Franc Zabkar wrote:

(...)

As for R22, I don't understand how an open circuit could reduce the
voltage on the base of the transistor.


Emitter current through R24 is adjusted by the setting of
R22, per the note to the right of R24. As R22 is adjusted
higher in resistance, we can expect less current to flow
from the base to emitter, causing AF126 to tend towards
cutoff. (The base becomes less negative *in relation to
the emitter*).

What if we broke the connection to R22 by lifting it's rotor
or fracturing either of it's solder connections?
The base of AF126 will be biased more positive in relation
to it's emitter (towards cutoff) because R22 isn't there to
provide a parallel current path to ground.

See how the AGC rectifier (AA130) biases the base of AF126
more positive (via R21) as IF voltage increases? This
also tends to push AF126 towards cutoff. It's a negative
feedback loop that keeps IF voltage constant.


--Winston