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The photo isn't hi-res enough to read the labels on some of the chips, but those might be VRAM chips to the right of the 68000. However I'm not sure whether the rest of the circuitry to drive an external display exists on this board. In particular there's no sign of another crystal or...

Interesting. What would be the best way to artificially load it to see what happens? Add capacitance?

Yep, definitely a stuck line in the video address circuitry somewhere. Depending on which line it is, you could get any pattern of doubling from 8-pixel vertical stripes (bit 0) to two 256-pixel vertical stripes (bit 5) to horizontal stripes of any number of lines (bits 7+). My Plus logic board...

Did you swap in a known good replacement for the TDA? As far as I can tell it's the TDA chip itself that would limit the voltage appearing at pin 6. If it fails open circuit, you wouldn't see any difference removing the defective part.

Perhaps a bad TDA chip? I don't see anything in the late-model schematic that would clamp the voltage so perhaps that happens in the TDA itself, as a result of its current draw through the 22k resistor. Edit: also check bridge rectifier DP9.

So, some voltage measurements on changing the sweep frequency. We start with CL6 = 0.018uF (smaller than the factory 0.028uF), and with the HOT and the flyback diode both replaced with parts tolerant of 800V spikes. I measured three flyback secondary voltages: across CL1, CL2 and CL5. CL1 was...

Nice work! I'd be interested in the design files for my resolution hack project (PM me?). I'm curious about the voltage level of the video signal when it arrives from the Micron to this board. Is it more similar to VGA (0.7V) or to the TTL video (5V)? Are you able to measure it with a scope...

The stripes are definitely a logic board problem. If tapping it fixes it, then it's probably bad connection. I'd look at the +5V and ground lines from the analog board to the logic board. It's hard to think what else would restore both the chime and the video simultaneously. Really dumb...

Check UB11 too (Sony sound chip) which for obscure reasons controls the reset line. If the CPU is held in reset (pin 5 on UB11 low) you'll get the result you see. Of course a dozen other things could be causing it as well.

RL1 is 220 ohms on either the early or late analog board. PM me for more details.

According to the BOMARC schematic DP4 is a 1N4148, just a standard signal diode and not a zener.

There is probably some cap goo left on the board after the recapping. It gets into every tiny space and can be very hard to clean out, and it keeps corroding the traces over time. Try washing the board if you haven't already (everyone has a different recommended approach and set of solvents). If...

Using a bench unit for B+ was my plan once I start changing frequencies. The problem is that B+ controls both the secondary voltages and the screen width. On the late Classic board there's no adjustable inductor at all, only a changeable B+ for controlling width. Higher resolutions will...

More progress: on the late Classic analog board, changing capacitor CL6 does indeed change the retrace time in the way I would expect. Here it was with the original .028uF capacitor: And here it is after replacing CL6 with .018uF: Theory suggests that reducing CL6 by a factor of N would...

Well I've changed the IRF740 for an IPP80R280P7 (800V rating), and the Mac still works. No change in picture or in signal at the gate. Next up, messing with the flyback capacitor and seeing what happens to the retrace time and the secondary voltages.

What about just the logic board, or just the drive? You'll want to make sure there isn't a fault in one or the other that is dragging the voltages down. The next step would be to replace the rest of the electrolytic caps on the analog board (especially the PSU part). You say you did 8 of them...

What's the DC value of the 5V line? I just recapped a Classic analog board; all the hi-temp caps in the power supply section had leaked and the voltages were very unstable. A bad 12V rail will affect the screen and the drives, but won't be the source of the checkerboard. You'll also need to...

Though recapping everything is always good, with the SE/30 the most urgent is to change the SMD electrolytic caps on the logic board. That is certainly the source of your problem. The thru-hole caps on the analog board and PSU tend to last a little longer, so if you're not having problems you...

The 128k used a CRT socket with wires coming out the sides rather than a neck PCB. So in principle I guess it's possible to move most of the neck electronics somewhere else, if kept well away from other sensitive parts of the circuit. On the other hand, it might be easier just to cut a hole in...

When you take a screenshot (Cmd-Shift-3), do you see more on the right side of the picture than actually shows up on the screen? That would point to something similar to the other thread. It's normal for the cursor to go almost totally off the screen -- I think you should still see one column...