Wednesday, May 8, 2013

Polysix - Bending Over the Capacitors

Dear Readers, I'm sorry.  Back when I described how I prepared my new keybed for installation into my Korg Polysix, I made it sound fairly straight-forward with no real hiccups.  Sure, at the end, I mentioned something about how a couple of capacitors might have been a little too tall, but this was actually quite a large understatement.  It turns out that those "too tall" capacitors required that I boost up the keybed even further than shown originally, which then prevented the Polysix's control panel from closing.  It was ugly and frustrating and disheartening.  Here's a description of how I fixed the problem.

Here Are the Offending Capacitors That Interfere with the Keybed
As you can see in the picture above, there are two groups of offending capacitors.  Both sets of caps happen to be on KLM-366.  The first group are the six tall Mylar caps (C31-C36) that are part of the "hold" circuits between IC29 and IC25.  The second group of interfering caps include the tall Mylar cap C23 and the stout electrolytic cap C22, both of which are part of the pitch-correction feedback circuit.  At first, I thought that the solution might be to replace all the caps with smaller ones (such as ceramics).  But after a brief exchange with the Polysix Yahoo Group, I considered alternate approaches.  With fresh eyes, it seemed like the best plan was to simply bend over the capacitors so that they weren't as tall.  Easy!

Could I Just Tip the Capacitors to the Side to Make Them Less Tall?
Unfortunately, the caps were mounted very close to the PCB and didn't have enough of their leads above the board to enable me to tip them over.  Since I just replaced an IC on this PCB, though, I have the synth all apart and, therefore, I have full access to the bottom side of KLM-366.  Looking closely, I saw that each cap had a little bit more of its legs sticking out below the PCB.  So, I got out my soldering iron, I applied some heat to the tips of the legs until the solder melted, and then I pushed until I couldn't see the tips of the legs anymore.  See the picture below.

Looking at the Underside of the PCB.  Making the Caps Taller By Pushing on the Tips of their Legs.
Once I did this, the first cap was now taller than it had been because of the extra bit of each leg that I pushed back above the PCB.  Now it was tall enough that I could tip it over with ease.

Bending Over Worked well for the First Cap.  Now for the rest...
After repeating the process for the remaining five capacitors, things are looking pretty good.  Now it's time for the other two caps.

The Six "Hold" Caps Are Done.  Now It's Time for C22 and C23.
The tall Mylar cap on the left side of the picture above was easy to address -- I simply did the same trick of re-heating the solder joint and pushing up the extra bit of each leg.  Then, I bent him over.  The electrolytic cap, though, was too stout (too fat) to push over.  Having a bunch of electrolytic caps on hand, I decided that it would be easier to just replace this guy with one that was skinnier and that had long enough legs to bend over.  So, I de-soldered the old one and soldered in the new one.  I bent it over and, as you can see below, everything is shorter than it was before.

All the Caps are Now Much Shorter Than Before.
With all of the steps complete, I put the keybed back into the synth.  Did it fit?  Well, the part of the keybed that was near the caps fit great.  That problem was solved.  But, with that part of the keybed sitting correctly, it now revealed that another part of the keybed was interfering with something within the case of the Polysix.  A little poking around showed that the long white bar of plastic that I added to my keybed wanted to sit right on top of the rail that holds the whole front side of the KLM-366 and KLM-367 boards.  That's an easy fix that I'll talk about later (just cut that darned plastic bar!).  For now, though, we can be content that this cap-bending job was the key.  It was the key to enabling the keybed to sit properly in the Polysix's case and for the lid to close properly and for the whole thing to be beautiful.

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