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Bad idea? Swapping the crystal in a Macintosh Classic

arnold.exe

New member
I'm not any kind of expert on this, so bear with me if I'm asking something completely bonkers. I have this extra Macintosh Classic logic board, that I managed to raise from the dead after a recap, a couple of washes, some reflowing, reseating, and so on. I should be content with having a spare for if or when the other board I have in my Classic has some sort of fault. But I was thinking about how the SWIM/BBU have this 15.3MHz crystal for driving the peculiar CRTs that were used in these, and reading the documents here https://archive.org/details/SWIMDesignDocs, specifically the SWIM Chip User's Reference, that it's essentially an extended IWM from the Apple II days.

The IWM chip in that machine of course used a 14.318MHz cyrstal, being intended for use with a NTSC display (and I assume a different crystal for our PAL pals). Could I just....swap the crystal for a 14.318MHz one and get something like CGA compatible video signals out of this board? I assume voltage levels would have to be corrected, buffers added, and so on, but the core syncs would at least be correct. I would hazard a guess the floppy drive would function strangely, and probably the sound too, being part of the same chip. And of course, without an analog board, I would have to power the works with a different supply, but I happen to have a Mean Well that can output +12/-12/+5 from a failed venture into building modular synthesizer boards by hand.

This is absolutely too ambitious for me and the projects I do, but I'm genuinely curious if this kind of hack could work and be useful to anyone else if it did. I think it would at least be reversible, being a simple through-hold soldering swap.
 

olePigeon

Well-known member
According to Low End Mac, none of the compacts can be overclocked. Perhaps because of too many interdependencies on the crystal.

But LEM isn't always correct. I'd wait to see if someone else knows.
 

cheesestraws

Well-known member
Underclocking like you're suggesting is more likely to be successful than overclocking; if you wanted to try it, actually I'd be inclined to just swap the crystal and see what broke.

That said, I'm not sure how useful CGA-compatible output is these days, given that a number of scan converters can deal quite cheerfully with the sync rates out of the unmodified LB (see http://www.waveguide.se/?article=compact-mac-video-adapter for settings).
 

arnold.exe

New member
Underclocking like you're suggesting is more likely to be successful than overclocking; if you wanted to try it, actually I'd be inclined to just swap the crystal and see what broke.

That said, I'm not sure how useful CGA-compatible output is these days, given that a number of scan converters can deal quite cheerfully with the sync rates out of the unmodified LB (see http://www.waveguide.se/?article=compact-mac-video-adapter for settings).
The OSSC is the more sober option here, for sure. I should finally get one.
 

cheesestraws

Well-known member
I mean the OSSC or the RGB2HDMI is the sensible option, but that doesn't mean it's the fun/interesting option :). Depends what you want to get out of the project, really!
 
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