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Refurbishing a Horizon 4000 RAMdisk

TI-99/4A 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.

I 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 someone else ends up working on a similarly broken board.

It turns out that U9, which was specced as a 6264 on the schematics, was a 62256. Although the 62256 passed a quick test with the TL866, it wasn't working with the 32/8 jumper in either position (and, per the documentation, that jumper was the selector between a 6264 (8) and a 62256 (32)).

There's bad weirdness going on with the 32/8 switch. It's on the schematic right next to R3, and if the schematic can be trusted, that jumper merely switches the power source for one of the chip selects on the CSR RAM from mains to battery. You don't want that. That doesn't switch pin 26 from a chip-enable (6264) to an address line (62256). Leave the jumper at the 8 position to keep it wired to the main voltage rail and avoid unnecessary battery drain ...

... the other CS (or A13 if you're using a 62256, which is what the aforementioned 32/8 switch should be frobbing) is more or less held high by the power LED, a diode, and a resistor before it hits the base of Q2. That's not a reliable way to do this. I think it works on a 6264 because TTL is voltage-tolerant, and chip-select is really tolerant. I can see a 62256 address line having issues with this. If I were redesigning it, I'd wire it high to the main voltage rail through a dedicated 4.7k resistor and depend on the !CS2 mentioned in bullet point one. No need for a 32/8 switch if that is done.

The '156 that is ultimately handling the !CS2 signal is open-collector, so it needs to stay connected to the main voltage rail. That bit I wouldn't change.

The power circuit needs to have R9 removed, and both CR10 and CR11 replaced with wires. This achieves several things: makes the circuit compatible with modern switching 7805 replacements, removes the voltage drop from the main power rail, and eliminates the need to add the same amount of voltage to the regulator's ground connection to compensate for the previous diode. CR12 handles any direct backwash from the battery power circuit (+5B), so CR10 and CR11 are both unnecessary and harmful.

After replacing U9 with a 6264 and moving the jumper to the 8 position, the RAMdisk fired up fine. Everything past that was either cosmetic (fixing the power circuit so that it didn't shed scads of waste heat) or investigative. These are weird boards, designed weirdly, and there's room for improvement. For example, the 74HC154s should not be HC, they should be HCT and they should not be on the battery-backup circuit.

I'm sure there's more weirdness in the design, but that's good enough for one post. Hope this helps someone else trying to fix one of these boards.

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