Korting Transmare A500 - vintage German amp, with issues!

[wunderlick]

Hello there. Been browsing the forum a while with a view to tackling a DIY problem, but could really do with some new pointers. Basically, my old - early-70s? - German-made solid-state amp (a Korting-Transmare A500; interior picture below) is showing possibly terminal signs of age. On powering up it makes a loud "growling"/"buzzing" noise in one channel which continues whatever input and/or volume is selected. If I connect a sound source and disconnect the right speaker, though, the music can be heard clearly through the left speaker with the buzzing only registering faintly in the background.

Thinking it might be the electrolytics, I’ve been through and replaced all the caps, and rectifier, on the power supply board (top left, below) and all the caps on the main amp board (centre, below). Result? No change! Next step was to check the four germanium transistors at the back with a multimeter, all of which seem to be functioning OK and register around 150mV emitter to base. Question is, what’s my next step? I’m no virtuoso with a soldering iron, but if the job is DIY-able I’d like to give it a go - it’s a really great sounding amp... when it’s working!

Thanks in advance for any suggestions...