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GnatGoSplat

Mac Classic B&W grayscale & VGA mod?

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I've been curious what it would take to convert a B&W Mac Classic to grayscale and VGA with the ORIGINAL CRT and analog board.

 

I haven't really been able to find many details on how the Color Classic VGA mod works, it seems all that's required to support a 31.5kHz horizontal scanning frequency is just to boost horizontal deflection voltage from 60V to 68V, then tweak some trimpots? Is that all it takes? If so, couldn't in theory a similar mod be performed on the B&W Macs as well?

 

The B&W analog board only supports a TTL video input and I admit I don't really understand how the video amp works, but being that the CRT itself is analog and so is most of the circuitry in the video amp circuits, I would think it should be possible to modify it to accept an analog VGA signal for a rather low cost. The Classic doesn't have the NAND inverter like the 128k/512k/Plus, so I tried feeding it variable voltage. It appears as though the screen will display grayscales between 1-2V, although it's difficult to tell since I don't have a proper picture.

 

Thoughts?

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You've got two separate challenges there - VGA scan rates and greyscale. Try to separate the two in your thinking as much as possible.

 

For example, are you feeding the analog board variable voltages whilst still using it as a scan generator? So I assume you're looking at a blank (grey) screen, and that when you tweak the voltage up and down, you can see the screen getting brighter and dimmer. That would seem to prove that greyscale is possible.

 

If you have a Windows or Linux machine handy, there's software about that will give you pretty fine control over the video card. I don't recall offhand any names. But anyway, if you can rig up an adapter cable from the (cheap, expendable) video card to the analog board, and set the screen size and refresh rate to match the existing setup in the Mac, you can prove further that greyscale works, by looking at an actual image.

 

As far as adjusting the scan rate for VGA, I can help not at all there.

 

BTW bolle mentioned the poor man's greyscale project- link

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Yeah, I recognized the scan rate and greyscale issues as separate, although it does seem there is some kind of sync signal in the video signal itself, perhaps to signal video start/stop or line start/stop? I don't really know since I only know a very basic concept as to how CRTs work.

 

What I've tried is desoldering only the video input wire from the analog board and then tweaking the voltage up/down. Without a video signal, there is raster and the screen is bright white, but there are bright tilted horizontal lines. I think it's bright white because 5V = white and 0V = black. If I ground the video input, the screen goes completely blank. When feeding just a DC voltage, below 1V the screen is blank and over 2V the screen is at full brightness. Between 1-2V, the brightness varies directly with the voltage.

 

I tried disconnecting the analog board from the logic board and connecting it to my laptop's VGA output. I was curious if the analog board would have any response to my laptop's HSYNC and VSYNC in VGA mode. Unfortunately, I didn't even get raster, just a blank screen. I think if it could recognize the sync signals at all, it should display some kind of raster like it does when hooked up to the logic board. Is the polarity of the sync signals different than VGA?

 

I've seen the poor man's greyscale project. It's a pretty cool idea, but seems like a lot of work from a mechanical standpoint to retrofit all that to fit, and I also would hate to waste a perfectly good monitor just to use its parts. I've also seen some people buy a 9" Point Of Sale monitor and transplant all its guts into a classic Mac to get the VGA greyscale. It would be slicker, easier (at least mechanically), less wasteful, and cheaper to use all the original parts, if possible.

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AFAIK there's no way you'll get a response from the HSYNC and VSYNC signals, at least without modifying the video card's timing to match that of the Classic. Even then I would doubt it.

 

You may need to make up a circuit to feed the sync signals elsewhere into the analog board. I'm reasonably sure I've seen such circuit schematics around on the web. You'll need to make up something to adjust/invert the voltage levels too by the sound of it.

 

http://www.epanorama.net/links/videocircuits.html#syncconv

 

It sounds like it's time to grab a copy of the service manual for your donor Mac, and perhaps the Larry Pina book too. The "ugly Mac" project from that book (classic motherboard in a box with external video) is documented somewhere on the web as well, and may have valuable hints.

 

What Mac are you using?

 

Also useful:

 

CRT interchangeability

SE/30 repair - helpful if you're using a 128, 512, Plus, SE or SE/30

VGA to RGB includes a section on VGA to TTL

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Also, I'd strongly recommend a cheap VGA card you don't mind blowing up for futher experiments

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I haven't had much time to look into this, but I do have the new "innards" working and would really like to experiment.

 

The donor is not a Mac, but a Jetway J9F2-KHDE Mini ITX board. I have the thing running Leopard 10.5.2. I have myself convinced that I want a Mac Classic that runs Leopard, and this is the least expensive way to do it.

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By far the simplest solution is going to be sourcing a 9" greyscale POS monitor. I can see the attraction in the gracefulness of using the original components though. If you do proceed with this, I hope you'll document your experiments here for future reference. Good luck.

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The B&W analog board only supports a TTL video input and I admit I don't really understand how the video amp works, but being that the CRT itself is analog and so is most of the circuitry in the video amp circuits...

 

Not sure where you got that idea, but it's not correct. The video amp is a binary video amplifier, not an analog one. If you try to run the video amp with non-digital signals, it will cook. The video amp is set to work with binary signals, and so it dissipates little with either black or white (the transistor is either full on -- low V -- or full off -- low I -- so it's never dissipating much power). But ask it to amplify an analog signal in between black and white, and the video amplifier will rapidly self-destruct. That's why the grayscale mod for the SE/30, for example, requires an entirely new video amplifier. So, you've got a little more work to do.

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Here's another suggestion: breaking it down further into steps, and taking the above into account

 

1/ Find a multi-sync 640x480 monitor cheap, say an old VGA NEC. Get your motherboard to output to that in greyscale/colour.

 

1.5/ See if you can stand Leopard at 640x480

 

2/ See if you can get that monitor and your video card/motherboard video to sync at the Classic scan rates.

 

Then you can proceed to the next step.

 

3/ Make up a buffer circuit to convert the analog video into binary TTL one bit black and white. Fast comparators, maybe. Test this on some other TTL monitor if you have access to one.

 

4/ Sync to Classic circuits. Mystery to me

 

Viola - 1-bit VGA

 

As for getting all this working with an OSx86 install (*snort*)

Edited by Guest

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Alternative version:

 

1/ There are some older Apple monitors that will do both 512x384 and VGA. Dig up one of those as a test bed, and make/find a reverse PC to Mac monitor converter.

 

2/ Sync that monitor and your video at the Classic scan rates and 640x480.

 

If that works, you *might* be able to scavenge the video amp from that and hook it up to the Classic CRT. Maybe.

 

I make no guarantees.

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Yeah, I've seen the 9" POS monitor done. It's electrically much simpler, but mechanically more difficult to install with minimal modification to the casing. I figured if I want to do a lot of mechanical modification, I might as well retrofit an 8.4" SVGA color LCD - which I may do if this idea doesn't pan out, because my motherboard has an LVDS connector.

 

It's possible my experimentation that resulted in what appears to be a grayscale raster may have been just operating the existing video amp on the fringe of its limits since it didn't self-destruct. The Classic video amp doesn't have the IC inverter that the Plus and older classic Macs have. I will have to see if I can get a schematic of the SE/30 grayscale card. I've seen photos of it and it looks fairly simple.

 

I think the biggest problem will be sync. I've still not verified that all that needs to be done to support a higher resolution is to increase the horizontal deflection voltage. It seems that's all that's necessary to get VGA out of a color Classic, but it could be the tutorial I read didn't get very detailed. Not having a schematic of the Mac Classic analog board makes it more complicated to even figure out how to raise the voltage. As for the sync signal itself, I'm not sure how complicated that would be. Isn't it going to be TTL and either negative or positive triggered? If so, I think I would be able to just simply invert the sync signal as necessary.

 

My goal would be that the built-in monitor could work as the primary with no external monitor connected, but for practical purposes, an external monitor would be primary and the built-in B&W would a secondary that I could throw a clock, calendar, and other things onto. In that case, I don't think 640x480 would be bad at all.

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