The Impossible Repair: Ribbon Cables

It’s a problem that faces many a piece of older equipment that ribbon cables of the type used on membrane keyboards start to fail as they become older. These cables are extremely difficult to repair as they can’t be soldered to, and since they are usually custom to the device in question. All is not lost, though, as [Spare Time Repair] shows us with the cable on a Honeywell heating controller broken by a user attempting to remove the battery with a screwdriver.

The whole process can be seen in the video below the break, and it involves the use of a vinyl cutter to cut the pattern of tracks in aluminium tape stuck on a sheet of acetate. This makes a new piece of ribbon cable, however it’s still a step short of being part of the circuit. His challenge is to make a clip tight enough to attach it to the intact part of the broken cable and maintain contact, then to hope that the new piece of cable bent back on itself can make enough contact for the device to work.

At the end of it all, he has a working Honeywell controller, though as he points out, it’s a device he has little interest in. Instead, this opens a window on an extremely useful technique that should be of relevance far beyond the world of heating. There’s one machine close to home for us that could use this technique, for example.

31 thoughts on “The Impossible Repair: Ribbon Cables

      1. Copper foil tape with conductive adhesive also exists, and can be found in guitar repair shops and the usual electronics parts suppliers.

        You tape the copper foil over the existing broken ribbon cable with the strands exposed, rub it down hard, then use a ruler and sharp knife to score the individual strands apart, then spray it down with some PCB varnish.

  1. I would call that a “flex cable”, not a ribbon cable. Ribbon cables are made with wires, not with flex printed circuits.

    Maybe it’s a regional difference in terminology.

  2. Custom to the device?

    Are they though?
    Those connectors have to be standard. I’ve seen too many of them. Different connector pitches, sure. Likely to match the pin pitch of the surface mount connector.

    Does anybody know where to find wide, long, cuttable cable of this type?
    Likely just knowing the name to search for.

    1. Digikey sells those cables in varying lengths.
      They’re not ribbon cables (which have actual wires in a plastic covering), they’re FFC – flat flex cables

    2. I’m pretty sure I’ve come across a few of these cables, probably in cell phones or LCD monitors, that had custom curved/bent traces, to neatly route between sockets that don’t line up. Maybe that’s what they had in mind.
      In a tight place, you might not be able to fold a generic FFC into your clam-shell, plus you’d have to fold it an even number of times to maintain the pin order.

    3. They can simply be a cable with a connector on each end, but it’s also very common for them to be a fully integrated tail on a larger pcb assembly, especially on LCDs, touch panels, and membrane keypads.

      Oh look, that’s exactly what this is.

  3. I used to fix ZX81 membrane keyboard cables by embedding a thin wire over the broken part of cable, with a hot soldering iron. Sometimes it worked, sometimes not. But then the spare membranes were not readily available, so it was worth trying.

  4. OK, nice one…. but I have this beat, I think.

    Our stupid dishwasher has a membrane switch and the same sort of printed ribbon connection, crimped into a connector that of course corroded in humidity. Replacement switch assembly with connector is like $200, naturally…… After trying to restore this connection, I ended up cutting the ribbon loose, poking several tiny holes in each ribbon conductor track, weaving stripped wire-wrap wire in and out of the poked holes, then soldering the ends of each wire to the pc board. It… worked and has hung in there for about 4 years now.

    1. I stuck tiny copper wires down on cellofane tape very carefully spaced out, it took ages to do but fixed my expensive waterdamaged keyboard. If I had known copper foil tape existed that would’ve made things so much easier!

  5. The best way I have found is if you can find a thicker ribbon (more leads) with the same size and configuration you get a razor count one lead past the amount of leads and you pull up this lead at the end of cable and then pull it all the way to other end of cable…..basically cutting the cable this way you are left with a exact replica of the cable you need…..configuration and size and spacing of leads can be easily matched from similar hardware devices.

  6. Still looking for a solution to replace large keyboard membranes for a vintage computer.
    The plastic sheet crack as soon as it is bent. The whole flex membrane (both sides) is digitized and vectorized already.
    Ordering a flex PCB this size is horrendously expensive ! If I would find large enough polyimide-copper sheet, I would attempt etching the tracks…

  7. Y’all sure do spend alot of time reinventing the wheel.
    The IPC 7711/7721 Rework, Repair and Modification document has a procedure for repairing flex circuits. You should check it out it has procedures for almost any repair that can be done on an electronic assembly.
    I do not make any money from you buying the document but my company does teach the course for this document.

  8. I don’t think aluminum foil is going to work very well. Even if it works initially.. aluminum likes to coat itself in a thin layer of oxidation.

    I’d use copper foil.

    And if you really want to to it right… apply tinning solution to the ends too.

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