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Tandy TRS-80 Color Computer (CoCo)

Tandy/Radio Shack's 6809 line (1980) — an elegant architecture (the 6809 is one of the finest 8-bits), with optional OS-9 multitasking. Three generations: CoCo 1/2 (VDG MC6847) and CoCo 3 (1986, GIME chip, 128/512 KB, 640×192). Standard 8-bit repair: recap, video, RAM.

TRS-80 Color Computer

TRS-80 Color Computer — photo by Bilby, CC BY 3.0 (Wikimedia Commons).

Identification

Model Year Video Note
CoCo 1 1980 MC6847 (VDG) "chiclet" case then full keyboard
CoCo 2 1983 MC6847(T1) compact, many revisions
CoCo 3 1986 GIME 128/512 KB, 640 px, extended palette

CPU: Motorola 6809 (6809E); Sound 6-bit DAC; cartridge (Program Pak), cassette, floppy (FD-500), serial ports.

Clocks & frequencies ✅

Component Frequency Role / scope
Motorola 6809 0,895 MHz CPU — CoCo 1 et CoCo 2
Quartz 14,31818 MHz (NTSC) — quatre fois la fréquence du color burst
VDG 3,579545 MHz (NTSC) — le quartz divisé par 4 alimente le générateur d'affichage

Everything derives from a single 14.31818 MHz crystal: divided by 4 it gives the VDG's 3.579545 MHz. That is the measurement point to reach for first on a dead machine.

Why this matters in repair

A clock that is missing or off-value on a frequency counter points straight at the crystal, its divider chain or the component it feeds — the fastest starting point on a machine that will not boot. Source: Wikipedia (CC-BY-SA).

Repair (technician)

  • Recap (PSU + board) → Recap.
  • Video: RF/composite (CoCo 1/2) or RGBA (CoCo 3) output → composite/RGB mods are common.
  • Heat (CoCo 1/2): the MC6847 and regulators run hot → heatsinks, check joints.
  • RAM: failing DRAM → substitution / test.
  • Cartridge contacts: clean.
  • CoCo 3: the GIME chip is the weak point (unobtainable new) → handle with care.

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.

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
No / garbled picture video (VDG/GIME) / RF Mods, substitute
Crashes when hot regulator overheating Heatsinks
Memory corruption DRAM Test / replace
Instability caps Recap

Modernisation & mods — what to buy, how

CoCo 1/2 run hot; the GIME is unobtainable

The CoCo 1/2 run hot → heatsinks on the MC6847 and regulators, test DRAM. Recap. The GIME (CoCo 3) is unobtainable new → handle it with care (no spare chip).

Modern storage - CoCoSDC 🟢: an SD-card floppy controller → mounts .dsk images, replaces the FD-500 (in the Program Pak). - DriveWire 🟢: a serial-to-PC cable = virtual disks (shared with the Dragon). - Multi-game cartridges / flashcarts 🟢 on the Program Pak.

Video - CoCo 1/2: RF output → widespread composite mod 🟡; CoCo 3: RGBA output 🟢 → RGB2HDMI (Pi) or an upscaler for crisp HDMI.

Expansion - RAM upgrade 🟡 (512 KB, even 2 MB via Disto boards); accelerators/clocks (Cloud-9 et al.) 🟡.

⚠️ Do not host ROMs/games — 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.

  • CoCo 2 — 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.