Great, my VT52 is shot.

[Chris Zach]

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.
>