Amiga 600 Keyboard Repair

The right-hand end of an Amiga 600 keyboard, with the power, floppy and disk LEDs.

The Amiga 600 keyboard is a Mitsumi membrane whose flat tail plugs into CN13, a 30-pin locking ZIF connector on the main board. The keys are read by the keyboard controller U13 on the main board, and the LEDs have their own four-pin connector, CN14. Most keyboard faults are in the membrane: worn or damaged contacts at the end of the tail, traces broken by bending, or traces eaten by spilled liquid. A keyboard that is dead from a cold start but works after a soft reboot is a reset fault on the main board, not a keyboard fault.

Symptoms

A600 keyboard faults reported by repairers

Symptom	What was found	Source
Whole keyboard dead after a cold start, working after a soft reboot	The power-on reset; see Amiga 600 Reset Circuit Repair	LinuxJedi
Keys 2, W, S, X and 4, R, F, V dead	A membrane trace broken where the membrane had been bent too far	Retrohax
Keys 5, T, G and B dead	One bad line on the membrane or its connector; the contacts showed wear and grime	8Bit Retro ReFix, via The Oasis BBS
The Caps Lock row dead	Two traces oxidised where the tail joins the keyboard, after a liquid spill; the keyboard tray and RF shield had rusted at the same place	My 8 bit revival
Function keys, right Amiga and Alt unreliable	Wear on the flat cable at the main board connector and liquid damage on the back of the membrane	RobSmithDev, via The Oasis BBS
No keys work on spare keyboards	Someone had tried to sand the green foil off the contact end and damaged the silver traces	Retrohax

RetroPassion's rule of thumb: if only one or two keys fail, the plunger may need cleaning; if a whole row is missing, the membrane very probably needs replacing.

The membrane and connector

The white ZIF keyboard connector CN13 at the rear of a Rev. 2D board, with the audio jacks above it and the keyboard controller U13 below.

RetroPassion lists three Mitsumi membranes for the A600: a blue one and two green ones (model 56C47A or 56C471A, perhaps stamped A300-A, and 56C47B or 56C471B, perhaps stamped A300-B). Blue and green membranes are not interchangeable, so check which is fitted before ordering a replacement.

The contact end of the tail is covered by a very thin green foil over silver traces that are easily damaged. Commodore's schematic shows the lines of the tail going to U13, with three of them also going to U36, which provides the keyboard reset. Commodore's key components table lists U13 as a 6570-30 "Amiga Keyboard MPU", part 391079-01; the parts list gives the same part number as a 6571R4.

Tools and parts

• Multimeter, and a magnifier or microscope for following membrane traces. Retrohax needed a microscope to find a break that could not be seen by eye.
• Isopropyl alcohol and cotton buds.
• Kapton tape.
• For a broken trace: conductive silver glue and a hot-air tool to cure it.
• A replacement membrane of the right type, if needed. Modern replacements built on a rigid board with a detachable cable are also sold.

Procedure

Narrow down the fault

1. If no key works from a cold start, press a key after a soft reboot (for example from Amiga Test Kit's menu, which works with the mouse). If the keyboard then works, the fault is the reset circuit; LinuxJedi also tried another A600 keyboard to rule the keyboard out.
2. Otherwise, note exactly which keys fail. A set of keys that fail together points to one line of the membrane or its contacts.

Check the connector

1. Open the case. RobSmithDev points out a cable that has to be unclipped before the top comes free.
2. Check that the tail is pushed fully into CN13.
3. Check that the connector's locking cap is present. LinuxJedi found one missing and fitted one from a scrap A600 board.
4. Unlock the connector before pulling the tail out. My 8 bit revival pulled a membrane out without unlocking it; the connector itself proved fine when the keyboard was tried in a second A600.

Open the keyboard and inspect the membrane

An A600 with the top case and keyboard removed (a Rev. 1.5 board).

1. Remove the keyboard. It is held together by many screws, with a metal plate over the membrane.
2. Look at the contact end of the tail for wear and grime.
3. Look along the tail, especially where it joins the keyboard, and at the back of the membrane for liquid damage. Rust on the keyboard tray or the RF shield is a sign of a past spill.
4. Check the suspect lines with a meter. My 8 bit revival confirmed two oxidised traces with a multimeter; Retrohax traced a broken line under a microscope.

Clean the contact end

1. Do not sand the green foil; the traces underneath are too fragile.
2. Retrohax held the membrane in place with Kapton tape, softened the foil with an epoxy solvent (methanol only partly worked) and scraped it away with a brush. When the Kapton tape came off, the foil stuck to it and left the silver traces evenly exposed, so Retrohax then used a tape with a stronger adhesive to lift the foil, and repeated the method on a second membrane.

Repair a broken trace

1. Expose the broken trace with the tape method, covering the good traces.
2. Bridge the break with conductive silver glue and cure it with heat. A desk lamp was not hot enough for Retrohax; a hot-air tool cured it.
3. Check the traces next to the repair; Retrohax found and repaired a second, nearly broken trace beside the first.
4. Retrohax added a strip of thick rubbery door-sealing tape to stop the membrane bending at the same place.

A reader of the Retrohax article reports that conductive pastes and glues tend to dry out, flake off and conduct poorly over time. If the traces are badly damaged, replace the membrane: My 8 bit revival judged two oxidised traces beyond repair.

Replace the membrane

1. Check whether the old membrane is blue or green before ordering.
2. Clean the key pads with isopropyl alcohol on a cotton bud, lightly.
3. Fit the new membrane, aligning it on the locating posts, and refit the screws.
4. If the keyboard tray has rusted, My 8 bit revival sanded it, washed and dried it, and painted it with zinc galvanising paint.

Reassemble

1. Push the tail fully into CN13 and lock the connector.
2. Refit the top case, reconnecting the cable that was unclipped.

Testing

1. Start the machine from cold and check that the keyboard responds straight away.
2. Test every key, including the ones that failed before. The 8Bit Retro ReFix repair was checked key by key until 5, T, G and B registered normally.

Board revisions

Commodore's connectors table lists CN13 as the keyboard membrane connector and CN14 as the keyboard status LEDs. The Rev. 2D board in the photograph has CN13 in the same place at the rear of the board. The membrane types are listed under The membrane and connector.

Related pages

• Amiga 600
• Amiga 600 Maintenance Guide
• Amiga 600 Troubleshooting Guide
• Amiga 600 Reset Circuit Repair
• Amiga 500 Keyboard Repair