Category Archives: Horizon

Horizon HRD RAM disk technical information

Finished and verified: SMD replacement boards for Horizon HRD+

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 … Continue reading

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The HRD+ upgrade design is validated

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 … Continue reading

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Replace your #$%& console power switch!

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 … Continue reading

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Well, it wasn’t the 32k card

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. … Continue reading

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Notes on reworking a low-serial-number HRD into a HRD+

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 … Continue reading

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Refurbishing a Horizon 4000 RAMdisk

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 … Continue reading

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Internal power / disable switch for the Axiom Parallax printer interface

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 … Continue reading

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Two UberGROM images: Logo/Logo-II/Multiplan/TurboForth and Plato/Return to Pirate’s Isle

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, … Continue reading

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HRD+ 74259 Daughterboard verified good

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 … Continue reading

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Daughterboard replacement for Horizon HRD+ chip stacks

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 … Continue reading

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