As previously mentioned, I’ve been working on an old Horizon HRD+ RAMdisk card for the TI-99/4A. Three of the chips (a 74LS138, a 74LS154, and a 74LS259) had other chips stacked and soldered on top of them, with leads bent out and wires connecting to various places on the board. One chip (the 154) is […]

The final piggyback board arrived today; this one eliminates the open-collector 74xx156 with a 74xx155 and a 74xx08 to handle what the pullup resistor used to do. Everything works well, with one caveat: I never could get reliable operation out of the low-power CMOS 6264 handling the DSR. The old [HN]MOS 6264 that came with […]

If “the solder joints are always bad” is rule #1 when repairing old gear, “the power switch is worn out and must be replaced” is rule #2. The TI-99/4A in-console power supply has a DPDT switch that handles +5VDC and +12VDC. The switch internals will wear grooves in the contacts over time. After nearly forty […]

I spent an unreasonable amount of time trying to get an in-console expansion working. I’d previously modified my main console with the 16-bit mod, which worked fine, and I thought I’d give Tursi’s scheme a test drive. I wasn’t successful. I used the PCB that fellow (name forgotten) had put up on OSHPark, and I […]

Along with the Horizon 4000 (that I repaired and documented in the previous entry), IM also sent an original HRD. It uses the same board as the HRD+, but had a bit of extra glue logic and was missing the ‘259 and ‘138 stacks. It was populated with a number of stacked 6264s, and had […]

Ksarul and IM sent over a Horizon 4000 that wasn’t working right. To wit, it wouldn’t be detected at all by what passes for an OS on the TI-99/4A. got it working, and it didn’t take too long. Along the way, I noticed some odd things about the design. My notes are below, in case […]

The Axiom Parallax sidecar Centronics printer interface has only nine chips inside, and they’re all small LS TTL glue. The console +5VDC supply should be able to handle the additional (minimal) current draw, especially if the console has an F18A and the now-useless 4116s were removed. One less wall wart to get in the way, […]

Thanks to Tursi, Ksarul, and a few others, it’s now possible to create bankswitched GROM/ROM cartridges for the TI-99/4A. Most of the game cartridges were converted long ago to disk-loadable EA/5 images (most needing the 32k RAM expansion). The languages, courseware, and things that wouldn’t fit into a RAM expansion have not been converted. The […]

The subject says it all, really. I got ahold of a couple of 74LS259D SMD chips, soldered them into the adapter board, and replaced the ‘259 stack in the HRD+ with the daughterboard. Since nothing ever works correctly the first time, I botched a solder joint on one of the chips and spent an hour […]

Earlier, in my article about refurbishing an old Bud Mills Horizon HRD+, I asked why TI gear (both official and homebrew) tended to stack similar chips on top of each other, straightening pins out, and using flying wire to connect to other stacked chips. The question was mostly rhetorical because it was pre-electronic-CAD … so […]