Great, my VT52 is shot.
bhilpert at shaw.ca
Tue Apr 21 12:23:00 CDT 2020
On 2020-Apr-21, at 7:57 AM, Chris Zach via cctalk wrote:
> Well I'm starting to walk through this. First I took an IR picture of the boards in operation, then started troubleshooting.
> First I started checking voltages. J2 is easily accessible so I put a ground probe on pin 10, and checked voltages as follows:
> J2-3 Should be -5v, reading -3.8v
> J2-2 Should be -12v reading -15v
> J2-4 Should be 5v, reading 5.04v
> J1-10 Should be 15v reading 14.84
> Hm. That's odd. Looks like the negative voltages are a bit off.
> According to the schematic the key transistor on the 5v rail is Q6, which is a 2n3055. The key transistor on +15 is Q10 and that circuit looks ok. However whatever is running the -5 and -12 volts is not working right.... Looks like Q12 is the key power transistor there with E2 serving as the control.
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).
I wouldn't immediately anticipate -12 going to -15 alone would produce the upset you're seeing on the screen but there could well be interrelated failures between the vertical deflection and the over-voltage -12, so the excessive -12 should probably be addressed.
In the power supply, you could measure some voltages around the -12 regulator.
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.
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.
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