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MGT SAM Coupé

The late "super Spectrum" (Miles Gordon Technology, 1989): a Z80B @ 6 MHz, extended graphics, SAA1099 sound (6-voice stereo), optional 3.5″ drives — and partial ZX Spectrum compatibility. It arrived too late against the 16-bits, but is a cult machine. Standard late-80s repair: recap, ASIC, drive.

SAM Coupé

SAM Coupé — photo by Simon Owen, CC BY-SA 3.0 (Wikimedia Commons).

Identification

  • CPU: Zilog Z80B @ 6 MHz; 256/512 KB.
  • Video: extended modes (up to 256×192 in 16 colors, mode 4) via a custom ASIC.
  • Sound: Philips SAA1099 (6-channel stereo).
  • Media: cassette, 3.5″ drives (internal disk interface), cartridge/expansions.

Repair (technician)

  • Recap (external PSU + board) → Recap.
  • Custom ASIC: the graphics/logic heart, unobtainable new → handle with care; diagnose by substituting the rest.
  • 3.5″ drive: mechanism / belt → clean; Gotek is very common on the SAM.
  • Keyboard: membrane / contacts → clean / replace.
  • RAM: DRAM → test / substitution.

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 / corrupt picture ASIC / video RAM Substitute the rest, check the ASIC
Floppy not read 3.5″ drive Clean / Gotek
Dead keys membrane / keyboard Clean / replace
Instability caps Recap

Modernisation & mods — what to buy, how

ASIC = sensitive, unobtainable part

The SAM's ASIC is unobtainable new → handle it with care (no spare), diagnose by substituting the rest before suspecting it. Recap + check the external PSU.

Modern storage - Gotek + FlashFloppy 🟡 (3.5″ drive emulation) = the simplest. - Atom Lite / trinity 🟢: modern interfaces (IDE/CF/SD, and Ethernet on trinity) → mass storage and networking on the expansion port.

Video - Clean RGB output 🟢 → SCART then OSSC/RetroTINK or RGB2HDMI.

Expansion - RAM upgrade (256 → 512 KB) 🟡, sound cards 🟡.

⚠️ Do not host ROMs/games — 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.