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Tandy TRS-80 Model I / III / 4

The original TRS-80 (Model I, 1977) — one of the "1977 trinity" with the Apple II and PET — then the all-in-one Model III (1980) and Model 4 (1983). Z80, monochrome, CP/M on the Model 4. Not to be confused with the CoCo (6809, a separate color line).

TRS-80 Model III

TRS-80 Model III — photo by Zalasem1, CC BY-SA 4.0 (Wikimedia Commons).

Identification

Model Year Note
Model I 1977 separate CPU + monitor + "Expansion Interface"; notorious RFI
Model III 1980 all-in-one (built-in CRT), fixes compat/RFI
Model 4 / 4P 1983 128 KB, CP/M, 80 columns; 4P = portable

CPU: Z80 @ 1.77–4 MHz; monochrome; cassette then floppy (5.25″).

Repair (technician)

Built-in CRT (Model III / 4)

These models are all-in-one units with an internal CRT and PSU → CRT safety, discharge the EHT before working.

  • Recap (PSU + board) → Recap.
  • Model I: wiring/grounding and the Expansion Interface = fault/instability sources → joints, grounds, contacts.
  • RAM: period DRAM → test / substitution.
  • Floppy drive (5.25″): mechanism / belt → clean, Gotek possible.
  • Keyboard: contacts → clean (classic Model I key "bounce").

Procedure — a dead computer, step by step ✅

There is a CRT inside this machine

The tube holds its charge with the machine unplugged, and on these all-in-ones the analogue board sits right where your hands go. Discharge it, or work on the logic board out of the case. → CRT safety

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.

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
Instability / bounce (Model I) grounds / Expansion Interface Joints, contacts
Dead screen (III/4) CRT / PSU CRT safety first
Memory corruption DRAM Test / substitute
Floppy not read mechanism / belt Clean / Gotek

Modernisation & mods — what to buy, how

Model III/4 = integrated CRT; Model I grounds

The Model III/4 have an integrated CRT: high voltage, discharge the tube before opening → CRT safety. On the Model I, cold grounds/joints are the #1 instability source → reflow them. Recap; test DRAM.

Modern storage - FreHD 🟢: an SD-card hard-disk emulator (Model I/III/4) → mass storage, very common. - Gotek + FlashFloppy / SD interfaces 🟡 to replace the 5.25″ drives. - Model I: modern boards (RAM + interface) 🟡 that replace the finicky Expansion Interface.

Video - Integrated monochrome screen (III/4): no color mod; service the analog board/CRT. A Model I with a separate monitor → a modern display via its video output 🟡.

⚠️ Do not host ROMs/software — method only.

Archived technical documentation ✅

Manufacturer documentation archived locally: schematics, block diagrams, expected voltages, parts lists. The reference when a symptom matches no known fault.

  • Model II — PDF
  • Model 4 — PDF
  • DT-1 — PDF
  • Tandy 102 — PDF
  • Tandy 2000 — PDF
  • TRS-80 Model 16 — PDF

Status

Manufacturer documents kept for preservation (machines out of production for decades), with no explicit licence — the same position as for the cabinet manuals already archived here.

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.