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Texas Instruments TI-99/4A

The first consumer 16-bit micro (1981): a TMS9900 16-bit CPU, TMS9918A video (the VDP that became the MSX/ColecoVision/SG-1000 one), TMS9919/SN94624 sound. GROM cartridges, an iconic speech synthesis module. Standard early-80s repair: internal PSU, cartridge contacts.

TI-99/4A

TI-99/4A — photo by Thomas Vogt, CC BY 4.0 (Wikimedia Commons).

Identification

  • TI-99/4 (1979) then TI-99/4A (1981, fixed keyboard, improved VDP).
  • CPU: TMS9900 @ ~3 MHz (16-bit, rare then) — but system RAM on an 8-bit bus (hobbled).
  • Video: TMS9918A → RGB/composite/RF; Sound: SN94624 (close to SN76489).
  • Ports: cartridge (GROM/ROM), cassette, Peripheral Expansion Box (PEB), speech synth.

Repair (technician)

  • Internal PSU: block inside the case → recap + RIFA (mains filter) → Recap; the RIFA smokes at end of life.
  • Black screen / crash: bus buffers, the TMS9900 or TMS9918A → substitution (socketed or desoldered parts).
  • Cartridge contacts: oxidation → clean the connector (the #1 "won't start" cause).
  • Keyboard: contacts / membrane → clean.
  • PEB: the fragile interface card ("Flex cable") → re-check.

Procedure — a dead computer, step by step ✅

What this adds

Ordered for a home computer: what kills these is its power supply, its battery, its capacitors, its socketed RAM and its keyboard — roughly in that order, and well before the CPU. → PCB diagnosis

1. The power supply — measured before it is plugged in. This is the one step not to swap around. On a machine of this age a supply does not simply stop: a 5 V regulator that fails high puts 7, 9, 12 V onto the bus and takes the RAM and the custom chips with it. Measure every rail off the machine, under a dummy load if you can, and only then plug it in. This page documents RIFA capacitors on this machine: the mains filter caps that crack, then let go in smoke on the first power-up. Replace them before you switch on, not after.

2. The battery, if it has one. Any RTC or backup cell soldered to a board is a future leak. If yours has one and it has never been changed, take it out before it takes the tracks with it. → Backup batteries

3. Capacitors. Recap is the default on these machines, not a last resort — and on the supply first, since that is what protects everything else. → Recap

4. RAM, on its sockets. These machines were built to be repaired: the RAM, and often the logic, sit in sockets. Reseat everything first — thirty years of oxide is a classic no-boot. Then swap chips: a bad RAM chip usually gives a repeatable pattern of garbage or a screen frozen at a fixed character.

5. The keyboard. A console does not have one; a computer's is a wear part. A membrane keyboard fails as whole dead rows or columns — that is the membrane, not the machine. A mechanical one wants its contacts cleaning, not replacing. Check this page: the failure mode is often documented per model.

6. Storage — tape or disk. Both often come down to the same things: a dirty head, and for tape the azimuth and a belt turned to tar. ⚠️ On the floppy side, check whether the mechanism actually has a belt before looking for one: many do, but some drives are direct drive, and hunting a belt that isn't there means stripping a drive for nothing. A machine that boots but loads nothing is a drive fault, not a logic fault — do not go looking on the board for it.

7. Video output. RF, composite, RGB or a dedicated monitor, depending on the machine (see Identification). Test with something else before you conclude: a period modulator and a modern TV disagree far more often than either is broken.

8. Still nothing. Now the logic: rails on the board, reset, clock, bus. → PCB diagnosis

Common faults

Symptom Likely cause Lead
Won't start on cartridge cartridge contacts Clean the connector
Black screen buffers / VDP / CPU Substitute
Smoke at power-on PSU RIFA Replace the RIFA
Instability PSU caps Recap

Modernisation & mods — what to buy, how

Internal PSU: RIFA + recap; cartridge contacts

The internal PSU has RIFA caps that smoke at end of life → replace them + recap. A dirty cartridge connector is the #1 "won't start" cause → clean it first.

Modern storage - FinalGROM 99 🟢: an SD-card cartridge → loads any title (ROM/GROM) without a PEB — the simplest mod, plug-and-play. - TIPI 🟡: a Raspberry Pi interface (PEB bus) = virtual disks, networking, clock. - nanoPEB / CF7+ 🟢: CompactFlash as floppies (emulates drives) — compact, no PEB.

Video - F18A 🟠: an FPGA replacement for the TMS9918A VDP (desolder/remove the VDP) → crisp digital VGA/RGB output (goodbye RF), plus extended graphics modes.

⚠️ Do not host ROMs/GROM — method only.

See also

Sources & attribution

Review & corrections

How this space is used

Spotted a wrong value, an outdated procedure, a chip reference that does not match your board? Say so here, with what you observed (model, board revision, serial number, measurement). Every report is cross-checked against a source before anything changes — an unverifiable correction is published as “reported by …, not cross-checked” rather than silently applied.

Reading is open to everyone; posting requires signing in with Discord. Reports from the wiki's declared reviewers are handled first; anyone else's are read too, but go through a human before anything is changed.