Repairing a torn flat flexible cable on the typewriter

The Selectric typewriter and the computer attached versions (IBM 2741, IBM 1053, IBM 735 etc) attached solenoids to pull on the same mechanical linkages that activate various functions in the almost purely mechanical typewriters, as the only electrical items in a Selectric typewriter are the motor, power switch and power cord. A few microswitches provide feedback on what is occuring with the physical mechanism, necessary to ensure that simultaneous actions are not attempted that would jam the machinery - for instance if one were to attempt to print a character while the carriage was still moving as part of a carriage return operation, or even worse if one tried to shift the typeball to upper case when it was already in that condition.

The Electronic Model typewriters, although still based on the 'golfball' type element and basic Selectric approach, adopted some very different approaches within the machinery. The one example that is important in this project is how they accomplished the rotation and tilting and striking of the type element onto the paper. In the Selectric models, metal 'tapes' are routed around pulleys and connect the levers causing a tilt or rotate, situated on the stationary frame of the typewriter, to the carriage that moves left to right across the width of the paper. A lever will pull on a tape, that tape will move parts in the carriage, and the type element will be rotated or tilted. In the Electronic Models, miniature solenoids are mounted in the moving carriage itself, under the type element, and are connected by a flat flexible cable (sometimes called a ribbon cable). Electrical signals are sent over the cable to activate one or more of the solenoids in the carriage; those solenoids move levers that cause the element to rotate, tilt, and strike toward the paper.

Thus, there is a long brown cable, formed as a sandwich of two strips of polyester plastic with copper 'wiring' laid in between the layers. This flexible 'printed circuit' is roughly 1/2" wide and 19" long, allowing the carriage to move across the wide platen (I am using a 19" wide model of the typewriter) while remaining connected to a logic board in a fixed position underneath the machine. The end of the cable that connects to the logic board takes a 90 degree turn, with the plastic end curving just before the connector is attached.

Somehow, either due to repeated stress and age before I bought the machine on ebay, or while I was disassembling it, the cable was damaged.  At the inner part of the bend (small radius on the inside, opposite the large radius of the other edge), the cable suffered a tear, projecting about 3/16 of an inch from the bend inward and severing two of the copper conductors. These two conductors control two solenoids that determine the velocity with which the type element is hammered onto the paper (actually onto the ribbon which in turn strikes the paper). If the default velocity with both inactive were readable, this tear would not be a problem but sadly, that state of the solenoids corresponds to a space - the ball rotates and moves toward the ribbon but does not actually contact it so no image is typed on the paper.

Repairing these is tedious and challenging - the copper traces are buried inside the laminated pair of polyester strips, and the trace is just 1/50th of an inch wide. I made several attempts before I achieved a satisfactory repair, with the mandatory first step of exposing the copper traces in order to modify them. I had to sand off a layer of the plastic without sanding through the conductors in the middle -- and they are very shallow traces of copper printed on the face of one of the layers.  I bought some 800 grit sandpaper, extremely fine thus very minute amounts of material removed by each stroke. I carefully sanded to expose the area in which I had to make the repair, spanning the tear and enough of the conductors on each side to accomplish a successful bridging of the circuit.
Working on the torn cable end with magnifying lens

Any repair I make is going to be more fragile than the original cable, which has already suffered damage at this spot, so I chose to brace it first. I used epoxy and part of a cut up expired credit card, applied to the other side of cable from the face I had sanded open. I roughed up the surfaces but the epoxy bonded only to the polyester cable, failing to adhere at all to the credit card material. Many plastics, particularly polyethylenes like LPDE and HPDE or materials like teflon, are almost impossible to bond with glues; seems the credit card is not a polyester or acrylic that can be glued with epoxy.

I switched to a higher power glue, one based on methyl methylacrylate, and found another material for the stiffener to replace the credit card fragment. This time I got a high quality bond, with the critical section of the cable now well braced both for the repair work and for permanent operation afterwards.

Some have repaired these types of cables by using a pen containing a special silver ink - containing real silver to produce a good conductive path with low resistance. The pen tip is wider than the trace size, but I finally got the traces bridged with some silver ink yet not touching each other which could short the signals together. It was not, however, going to be mechanically reliable with all the vibrating, moving and flexing.

The most straightforward repair, I should mention, would be to have purchased a replacement cable from IBM, tossing the torn one in the trash, but they are no longer produced by Lexmark and none could be found. Repair was the only option to save this typewriter.

I would need to solder some wire or copper tape onto the copper traces to bridge the gap created by the tear. Polyester melts and twists if heated too much, thus this needs to be done very, very carefully to minimize heat. The trace width of 1/50 inch is also very tiny compared to most soldering iron tips and even to most wires or copper tapes.
Solder paste and wire to be applied to the connector

My strategy was to make use of a special low temperature solder paste, one that melts at a much lower temperature than ordinary solder, and set my soldering station to stay under 500 F. Coupling that with quick, careful application of heat and I would be able to make the soldered connections. For wire, I chose some thin wire used for wire wrap, a technique where wire is wrapped around pins on a backplace to make connections between pins - this was a common construction method in early computers.

Soldering station set at 500F to minimize heat damage
It took many tries to get the wire held in the right place and the proper amount of solder paste applied at the correct position. Too much and it would form a blob bridging multiple copper traces. The wrong place and it would put solder on unwanted traces. The wire had to align well and sit exactly on top of the trace, not touching the adjacent traces a less than 1/20 of an inch away. I had to apply the soldering iron quickly and accurately. Any time the wire, paste or iron was too far off, I had to abandon the try, remove everything and set it up anew. Ultimately, however, the two gaps were bridged by soldered wires with the ohmmeter proving I had good circuits with no shorts among the traces.

.01" soldering iron tip and the tinned wire


Wires soldered in place to restore the torn circuits
The cable will plug into the connector just visible on the board below
The final step was to cover the traces to protect them and I am now applying a stiffener on this face of the cable that will minimize flexing during subsequent operation of the typewriter. The completed cable will be attached to my circuit board,  in place under the typewriter mechanism.

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