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Macintosh Plus strange vertical/PSU failure

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Hello all - 


For the past few days I've been wrestling with this Macintosh Plus that has some serious analog board issues. When turned on, the system does not chime - it is completely silent. It appears to be dead, however, if the brightness is cranked up nearly to the max, a very distorted picture can be seen. It appears to be a partial vertical collapse, but on the top of the screen rather than in the middle. I've never seen a Mac exhibit this symptom before and am stumped as to where it is originating from. The small bit of vertical picture left also occasionally changes, as can be seen in one of the photos.


Of note is the fact that measuring voltages through the mouse and external floppy ports, I get noticeably weak voltage - approximately 3 volts on the +5 volt rail, 8 volts on the +12 volt rail, et cetera. The floppy drive is also completely non-functional. I'm not sure if the problem here lies with the power supply or something in the video circuit.


I've replaced most of the capacitors except for a few which I did not have replacements for, and those I verified with my meter to be within spec. (I did not replace C1, but it measured close enough to 3.9 uF that it seems to be fine). I've reflowed the yoke connector J1, the logic board power/video connector, and the flyback transformer solder joints and made sure they all have proper continuity. I've also tested the logic board with another known good Plus, and it boots and functions as normal. If anyone could point me in the right direction in fixing this issue, I would be extremely appreciative.







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Alright, I've been working on this Plus for the past few days, and some small updates:


After making an incredibly stupid mistake on my part (do NOT work on computers when half asleep, kids) the analog board would no longer function at all, and immediately blew the fuse when the power switch was flipped. I eventually tracked this down to a pair of shorted diodes (CR25 and CR26, respectively), which, when replaced, caused the board to spring back to "life" (still exhibiting the mysterious symptoms, but displaying a picture as it did before). 


The problem is NOT the flyback or CRT/yoke - I've swapped in spare flybacks and tubes, and nothing changes. These parts are all known working, and booted up fine in another chassis.


After experimenting with measuring voltages via the floppy port on the logic board, I discovered that turning up the contrast knob makes the voltage visibly drop by around a third of a volt. It only starts dropping once the horizontal bar is visible on the screen. This seems like a substantial drop, and like nothing I've ever seen before.


I am still unsure as to what or where the problem component is, and any help would be greatly appreciated.

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