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revive a macintosh 512k

weldum

Member
Hi, I have a macintosh 512k that has some modifications (an 1mb+scsi+rom upgrade board) that works fine
at least the logic board does, as the analog board is currently broken, it makes noises and squeaks and the image on the crt is way off centered, distorted and out of focus
also, the keyboard is not working right, most keys doesn't work
the possibility of getting these parts replaced is null, i'm from Argentina and there are a lot of issues for importing stuff

so i'm wondering in which ways one can replace or bypass the crt and analog board and use it with an external, standard vga monitor
i've got the logic board up and running by the modification of a standard atx psu, but the image thing is a lot trickier
i've found out that the signals used are very similar to a true EGA signal with some signals inverted and so on
managed to buy a gbs8800 board (without modifications) that should take that signal and feed it into a standard vga crt/lcd monitor, but it doesn't sync correctly

also i've tried to make a vga adapter thing with some resistors and a logic gate. this fared better as the signal does show up in one of my lcd but off centered and it loses sync sometimes
in one crt that I have lying around it shows full screen but the image is all messed up, like it needs to correct the horizontal sync, which is not capable of as it's a pure vga monitor (with knobs in front and no multisync)

sorry by all that text but, what are my options? thanks.
 

cheesestraws

Well-known member
Yeah, recap the AB before giving up on it. Those ABs have underspecced capacitors especially, and the solder joints go grotty. There's lots of stuff you can try before replacing the whole lot.

That said if that all fails, @Stephen's suggestion of the hoglet/IanB RGBtoHDMI is a good one, if you can get hold of one. Those single-gate adapters work (like http://www.waveguide.se/?article=compact-mac-video-adapter) but the problem is that all it does is invert the signals, and doesn't do anything about the fact that the timing is kind of weird. Whether or not the built-in scaler in an LCD will pick it up is anyone's guess; some do, some don't. You'll certainly have no luck with your VGA CRT.
 

Stephen

Well-known member
That said if that all fails, @Stephen's suggestion of the hoglet/IanB RGBtoHDMI is a good one, if you can get hold of one. Those single-gate adapters work (like http://www.waveguide.se/?article=compact-mac-video-adapter) but the problem is that all it does is invert the signals, and doesn't do anything about the fact that the timing is kind of weird. Whether or not the built-in scaler in an LCD will pick it up is anyone's guess; some do, some don't. You'll certainly have no luck with your VGA CRT.
I haven't had any timing issues with RGBtoHDMI, but to be fair I've only tested on the 128k, 512k, Compact Mac, and "dabbled" with the SE. It's just too bad it's not easier to buy the RGBtoHDMI premade or they would be a massive hit.
 

cheesestraws

Well-known member
I haven't had any timing issues with RGBtoHDMI

Sorry, for clarity, I meant timing would be weird with one of those "single logic chip" converters, not the RGBtoHDMI. As far as I know the RGBtoHDMI works fine but as you say might be harder to get hold of / get hold of the bits for, especially as OP mentioned problems importing things. I could have structured those sentences better, sorry :)
 

weldum

Member
yeah, the analog board is recapped, with better and/or higher voltage ones where it's neccesary
sadly the problem never went out, i'm suspicious about the flyback or the tube itself
also there's a little of a burnt smell when you power on the thing

the RGBtoHDMI thing should work, also i've been told that an OSSC should work too but there's the import/exchange rate/taxes problem, and also I should buy the RGBtoHDMI hat premade as I can't do it myself

that's why I've been looking for alternatives
http://www.waveguide.se/?article=compact-mac-video-adapter
this one seems to work but it uses the OSSC and also it's a SE30 (don't know if there are any differences in the video signal)
 
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