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.
