Mac Plus CRT voltage issue

[d_a_parker]

Hi All,

I recently acquired a non-functioning Mac Plus 1Mb in good physical condition. It boots and gets the chime on startup, all of the usual signals and voltages are present, but it has no image on the screen. I think I have traced the issue to a weird voltage reading on the CRT cathode. Voltages and signals on J1 seem fine. Everything except pin 9 on J2 is also fine. J2-9 (CRT cathode) consistently shows +50VDC, but I believe it should be -50V. I tried following the schematic found here but can't figure out which component would be causing an inversion from negative to positive (assuming that's actually the problem here).

Does anyone know where the issue might be?

Thanks!

 

[bibilit]

Are you sure there is no broken solder at J1… most common issue on those boards ever.

 

[d_a_parker]

I reflowed J1 but that didn't fix it.

Also, ChatGPT says that the cathode is expected to be positive since the bias is negative, and that +50V DC is normal. If that's the case, then all voltages on both the analog board and logic board are correct. But still the screen is black

 

[d_a_parker]

After some extensive testing, I found that it's definitely a broken solder joint somewhere on the analog board. When I lightly press the upper corner of the board inward (at the location of C1), causing it to flex just a bit, the screen comes to life. I reflowed the major connectors (J1, J2 and J4) as well as C1, but so far the issue persists. I'll need to do some meticulous continuity tracing to find the exact break, but the good news is that there isn't a failed component.

 

[bibilit]

Can you post some pictures of the AB ??

 

[richmacr]

Pic of back of the AB and logic board could help.

Try reflowing J8 on the logic board (where the cable from the analog board attaches) especially the 2 outside pins.

Use magnifying glass to look for cracked solder joints all over the analog board - especially the ones for the flyback transformer. Have a low threshold to reflow any flyback joints that look sketchy.