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.
Hm....
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.