Franc Zabkar wrote:

(...)

AISI, the voltage readings on the circuit diagram are using the
positive terminal of the 9V battery as the 0V reference. Therefore all
the voltage measurements are negative numbers, which means that the
transistor is being turned on harder during the fault condition. The
voltage on circuit ground would be -9V.

Or am I having a brain fart? (Quite likely)

I had the fart.

You and Sylvia are much closer to the truth.
I now see that R5, R4, R28 and R21 would tend to turn on
the transistor (AF126 I) if the path from the positive
terminal of the battery, (through the diode "1.4 St1",
through pin 10 of F IV) were opened, because pin 10
of F IV is only one diode drop less than
the positive terminal of the battery normally.

So, I retract my diagnosis and now think that pin 10
of F IV and both pins of diode "1.4 Stl" should be
cleaned and re-soldered.

--Winston