On 6/03/2012 10:53 PM, Sylvia Else wrote:

So if the transistor has a beta of around 40, the behaviour can be
explained by an intermittent open circuit at, or in the vicinity of, pin
10 of F IV.

Belatedly, I realise that this cannot be correct. If pin 10 of F IV were
to become open circuit, there would be no sink for the current through
the diode 1.4 St1, and the bases of AF178 and AF124, and the latter
would rise to the positive rail.

I'm left wondering about the voltage on the collector of AF121. If it
turned out to be -4V (which seems a plausible enough operating point),
then there could be an intermittent short across the windings of FII,
thereby connecting the base of AF126 I more or less directly to the
collector of AF121.

Sylvia.