Mac SE/30 with a Raspberry Pi 4 inside - in greyscale. Can this work?

[Dooser]

[First post. First classic Mac. Amazed of the following these things still have.]

I was recently given a Mac SE/30 with its keyboard and mouse. Looks relatively clean on the outside; inside, a different story altogether. Battery has leaked and completely destroyed the logic board; I mean 'components falling off the board' level. HDD barely spins up and could not be detected by the SCSI card in another computer. FDD is gunked up and extremely dirty. Power supply voltages are mostly OK (-12 reads -10, but I'm not really worried about it). Analog board looks fine.

I want to give it a new life. I don't really care about the internals, but I like the small CRT. I was reading about a project which found a way to drive the CRT with a Raspberry Pi 4: https://nerdhut.de/2020/03/17/raspberry-pi-dpi-control-crt/ . What I don't like is that the stock screen is black and white; however, I see that it can be made to output in greyscale using a replacement neckboard (Micron Xceed), for which gerber files were posted on the forum: https://68kmla.org/forums/topic/31133-se30-micron-xceed-clear-plastic-goodie/ .

The plan is to replace the neckboard and make a custom cable in order to drive it directly from the analog board (the original cable sent the video signal to an add-on card and back into the neckboard). The Raspberry Pi will do all the heavy lifting and send a greyscale signal into the analog board. My question to all you classic Mac gurus is: does this make sense? Can it be made to work or is this just plain stupid? Let me know what you think.

 

[aeberbach]

It can work but it is not going to be great as a Linux machine. For one the CRT is old and you will probably have to recap the analog board and power supply and you might also want to align the CRT. Also there are a lot of high voltages on that board that make it less desirable to keep around in a Pi system. If you use a modern power supply and find an 8" or so LCD you will be able to pick a better resolution for your Pi (1024x768 or 512x384 vs the original 512x342, for example) and it will be much sharper. A 3D printed bezel can make the LCD fit into the SE/30 case very nicely.

But if you want grayscale probably start by reading Micron's patent: https://www.freepatentsonline.com/5307083.html

This page might help too: https://sites.google.com/site/micronxceed/

ymk has been using an internal CRT with DIY electronics:

 

[Dooser]

I don't intend to use it as a Linux machine; I have enough of those. My plan is to install a classic Mac emulator such as Basilisk II and make it run full-screen; this will let me run period-correct applications. The resolution shouldn't be too much of a problem in that case. I know all of this sounds impractical, but I'm not looking for something practical

The greyscale part is the challenge here. I'll have a look at Micron's technical documents and see what kind of signal that CRT board expects. It's not very clear to me why the original CRT board cannot be used for greyscale, nor is what happens if the original Apple video signal is sent to the Micron CRT board (with everything else wired correctly).

ymk's method is really interesting, since I saw that the Pi 3 cannot do the job by itself. Unfortunately he is not sharing a lot of details yet.

 

[cheesestraws]

The greyscale part is the challenge here. I'll have a look at Micron's technical documents and see what kind of signal that CRT board expects. It's not very clear to me why the original CRT board cannot be used for greyscale, nor is what happens if the original Apple video signal is sent to the Micron CRT board (with everything else wired correctly).

From memory, @Bolle has reproduced the neckboard/wiring stuff for the Xceed, and might have some insight on why it's different from the stock one.

(If it wasn't you, sorry)

 

[Dooser]

what happens if the original Apple video signal is sent to the Micron CRT board (with everything else wired correctly)

I figured this part out; it should be OK. Signal voltages are also the same. From the patent:

More preferably, frame buffer expansion board 32 contains a primary video switch (described below with reference to FIG. 5) which selectively produces either the motherhood binary video signal BVID or the frame buffer analog video signal BLU at primary video output VOUT to form primary video signal PVID, the computer's internal CRT therefore being responsive to a selected one of said two video signals.

 

[Bolle]

Signal voltages are also the same.

How come you think that’s the case?

The Macintosh outputs a 5V TTL signal, the Micron card outputs a VGA-like analog signal.

Why would you want to run the Micron CRT board with the Macintosh video signal anyways? It won’t just give you grayscale that way.

Find a way to get the RPi to output a 512x342 60Hz VGA-level video signal while keeping the sync signals at the original Macintosh TTL levels and feed that to the analogbord and Micron CRT board accordingly and you will get grayscale output. 

Be careful though, anything too far off in freqency and resolution is going to kill your analog board at some point.

 

[Dooser]

How come you think that’s the case?

The logic board outputs 5V TTL. The analog board turns that into a 1V signal. The CRT boards (both Apple's and Micron's) amplify it to 30V. The Micron expansion board also outputs a 1V signal, but greyscale.

Why would you want to run the Micron CRT board with the Macintosh video signal anyways? It won’t just give you grayscale that way.

I know. But if the video signal output by the analog board is higher than the one produced by the Micron card, it could damage the CRT. I now know that won't happen.

Find a way to get the RPi to output a 512x342 60Hz VGA-level video signal while keeping the sync signals at the original Macintosh TTL levels and feed that to the analogbord and Micron CRT board accordingly and you will get grayscale output.

That's exactly the plan, starting with what was done here: https://nerdhut.de/2020/03/17/raspberry-pi-dpi-control-crt/ . Also, many thanks for cloning the CRT board and posting the gerbers; originals are really expensive.

 

[Dooser]

I had some time for this recently, so I started by checking if the CRT and analog board are OK (by using the Pi to generate the hsync/vsync signals). Right away things are not looking good: the CRT lights up for a fraction of a second and then goes dark. During that short time I can see a white image with some retrace lines, which is correct. After the CRT goes dark, I can hear crackling and clicking noises from the flyback area. If I turn the power off and on again without waiting, the same thing happens except the CRT image is not as bright.

A few notes:

- I don't see any sparks.

- There isn't anything obviously wrong with the analog board. The electrolytics are all Nichicon and look like new.

- I haven't tested any components yet.

Any ideas of what the problem might be ? If the flyback is dead then this project will stop here, unless someone knows where I can get a cheap replacement.