Running a Mac Plus logic board on an SE analog board/PSU — anyone done this?

[litriSE]

I'm looking at running a Macintosh Plus logic board off a Macintosh SE's analog board, CRT, and PSU, instead of the Plus's own analog board. I've cross-referenced and mapped the power and video pins between the two harness connectors.

What I want to understand is the Plus's output video signal versus what the SE analog board expects on its input — how (if at all) they differ, and whether there's anything I'd need to do to match the two up. Has anyone gone through this specific combination before, or knows enough about both boards' video/sync characteristics to weigh in?

I'm comfortable building whatever circuit is needed, so any insight into the signal differences (or confirmation that there aren't any) would be a big help.

I see some adapters out there but it's unclear if I can utilize the SE's video circuit with the Plus's logic board.

Thanks! litriSE

 

[litriSE]

Update: got it working (mostly)!

Connected a known-good Macintosh Plus logic board (1MB RAM, booting off BlueSCSI) to a known-good Macintosh SE analog board, PSU, and CRT, with power and video pins cross-referenced and wired. Direct connection, video out from the Plus straight into the SE's video in — and it boots to a fully legible System 6.0.8 desktop. Menus, trash icon, cursor all render correctly, correct aspect ratio. So the core premise works: the Plus logic board can drive the SE analog board.

One remaining issue: there are persistent horizontal lines running across the top portion of the screen, like unblanked retrace sweep. I also tried inverting the Plus's video signal through a 74LS14 before feeding it to the SE — that made the lines worse (second image), so I don't think inversion is needed here (makes sense, since horizontal/video circuits are supposedly near-identical between Plus and SE).

Current theory: the Plus and SE use different vertical deflection circuits — the Plus has a discrete op-amp/transistor vertical sawtooth generator, while the SE (and later compact Macs) use a TDA1170 IC for vertical deflection. My guess is the Plus's VSYNC pulse width/timing doesn't quite match what the TDA1170 expects for its blanking/retrace window, even though the overall vertical frequency (60.15Hz) is correct.

Has anyone run into this specific symptom before, or knows the TDA1170's expected VSYNC pulse characteristics well enough to compare against the Plus's actual output? Photos of the result attached. Let me know if you have solved this problem. Getting close!

 

[litriSE]

Update: found the actual mismatch, working on a fix.

Scoped VSYNC directly at the connector on both boards for a real apples-to-apples (pun intended) comparison:

- Macintosh Plus logic board VSYNC low (retrace) pulse: ~900µs
- Macintosh SE's own logic board VSYNC low (retrace) pulse: ~200µs

So the Plus holds VSYNC low about 4.5x longer than what the SE's analog board (TDA1170-based vertical deflection) is normally fed. That lines up well with the lines appearing specifically at the top of the frame — looks like the SE's vertical circuit is getting held in what it thinks is an abnormally long retrace/reset condition.

Plan: building a 74HC123 monostable one-shot to reshape the Plus's VSYNC pulse down to ~200µs before it reaches the SE analog board, triggered on the same falling edge, output taken from the active-low Q output. Adding a small series resistor ahead of the trigger input too, since the Plus's VSYNC showed a real negative undershoot (~-1.76V) on the scope that's worth protecting against. Breadboarding it now before committing to anything permanent.

Will report back once it's built and tested on the actual hardware. If anyone's run into this specific 900µs vs 200µs vertical sync width difference between these board generations before, or has thoughts on the approach, would love to hear it.

litriSE

 

[fred1212]

Nice work I've thought about doing this in the past