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Composite video (probably) mod in my Mac Plus?

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I bought this Plus some time in the early to mid 90's from a university surplus sale.


I don't recall how much i did with it at the time but i am pretty sure it worked. 


It was modified to include a BNC port on the back. The BNC is on a circuit board that was potted in silicone caulking and wedged between the metal frame and the plastic back. It piggybacks the connection between the analog board and the digital board. 


With the availability of the blue scsi sd board at $25ish for the kit, i decided to try and revive this thing. I had oshpark make me a few of the internal scsi hack boards that will be here later this week. I didn't get in on the last batch of blue scsi sd boards but i will do my best to get in on the next batch. Yes he says they are pretty sure that the Plus will work (some bodges required to the current revision of board).


I don't currently know where the keyboard and mouse are, and i took it apart before powering it up for the first time in at least 20 years because i wasn't sure if there would be rifa caps that are going to smoke out or some other issue. Thankfully no rifa but i don't like the look of the seals on the flyback. I have a vacuum pump so i might try slathering it with epoxy and pulling a vacuum. 


But back to the suspicious BNC. I pulled much of the silicone off of it but unfortunately at the moment the chips are unreadable - because they have some abrasion from when it was probably wedged in without the silicone protection, and because they are also coated with some kind of clear lacquer. I am not sure where my acetone is at the moment but i will see if i can figure out what these are. One of the chip numbers is unfortunately possibly completely obliterated other than the "PC" suffix i can see at the end. 


The board looks very DIY. I got this from BYU surplus and they have long had a really good EET program, so, maybe it was even designed in-house. 


I suspect it is intended to connect to a DEC style monochrome monitor (as for like a VT1000) but i got no information with it when i bought it. 









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  • 68kMLA Supporter

That's quite fun!  I'd bet one of the chips is an inverter, because the Mac logic board does its video the other way around to everyone else for some reason that I'm sure seemed good at the time.


There were several hacks a bit like this around.  There was a reasonably well-known one to drive TTL monitors, for example.  I'd be really interested to learn what this one is driving and how, if you ever do find out.

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Posted (edited)

I have an oscilloscope so maybe i can find out. 


A lot of the LCD monitors that shipped with VGA inputs accept sync-on-green via the VGA port, but i have some doubts about them accepting the signal from something like this without some additional sync rate conversion. I might have some ancient doodads for that too, somewhere. I used to buy weirdware speculatively. 


If i get my Plus up and running i will try and find out. 

Edited by Timpanogos Slim
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Just a wild guess, the "PC" suffix chip could be a binary counter, similar to the ones seen on full-fledged video expansion cards.  Considering the other chip is a 16-DIP rather than a 14-DIP, I'd be suspicious that it is a flip-flop rather than a simple hex inverter or quad-NAND gate.  In any csae, those two chips together can provide just enough glue logic to do some simple signal conversion relating to the sync signals and serialized video data.

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