Right hand channel output transistors BDV 65B and BDV64B shorted and took out 0.22 ohm emitter resistors which very quickly went open circuit. NOS ST brand BDV parts acquired and fitted to the dead channel, along with new 1W wirewound resistors
Always wise to check output for DC offsets etc before connecting to speaker and also set quiescent current. Wise to cover up all mains components too to avoid any nasty surprises!
After these were replaced, it was a little tricky to get a fully stable quiescent current (and yes, the trimmer pots were perfect, not intermittent or scratchy in any sense), and started getting thermal runaway leading to going into protect mode and powering off at moderate volumes. I believe the transistors acquired were either fakes or faulty.
In the end replaced all four output transistors in both channels with new and easily obtainable TIP142 NPN and TIP147 PNP and problem sorted.
The unit also had a long-standing issue where it sometimes failed to come out of mute (even though no mute indicated) when turned on from cold – 1N4148 diode D14 had failed open circuit, and it was pot luck if the voltage was right to allow mute off or not.