My beloved SU-A4 pre-amp has been a bit troublesome ever since, as it was thermally unstable. I had it open for realignment multiple times; once the right MC side cut out completely (DC offset drifted from 0mV to a whopping -22.5V!), then it run insanely hot, and finally the right MC stage started to produce nastly popping noises every now and then.
I always suspected the reason for the pre going nuts being the crusty trimpots built into the machine: the slightly better ones are the big round blue dishes (you can see them in my SE-A3 in the post just above this one), but the MC stage has two very nasty little fellows that are virtually impossible to adjust correctly; a mere touch with a screwdriver makes them jump about.
So yesterday I was fed up and replaced everything with top grade stuff, being spindle trimmers by Bourns.
MC riser board with one trimpot replaced:
Job completed (top to bottom: MC stage, phono MM/EQ/amp stage, right output stage, left output stage):
All innards of the SU-A4:
The effort spent to create this unit back in 1980 was beyond imagination. Remember that this is only a pre-amplifier! It clearly shows that it is one of the best ever made: the phono stages are fantastic, the beefy output means an output resistance of 0.2 Ohms (!!), and the parametric EQ section is a dream to use.
Technics sure knows what they are doing.