Friday, June 6, 2014

Korg Poly-61 Key Contact Repair

Key contacts are the source of much headache on the Poly-61.  Much has been blogged and posted by others about dis-assembly of the keybed, membranes, and maintaining clean key contacts.  However in the case of THIS machine no amount of cleaning could bring back key functionality.  I believe this is due to the membranes simply wearing out and the gold contacts losing much of their conductance.  Replacement membranes are very expensive, and I wanted to explore cost-effective methods to milk the last bit-of-life from this keybed.

This repair involves re-coating the membrane contacts with a conductive pen, adding plastic shims to the keys, and replacing broken key bushings that have allowed the keys to sway too much side-to-side.  The biggest disadvantage to this repair is a small number of keys are STILL un-responsive.  Furthermore, re-coating the contacts in this manner will produce an effect known as bouncing.  Bouncing is when a key is pressed it may send two, or several, note-on signals in rapid fire succession.  This is because the contact, on its way to being fully depressed, will bounce several times on-and-off before settling.  To me I can live with this more than have a dead keybed that does nothing.  Anyway...on with the repair process.
Using a CircuitWriter pen from radioshack I coated the membrane contacts twice, with 4 hours between coats. I allowed the contacts to cure overnight. 
Using the pen takes finesse. Shake vigorously for at least 2 minutes after the little ball becomes free. Practice on something else first. Press until flow starts for two seconds, then swirl in circular motion to cover surface of contact.  Here's what the covered contacts now look like. Next to add plastic to the bottom of the keys. After experimentation, I found a 1mm thick plastic was required, cut to 8mm x 10mm rectangles. I searched far and wide, and found an Artists Loft brand graph ruler from Michael's was perfect material. Tricky to cut with an exacto knife, but can be scored and bent to finish the cuts. This photo shows the rectangle in place on one of the keys. To tape it in place I used good old fashioned duct tape, the perfect redneck engineering solution. Replace and test the key. Another piece of tape was sometimes necessary. I found three layers of tape to cause errant self-triggering of the key. Next I replaced some damaged key bushings. First, you guessed it, I used duct tape to build up a base layer on the bushing prong. In this photo you also see a bare prong, a good bushing, and another damaged bushing. Then I cut a small piece of 3/16" Heat Shrink Tubing to place over the prong. This photo shows the heat shrink tubing in place. I found the tubing fit snugly without any heat being necessary. It's not a perfect replacement for an actual bushing, but it greatly improved the keys swaying side-to-side. And that's it.

5 comments:

  1. Hey there can u tell me how to remove the keyboard. I couldn't find a way to slipped it out... Thanks

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  2. Replies
    1. Check out this video on the D-50 it's similar but the spring is in a different place https://youtu.be/-czedYcY1mA there is a long clear plastic strip glued underneath that keeps the keys from popping out. Over time the glue weakens, such as on mine, such that you can pop keys out without removing the keybed assembly. There's a little spring underneath the key, don't lose that. Then you can carefully press down on the key and pull outward at the same time. The back of the key will pop out of the socket, then you will have to lift/rotate the key out of the key bushing. Hope this helps.

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