Keyboard Membrane Trace Repair

The three layers of a computer keyboard membrane pulled apart: two clear sheets with printed silver traces and contact pads, and the perforated spacer sheet between them. Photo: Daniel Beardsmore, public domain, via Wikimedia Commons.

Keyboard membrane trace repair restores a broken conductor in the printed circuit of a membrane keyboard. A membrane keyboard is built from flexible plastic sheets printed with conductive ink, usually two sheets carrying the traces and a spacer between them with a hole at each key; pressing a key pushes the two contact pads together through the hole. The traces are thin and crack where the sheet is bent, especially in the tail that plugs into the main board. A crack is bridged with conductive silver ink, or a cracked tail end is cut back; a membrane with many breaks or perished plastic is replaced.

When to use it

A break in a membrane trace shows as a group of keys that fail together: every key on one row or column line of the keyboard matrix beyond the break. Television magazine's 1986 servicing series describes the pattern on the ZX81: an open circuit affects only the keys on that line, while a short between lines holds a key permanently down and disables the whole keyboard. On the ZX81 a bank or row of dead keys is usually a crack across one of the plastic tails.

Rule out the other causes first:

• The tail not pushed fully home, or its connector unlocked. See Amiga 600 Keyboard Repair for a locking ZIF socket.
• Worn or dirty contact pads under individual keys, which affect single keys. See Switch and Potentiometer Repair and the device's keyboard guide.
• A fault on the main board: the keyboard scanning chip, its buffers, diodes or resistors, or the connector's solder joints. Test from the board side with the membrane unplugged; see Continuity Testing and Signal Tracing.

Replace the membrane instead of repairing it when the plastic has gone brittle, when several traces are broken or corroded, or when a liquid spill has eaten the traces. DataServe Retro, which makes new membranes, attributes groups of dead keys on the Sinclair QL to membranes made brittle by years of heat from the computer.

Safety

• Switch off and unplug before opening the keyboard or unplugging its tail.
• Use solvents only where the source for your keyboard allows them. Television warns that some liquid solvent cleaners destroy the plastic of the ZX81's tails completely. Retrohax used an epoxy solvent containing xylene to soften the coating on an Amiga 600 tail, with the membrane taped down so the solvent stayed where it was wanted.

Tools and materials

A ZX Spectrum 48K keyboard membrane with its two tails at the top. The two traces that meet at the F key are marked in red, from the key back to each tail. Image: Alejandro Sánchez, CC BY-SA 4.0, via Wikimedia Commons.

• A multimeter with a continuity range and fine probes.
• The keyboard matrix for the machine, from its service manual or a reference, to turn a list of dead keys into the line that serves them. Atari 800XL Keyboard Repair has the 800XL's matrix by connector pin.
• A magnifier or microscope. Retrohax found its break only under a microscope.
• A craft knife for scraping lacquer from the trace.
• Conductive silver ink, paint or glue. Chemtronics' CW2200 pen, for example, is a silver-filled polymer sold for trace repair, rated to 205 °C, tack-free in 3 to 5 minutes and conductive within 30 minutes at room temperature.
• Fine copper wire for reinforcement: hackup.net used single strands from 30 AWG stranded wire.
• A covering for the finished repair: hackup.net used a clear UV-curing resin.
• Kapton tape for masking good traces and holding the sheet.

Procedure

Find the break

1. List every key that fails. Use the machine's own keyboard test if it has one, as the Atari 800XL's self-test does, or type each key at a prompt.
2. Look the failing keys up in the matrix and find the line they share.
3. With the keyboard disconnected, check the main board side first. Television's ZX81 test bridges one contact of each keyboard socket in turn with the keyboard unplugged and watches for the character on screen.
4. Open the keyboard and trace the suspect line from its contact at the end of the tail to the keys. The marked traces in the Spectrum image show how each key sits on two lines that run back to the two tails.
5. Inspect the line under magnification, especially at folds, at the point where the tail leaves the keyboard and round any liquid marks.
6. Measure continuity along the line. Where the trace is covered in lacquer, hackup.net scraped tiny spots of lacquer off at chosen points and checked continuity at each, halving the length under suspicion each time, until the crack was found.

Bridge a cracked trace

Finding the crack and the layers of the finished repair. Drawn for this wiki from hackup.net's Amiga 500 repair.

These steps follow hackup.net's repair of a Samsung Amiga 500 membrane.

1. Hold the membrane flat. If the trace lies between two sheets, spread them gently; hackup.net used a third-hand tool.
2. Expose about 20 mm of the trace round the crack by scraping off the lacquer with a craft knife, very carefully.
3. Mask the neighbouring traces with tape so the ink cannot bridge them.
4. Apply conductive silver ink over the exposed trace, covering both sides of the crack. Follow the maker's instructions for the ink; Chemtronics specifies a clean surface and shaking the pen for 30 seconds.
5. Lay two or three single strands of 30 AWG wire in the wet ink along the trace as reinforcement.
6. Let the ink dry. Hot air at 100 °C speeds this up; Retrohax's silver glue needed a hot-air tool, because a desk lamp did not cure it.
7. Cover the ink and wires with a thin layer of resin and cure it.
8. Stop the membrane bending at the same place again. Retrohax stuck a strip of thick rubbery door-sealing tape along the fold.

Do not solder to printed membrane traces. The author of the Retrohax article found the silver layers on the Amiga's keyboard strips so thin that they disappear under a soldering iron.

Cut back a cracked tail

If the crack is close to the end of a tail, Television's method is a clean square cut that removes the cracked part, after which the tail can be refitted if it is still long enough to reach the contacts of its socket. Terry Stewart trimmed and refitted a ZX81 tail torn while the case was being opened. Check that the printed contacts at the new end are intact before refitting.

Checking the repair

• Measure continuity from the tail contact to the far end of the repaired line.
• Check that the repair does not short to the traces either side.
• Test every key on the line, and every key on the neighbouring lines, with the keyboard plugged in. Retrohax found and repaired a second, nearly broken trace beside the first.
• Check the main board side as well: Television gives a few thousand ohms as the expected resistance between the ZX81's keyboard (KBD) tracks on the board.

Common mistakes

• Sanding the coating off a tail. On the Amiga 600 tail Retrohax repaired, an earlier sanding attempt had destroyed the silver traces underneath.
• Using a solvent that attacks the membrane plastic.
• Soldering to printed silver traces.
• Letting the ink spread across to a neighbouring trace. Retrohax masked the good traces with tape first.
• Treating the repair as permanent. A reader of the Retrohax article reports that conductive pastes and glues dry out, flake off and conduct poorly over time; hackup.net reinforced the ink with wire and sealed it with resin.
• Pushing a brittle tail hard into its socket. Television warns that pushing too hard buckles and splits the ZX81's tails.

Worked examples on this wiki

• Amiga 600 Keyboard Repair: a tail's coating removed with solvent and a broken trace bridged with silver glue.
• Amiga 500 Keyboard Repair: a cracked trace inside a Samsung membrane bridged with silver ink, wire strands and resin.
• Sinclair ZX81 Keyboard Membrane Replacement: cracked tails, trimming a tail, and fitting a new membrane.
• Sinclair QL Keyboard Membrane Replacement and Sinclair ZX Spectrum+ Keyboard Membrane Replacement: brittle membranes replaced.
• Atari 800XL Keyboard Repair and Atari 65XE Keyboard Repair: the keyboard test, the matrix by connector pin, and membrane repair.

Related pages

• Ribbon and Flex Cable Repair
• Switch and Potentiometer Repair
• Continuity Testing and Signal Tracing
• Connector Pin Replacement and Crimping
• Recommended Tools