted. Or it’s just super quiet. I swapped tubes and it’s still the same channel. Problem was occasional. Now it seems to be getting worse.

I’m about to pull it out and run through the testing again but I no longer have the assembly manual. Anyone have a copy?

Also welcome any ideas on how best to troubleshoot.
 
Finally got around to doing this
 

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