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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.

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

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