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

TI-99/4A 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 chip-enable wires flying everywhere. I intend to use this board as a test bed for a one-megabyte daughter board (I'm awaiting delivery from the fab), but in order to do so I figured I should bring it up to HRD+ specs.

That entailed removing U10 (and its socket), the socket from U1, and desoldering a stacked '138 from U19. After that, the 138/259/154 stack-replacement boards slotted right in. I fully-populated it with 62256s, reworked the power regulator as detailed for the Horizon 4000, connected the wires between U1, U10, and the various chips per the HRD+ construction manual, and fired it up for a test.

Nothing. The ROS config utility didn't probe it. MEGTEST would test the DSR RAM chip (the sole 6264 left on the board), but would lock up halfway through the tests.

It took about a day of dinking around with it, comparing the wiring against my other HRD+, and pulling chips for test in my TOP3000 (all chips passed) ... before I remembered rule #1 for repairing mid-eighties homebrew equipment: the original solder job is always going to be marginal.

I touched up (and, in some cases, resoldered) all of the pads for the address glue logic. Suddenly, ROS config could see the card. The DSR test would still crash halfway through, so I replaced the original 6264 with a new CMOS workalike. Not only did that not fix the crash issue, but it also started throwing read errors during the bit verification test. My best guess is the weird germanium-diode-driven chip-select logic that turns on the 6264 is generating a signal that's marginally bad.

It turns out that the 32k expansion card in the PEB has at least one bad chip. MEGTEST runs from expansion RAM. Removing the 32k expansion and swapping the console for a unit that has a known-good internal 32k expansion fixed the MEGTEST crash issue. All tests passed, the new HRD+ is happily chugging along at 384k, and all is well.

Rather than troubleshoot the 32k card, I'm going to replace it with an in-console expansion. There's no point in replacing a slew of 4116s; they're weird (need +12V, +5V, and -5V), obsolete, and power-hungry. Tursi's two-chip expansion scheme costs less than ten bucks; I'm going to go that route.

A side note: it's possible to rework the HRD+ to host the 32k expansion. There's a writeup on the mainbyte.com site; the author claims that it's for the HRD 2000, but the writeup and board pictures are obviously for the HRD+. There's a few discrepancies and/or errors on that site. I wrote about one in a previous entry here.

I did send an email to the guy running the site about the flex card issue. No response, which didn't come as a surprise as the site hasn't been updated since 2010. Hence, I document the issues here.

Anyway ... that's it for Horizon stuff until the one-meg board arrives from the fab. The design should be able to address four megs directly (with 512k x 8 SRAM) while eliminating the need for any '154s. I'll post the results here when everything is tested.

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