Result ! It's fixed ! Looking at all the schematics for the other marks,
they all seemed to use a fairly straightforward AC voltage measuring scheme,
employing two diodes in bridge with two resistors, for the rectification. In
view of the fact that I'd ****ted it with a high pulse voltage, I figured
that the most likely bet was those diodes. When I found them, they were
white glass ones type HG5808, so a fairly bog-standard germanium type.
Casting around in the parts drawers, I found a packet of brand new OA91s, so
I bunged a couple in, and voila ! - my good old AVO is now reading the right
thing again on AC volts

Arfa