Feeding VGA 31KHz to a Classic II CRT?

[Daniël]

Hello all, I had a bit of a crazy idea for repurprosing my dead Classic II. Now, to do this, I'd have to be able to feed a VGA 31KHz signal to the internal B/W CRT in the Mac. Of course, it doesn't take that, so I'm wondering if anyone knows how I could modify the CRT driver to allow this? One thing I wondered about is the Micron Xceed, what video signal does it feed to its proprietary neckboard? If it's anything analog, it would be a heck of a lot easier to convert a VGA signal to that, if it doesn't already use such a signal itself. With that, I could attempt to make a neckboard replica, then feed it the appropriate signal and use the CRT that way. If anyone has any idea on it, please let me know! TIA!

 

[Bolle]

You might want to read this thread:

https://68kmla.org/forums/index.php?/topic/26313-compact-mac-retina-display/

 

[Daniël]

Darn, the thread you linked has three threads with what looks to be good info on what I want to do as well, totally dead. I might actually not need to bother with KHz conversion. Basically, my plan is to stick a Bondi iMac board, salvaged from a dying CRT/PSU chassis, and stick it in the Classic to make a Macintosh Classic G3. I noticed when I had to board connected to my Multiple Scan 17 monitor, I could push it back to 512x384@60Hz, which I think runs at 24.5KHz? If I could knock it down to 22KHz, all I need to do is feed the right signals to the CRT circuitry and grayscale neckboard, it should work. I just need to figure out where the R/G/B/H/V signals go, and I suppose these three threads might have had some info on that:

https://68kmla.org/forums/index.php?/topic/19351-raspberry-piclassic-crt/

https://68kmla.org/forums/index.php?/topic/5045-mac-classic-bw-grayscale-vga-mod/

https://68kmla.org/forums/index.php?/topic/11043-vga-on-compact-crt/

 

[Trash80toHP_Mini]

I noticed when I had to board connected to my Multiple Scan 17 monitor, I could push it back to 512x384@60Hz, which I think runs at 24.5KHz? If I could knock it down to 22KHz, all I need to do is feed the right signals to the CRT circuitry and grayscale neckboard, it should work.

One fly in the ointment there, Mac resolution is 512 x 342 pixels and getting the circuitry up to 384 pixels would be the issue. We got all excited about running the Radius Color Pivot II/IIsi as an internal grayscale card in the SE/30 a while back and it turned out to be a dead end IIRC.

That said, another set of eyes looking at the problem from another angle is never a wast of time when it comes to hacking here. You never know what'll shake loose.

 

[Daniël]

Yeah, that might be a bit of roadblock.

Now I'm wondering, what if I could attempt to force the Bondi board to run at 512x342? That might get it on-spec to work with the internal CRT, but I don't know if this is a possibility. Just getting an (S)VGA 9" Monochrome monitor would be the easiest way to make the Mac Classic G3, but if I can manage to use the original guts, that'd be even better. Plus, I can't seem to find these monitors very easily over here.

 

[Trash80toHP_Mini]

One approach I've wondered about would be limited by parts availability on the Flyback front. Engineering a new control board in the AB form factor looks like the only possible solution from everything I've read. I wonder if some Chinese outfit is still making something that might work?

 

[Trash80toHP_Mini]

Looks like some may be available: https://www.alibaba.com/showroom/flyback-transformer-for-sale.html

First look: http://www.instructables.com/id/2n3055-flyback-transformer-driver-for-beginners/

I wonder if it might be possible to get an engineering sample of one? Minimum order would be a big problem, sourcing info on physical compatibility would be the first thing to do. Finding one for grayscale might be a problem.

PCB's not a problem, single sided blanks can of that size can be milled or etched easily enough or a smaller PCB could be sourced and bolted up to a dead A/B.

Can't sell the whole unit per FCC/UL reg compliance here in the US, I assume the same would be true in the EU? Heathkit used to sell home build color TV kits, so that may be a workaround.

 

[BadGoldEagle]

You may find this interesting...

https://nerdhut.de/2016/06/26/macintosh-classic-crt-1/

 

[Daniël]

You may find this interesting...

https://nerdhut.de/2016/06/26/macintosh-classic-crt-1/

It's quite neat, but sadly he hasn't done more than get the BeagleBone board to display a static image on the CRT.

It's a matter of either getting the Mac Classic up to 24KHz (people tried this before without real success from what I can tell), the Bondi board down to 22KHz (which I have no idea how to achieve this or if it is even possible), or doing a swap with one of those monochrome VGA CRTs. 

 

[BadGoldEagle]

or doing a swap with one of those monochrome VGA CRTs. 

It's probably the easiest solution... You just need the circuitry. It'll probably fit quite well where the Analog board sits. 

 

[Trash80toHP_Mini]

Indubitably, but out of curiosity, what besides the Flyback would need to be swapped out on the stock A/B to achieve workable frequency ranges for higher resolutions/grayscale?

 

[apm]

For what it's worth, with a Classic II the stuff I was trying to do on the "Compact Mac retina display" thread might actually be possible. The flyback used in all the compacts up through the early Classic basically can't be run much above 22kHz -- there's so much internal capacitance in the windings that you can't get a short enough flyback pulse to run at higher horizontal scan rates.

The flyback in the late-model Classic and the Classic II looks more flexible. See the end of the thread. I haven't had much time to look at it in the past several months but I still plan to revisit it when I get a chance.