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Dec 3, 2021 24 tweets 6 min read Read on X
look what I got! it's the first high resolution video card for the IBM PC! Image
handwritten serial number. Image
it's a rev C, so not particularly rare. Image
zytrex? weird, they seem to have been a parts distributor in the 80s. my guess is that these parts were remarked by them. Image
it seems to be missing the character ROM chip. Image
I'll try borrowing the ROM from this old MDA card. Image
no. Image
the address lines on the ROM are changing, chip enable is low, but the outputs are always high. 🤔
something tickled my memory about edge-triggered chip enable signals. sure enough, the IBM MDA ROM needs to see edges on chip enable! (the Hercules card ties this line low) Image
so let's give the chip enable a signal! Image
I literally dragged the alligator clip around until I got a display. 🤷‍♂️ the layout of the MDA ROM must be different than the Hercules, but it's basically working! Image
in MDA, the bottom half of the 7x14 font starts at 0x0800, and the CGA thin font (stored in the same ROM) starts at 0x1000. Hercules seems to map it differently, with the bottom half of the font starting at 0x1000 instead. Image
made a little adapter Image
probably won't work Image
well that was easy Image
here's the adapter pinout. Image
(all the other pins go straight through)
it's kinda ugly though, it sticks out really far from the card. since i probably will never find a Hercules character ROM, i think i'll modify the card... 🤔
graphics mode works fine btw. Image
i've written about the Hercules cards before.

removing the old socket Image
preparing the new socket. the blue wires will go through the board and solder to the through holes from the back. Image
you can see the blue wires poking through, before soldering Image
done! Image

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More from @TubeTimeUS

Jun 13, 2023
here's a weird object! it's some kind of foot pedal with an IBM logo. is it for a sewing machine? electronic synthesizer? what do you think? 🧵 Image
it's got a cursed connector! what IS that? Image
amazingly enough, I found something with the mating connector on it! but what is it? Image
Read 24 tweets
Jan 11, 2023
hey so remember the world's cutest oscilloscope i picked up last year? i think it's time to fix the probe socket issue.
i printed a new panel insert that has two BNC sockets on it (the original probes would have had captive cables)
the tricky part is that this oscilloscope has a 333K ohm input impedance instead of 1M like umm every other scope on the planet. fortunately someone designed a little conversion circuit...
Read 11 tweets
Jan 5, 2023
in honor of Sim Wong Hoo, here's a quick historical review of the Sound Blaster! 🧵
but first we have to go back to their first sound card, the Creative Music System from 1987. (image credit: Bratgoul on Wikipedia)
this card has some special "CMS-301" chips. if you peel back the sticker, they have had their top marks removed. they're actually SAA1099 synthesizer chips made by Philips. Creative wrote music composition software for musicians on the PC.
Read 23 tweets
Dec 31, 2022
Tube Time: a 2022 retrospective 🧵
(and stuff you might have missed!) Image
starting things off in january, i found a tube of super old 555 timer chips! i also decapped one of them.
i also started fixing up an old HP 150 computer. it's not PC compatible but it runs MS-DOS--how odd!
Read 65 tweets
Dec 29, 2022
today's mini project: fixing this Radius Thunder IV GX daughtercard. it looks fine, but... 🧵 Image
one of the chips has some broken pins! Image
looks like two of the pins got mashed and broke right at the plastic package. Image
Read 11 tweets
Dec 8, 2022
on the bench of healing tonight: my coffee grinder which decided to bite the dust. Image
symptoms: i push the start button and the motor just bumps a little bit and stops.
how to take it apart? hmm, i bet there are screws under the rubber feet. Image
Read 23 tweets

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