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 — 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").
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.
See also¶
- TRS-80 CoCo (the 6809 color line) · Apple II (the 1977 rival) · CRT safety.
Sources & attribution¶
- Photo TRS-80 Model III — Zalasem1, CC BY-SA 4.0, Wikimedia Commons — https://commons.wikimedia.org/wiki/File:Radio_Shack_TRS-80_Model_III.jpg
- Wikipedia — TRS-80 (Model I/III/4, Z80, CP/M) — CC-BY-SA — https://en.wikipedia.org/wiki/TRS-80
- Established TRS-80 community knowledge (Model I grounds, recap) — synthesis.
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.