When i repaired this F-2980 before I used some 24awg solid telephone wire, stripped the insulation, formed the wire as was the original trace, make small circles for eyelets (wrap around the intended lead diam to form) so the leads can poke through. Solder the ends first and then the middle parts. you can use the stripped insulation bits and slide back on, to use as spacing or insulation from other comps. Can glue if needed.
Follow the pcb pattern and the other channel.
While you are at it, you should do an exact 1:1 ohm readings comparing the good channel vs the bad channels. Follow paths like from a supply to a semi junction to compare 1:1.
I re-ran simulations for this design, using modern day parts,
As you see I have the DC ecap(C15 in the sim) installed in the FB loop, you should do the same, as it helps to tame the amp down and may fix your problem.
I believe this design has too much open loop gain for a DC amp.
Good luck
Rick