Solved: Repair of English Mark Darts Switch Matrix

[cdean]

I have developed an effective repair for older English Mark Darts matrixes (matrices?).  A little background:  Our English Mark Darts game began to show dead segments.  After a long afternoon with the (incorrect) matrix chart in the manual and an ohmmeter, I determined that the only place the circuit could be incomplete was at the junction between the connector and the flexible matrix conductor.

The matrix is built in 7 layers - from the outside: Mylar, silk-screened conductive paint, silk-screened clear mask, mylar mask, clear mask, conductive paint, mylar.  I removed the clear layer on two leads and confirmed that the switch did, in fact, work, but the signal did not pass between the conductive paint and the connector.

Discussing this problem with a wise friend, we came up with several possible solutions.  I went with this one:  Remove the clear mask (lacquer, enamel?) from the conductive leads near the connector and apply a small strip of conductive tape to bridge from the trace to the connector body.

In detail:
Using lacquer thinner, a cotton swab, and an orange stick, carefully remove the clear overcoat without damaging the conductive paint.  If you catch a reflection on the surface while rubbing with the cotton swab, the clear lacquer will begin to fog and break up.  At this point gently rub with the orange stick paddle end to remove the overcoat.  If you scratch the conductive trace, don't worry, this repair will fix that as well.

Open the connector to gain access to the metal swaged portion (I simply bent the connector back - ugly but works).

Cut a narrow (.050") strip of conductive tape about 3/8" long.  I put a 3/8" strip of tape on a block of plastic and pushed it up against the fence on the saw table.  Using a #55 (.052") drill bit as a guide, I cut small strips with an artist's knife.

Apply this strip to the connector, overlapping in to the swaged portion of the connector a bit.  Lay it down on the top of connector, down the face of the connector and along the trace.  Burnish well.

Test by checking resistance from the connector socket to the conductive tape.  I saw approx 20 ohm, give or take.

That's pretty much it.

Notes:
The matrix on the machine that originally sent me on this search had a few bad segments.  I had another matrix I swapped in that had zero good segments.  It was this second matrix that I repaired - going from 0% to 100% switch operation.

Both matrices are the three-legged variety.  But, I think this technique will work for the newer versions.

The two three-legged matrices I have are different - they have different width traces.  I have not determined if they are 'wired' exactly the same (see next paragraph).

The matrix chart in the available manuals has at least one error - there is an 'R' in the chart; that should be a 'B'.  Also, the chart does not go into enough detail to determine which trace to repair.  For example, there are three traces on the matrix that terminate in a pin labeled "E".  These pins are connected to each other on the interface board, but are not connected in the matrix.  For our purposes here, these traces are unique.  I renumbered them E1, E2, E3 in my matrix chart (and the same applies to many other traces).  By visually inspecting the matrix, you can determine which lead  or leads are causing the problem.  Or, just fix them all.

The only difficult part of this repair is a consistent connection to the connector body (the tape sticks great to the traces).  I am considering gluing the tape to the connector with conductive adhesive, but have not found it necessary yet.