Great, my VT52 is shot.

Chris Zach cz at
Tue Apr 21 15:00:33 CDT 2020

> It's conceivable there was a revision change where the -12V was changed to -15V, but it does seem unlikely.
> The -12 is used by the vertical deflection circuit, the CRT filament, (and the RS-232).

Well, vertical deflection seems to be my problem so this may make sense.

Until I figure this out I'm going to pull that jumper to remove power 
from the HV circuits. I'm guessing the circuit can regulate with no load.

> E2.2 and E2.3 should be around 0V.
> 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.

*nod* I just took a look at Q12 and it's not shorted which is good. It 
looks to be the regulator power transistor, the other components vary 
the base to keep the voltage at the output constant under load. The 
input voltage to the circuit is around -24v so it looks like the 
rectifier diodes are ok enough.

> The low -5 is odd, zener D14 looks to be a 5.1V type.

Yeah, that is really odd, especially if -5v isn't really used. I wonder 
if the zener diode is shorted, that could cause all sorts of weirdness. 
I'll check.


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