Ok, some test results below.
1) Are we sure J2-2 should be -12? It really seems to like being -15.
2) The voltage across the zener diode there is a nice solid -3.8-3.9v.
It appears to be a germanium diode (.1v drop across, both directions) I
could disconnect it and see if that makes things better.
3)
E2.2 and E2.3 should be around 0V.E2.2 is -.8 volts referenced to ground.
E2.3 is -22mv referenced to ground
E2.6 is around .6v. Ramps up from about .58 at startup to .61 then down
to .60something. Note that is a positive voltage wrt ground.
Q12 was reading
-24.4v emitter
-15v collector
-23.8v base
Q12 does not have a CE short
What is this using as a reference to get the -12v from the transistor?
Maybe the zener is just out of spec. Can someone with a VT52 cross check
my voltages to see if they are accurate?
Thanks all!
E2.6 I expect would normally be within +/- 2V of GND.
If E2.6 is way-positive (>Q4.E, > ~ +2.6V), then E2 is attempting to throttle
down the output but failing due to some failure around Q4/Q12.
Measure the voltages on Q4 and Q12.
They could be leaky or low-gain.
It's conceivable Q12 has a CE short, but I'd expect the input to the reg to be
more than -15.
The low -5 is odd, zener D14 looks to be a 5.1V type.
R45 is probably burning up.
I haven't spotted anything in the schematic that actually uses the -5,
it looks to be provided for some alternative character generator (schematic
pdf.42),
so that may depend on what character generator chip / daughter board is used in
your unit.
