Cleaning a PET 8032 Keyboard for Vintage Computer Festival East 10.0

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This is one of the computers I intended to run VisiCalc on for VCFeX.  It is a CBM 8032 that I bought locally from a Commodore executive who had stored it in a closet since 1985.  I unboxed it last year only to do a quick inspection and power on test.  The keys fought me while typing a hello world basic program so I knew for a while the keyboard was going to needed attention.  Now that a real use was at hand, it was time to make that keyboard sing.


So reviewing the situation again I found few keys were intermittent and “W” was  totally dead.  Getting down to business I removed the keyboard and took it to the bench.  First job is to remove the keycaps.

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I like to lay the key caps out in sequence and put all the parts in bowls.

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A bunch of small screws hold the circuit board to the key assembly.
There is also a set of wires to the shift lock key that need to be  UN-soldered in order to free the board.  Here is the business side of the circuit board.

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And close up.  The conductive rubber pads of the switch shafts bridge the interleaved fingers of the gold contact areas.

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You want to do a good enough job at this so that you only have to tear them down once.  Its not as bad of a job as I thought it was though.  

The pads were in good shape, still soft, and measured about 50ohms across on “W” and other pads that I spot checked.  I concluded most of the contact problems were dust and debris.  So I cleaned the contacts and the pads with 99% Isopropyl alcohol.  I put the keyboard partially back together, well enough to test it, and the intermittencies were gone.  Sadly the “W” key was still not functional at all.  This was looking like a wiring problem.

I took a look at the 8032 keyboard matrix from 6502.org to see what keys were on the same row of the matrix.  The chart there for the business keyboard says W is on row 4 line. However measuring from the side of the W contacts that should go to #4 edge contact showed that it was open (blue line), meter reading 0. Note my red meter lead goes to row #4 on the back side of the board, so I drew in the red line to illustrate that. See those wide traces on the board? They are jumpers that connect different sections of the layout to other sections.  I suspect that the hockey stick shaped one under my blue line was the culprit of this disconnect.   My solution was to run a jumper from a safe convenient spot of the W key’s #4 connection and the #4 row pad on the edge connector following the line of this color below.

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The green wire below is the patch wire for the W’s line to row #4 edge connector pad.  It was run underneath to avoid conflicts with key travel.

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With that patch wire done, the keyboard worked great. It took about 2 hours to complete.

I may revisit this to do a less obtrusive repair.  It was recommended to me to use very thin transformer wire to path the open from the other side.

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Filed under PET, Vintage Bitage

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