Polysix - MG Delay of Zero is not Zero

Earlier this week, when playing my Korg Polysix, I noticed that the LFO ("MG") was not having the effect that I expected.  I often like to set the MG to sweep the VCF very slowly, especially when playing the arpeggiator.  Based on my experience with the Korg Mono/Poly, I expect that the slow MG sweep would cause the arp notes to smoothly change their brightness from note to note.  Unfortunately, that's not what my Polysix is doing.  As you can hear in the soundcloud sample below, there is a clear transition at the start of each note where the VCF goes from its default value (as set by the VCF Cutoff knob) to the current value of the MG.  If you have a Polysix, does yours do this?


The settings for this sound are shown in the pictures below (MG Freq = 2, Delay = 0, Amount = 6, VCF Cutoff = 5, Resonance = 0, EG Intensity = 0, KBD Track = 0).

To show visually illustrate this unexpected audio behavior, check out the screen shots below.  These are screen shots of the audio in the sound cloud sample above.  Each segment shows four notes from the arpeggiation.  The top figure is when the MG is at the high end of its cycle, which means that the MG is opening the VCF beyond the setting from the VCF Cutoff knob.  As you can see in the screen shot, the note clearly starts at a lower VCF setting and then, after ~100 milliseconds, the filter opens up to the value defined by the MG.  This is with the MG Delay at zero!  It should not be like this.  The bottom figure shows the same thing, except where the MG is at the low end of its cycle where the MG is closing the VCF to a value lower than that set by the VCF cutoff knob.  Again, there is ~100 ms delay before it transitions to the MG's value.  In my mind, it should not be this way...the beginning of each note should be no different than the middle of the note.

Even with MG Delay set to Zero, There is Still ~100ms Before the MG Affects the Sound

So, I don't think that it is supposed to work this way.  Unfortunately, I don't know whether this is a new behavior of my Polysix (ie, it has become broken) or if it has always been this way (ie, it is a "feature" of the Polysix design).  If you have a Polysix, does it respond like this?

Update: I explored the MG circuit to find the cause of this behavior

Mono/Poly - Replacing a Trimmer Pot on KLM-398

Sadness strikes!  I went to turn on my Korg Mono/Poly the other day only to find that it was totally dead.  No sound and no LED lights.  The fact there were no lights was a pretty strong clue that it had no power.  So, opening up the Mono/Poly, I found that the fuses were blown.  When I replaced the fuse, I saw smoke pour out of one of the internal trimmer potentiometers.  Death spreads!  This post talks about the process of replacing a trimmer pot in a Mono/Poly.  Just so that you know that this has a happy ending, here are some clips of me playing my beloved Mono/Poly once I got it working again.


Replacing the Fuse:  The story starts with finding that the fuses were blown.  When I first opened my newly-dead synth, I probed around with my digital multi-meter to see if it had power.  I found that I had power on both sides of my transformer.  On the power supply PCB, however, I did not have voltage downstream of the two fuses that separate the transformer from the diodes and power caps.  So, I pulled the two fuses and visually saw that they were indeed blown.  So, I went to Digikey and found some fuses rated for 1.6A as stated on the schematic (Digikey P/N: 486-1882-ND).  When they arrived, I found that they were not transparent as suggested by the Digikey product page, but were opaque as shown in the picture below.  Oh, well.

Old (dead) fuse on the left.  My replacement fuse is on the right.

Burning a Potentiometer:  When I put the new fuse into the synth and turned it on, it only took a couple of seconds until I saw smoke issuing from one of the trimmer pots on KLM-398.  Smoke is never a good thing when working with electronics.  Usually, if you let out the magic smoke, it stops working.  So, I quickly turned off the synth, removed KLM-398, and took a close look.  The smoke came from VR2 (see photo below).  Very close inspection (2nd photo below) does indeed show charring (carbon) on the pot.  It needs to be replaced.

VR2 let out the magic smoke.  It is probably dead.

Close Inspection Reveals Some Charring (Black Carbon)

Removing the Dead Pot:  To install a new pot, you must first remove the dead pot.  That's a simple process of de-soldering the pot, pulling it from the board, and cleaning out the solder holes of any residual solder.  The pictures below show how I needed to use a pliers plus my soldering iron to remove the pot.  Once it was off the PCB, you could see a small burn mark on the PCB confirming that I removed the correct one.  To remove the residual solder, I chose to use a solder pump (see this post for more discussion of solder wick and solder pumps).  With the solder holes clean, I'm ready to install the new trimmer pot.

Removing VR2 by applying heat to the solder while pulling on the body of VR2 with pliers.

After removing VR2.  you can see burn marks on the PCB.

After removing VR2, you have to clear the holes of solder.  I use a spring-loaded solder pump.

Holes for VR2 after removing the solder.

Buying a Replacement Pot:  Of course, to install the pot, you must first buy the pot.  The schematic calls out a 100 kOhm pot, but gives little other information.  Digikey has tons of options for 100K pots, so I wasn't quite sure what to get.  Looking at the PCB, I clearly needed a pot with 3 legs in a triangular configuration.  Also, given that I smoked the original pot, I wanted a new pot that had more than the absolute minimum power rating.  I found a bunch that seemed reasonable and bought a few different models.  In the end, I decided to use the pot shown in the pictures below.  It is Digikey P/N 3362F-1-104LF-ND.  The only undesirable aspect of this pot is that the legs are not wide enough.  You need to use pliers to bend them outward, as shown in the right-hand picture below.  When you do that, the legs are now barely long enough to poke through the holes in the PCB.  Since they're so short, you have to be careful when soldering that you are able to heat the legs before applying the solder.

Replacement 100 kOhm trimer potentiometer.  The pot is small, so you have to bend the outward.

Soldering the Pot:  After bending the legs and inserting the pot's legs in the holes in the PCB, you are ready to solder it into place.  The soldering process is pretty straight-forward, as long as you can actually tough the pot's legs with the soldering iron.  If so, apply the heat, apply the solder, and you're done!  Since the new pot is not the same as the original pots, it does look a little funny on the PCB (see picture below), but that's OK with me as long as it works!

Soldering the new potentiometer onto the KLM-398 PCB.

Re-installing the PCB and Tuning:  With the PCB complete, simply screw it back into its spot inside the Mono/Poly and reconnect the wiring harness (see photo below).  Now it is time to cross your fingers and turn on the synth.  I got lucky -- there was no more smoke.  It seemed to run OK!

Mounting KLM-398 back into my Mono/Poly

Re-Calibrating the Mono/Poly:  Now that it's powered, I can go through the calibration procedure in the Mono/Poly Service Manual so that I can tweak the new pot so that it is properly doing its job.  I chose to start from the beginning of the Service Manual's calibration procedure, which begins with checking the power supply voltages (always start by checking these voltages!).  I then checked and corrected the voltages on the Key Assigner PCB.  After that, you reach the part of the procedure where you check the voltages produced by the KLM-398 PCB.  In my version of the manual, it says to check the voltage on the red, yellow, blue, and gray wires.  I disagree.  In my opinion, they are wrong about the Red wire...it should be the black wire.  Note that the pots are in a funny order on KLM-398.  I just replaced VR2, which is not associated with VCO2 (gray wire), but is instead associated with VCO1 (black wire).  Once I got all of that straightened out, I used my digital multi-meter set on mV mode and adjusted my new VR2 until the black wire showed 0.000 V +/- 1 mV (see picture below).

Adjusting the new pot to achieve the desired voltage on VCO1.

Playing the Synth:  After confirming the voltages produced by KLM-398 for all four VCOs, I closed up the synth, plugged into my amp, and started playing.  It works!  Oh the joy!

It's good to have the old girl back working again.

Polysix - Replacing a Push-Button

We all know that, given their age, most Korg Polysix's have problems.  Usually, the problems are related to the dreaded "leaking battery" problem.  My Polysix had that problem and I fixed it (I hope).  My Polysix, though, also had a problem with the push-buttons being very finicky.  I think that this is a common problem, too.  I've replaced many (but not all) of the switches when I first got the synth.  Recently, though, my "Arpeggio" button started acting up, so I decided to replace it.  For anyone who needs to replace any of their buttons, here's a picture tour of the process.  (By the way, the buttons on the Korg Mono/Poly are the same, so this applyies to Mono/Poly owners as well!)

I replaced the Arpeggio button and now it works great!

The general outline for replacing one of the push-buttons is:

1. Open the synth
2. De-solder the old button and the associated LED leads
3. Un-fasten the button's PCB from the synth
4. Remove the button cover
5. Remove the button itself
6. Solder in the new button
7. Re-attach the button cover and solder the LED leads
8. Re-attach the button's PCB to the synth
9. Re-fasten the PCB and close the synth

So, here we go with pictures and description and tips:

(1) Open the Synth:  I don't have any pictures of this because it is pretty straight-forward.  Unscrew the four screws on the cover panel and unscrew the four (?) screws on the bottom of the synth that secure the bottom lip of the cover panel.  Once you've got the synth open, you've got to find the PCB holding your button.  Since I'm replacing the Arpeggio button, it was pretty easy for me...they labeled it!  Note that they spelled it "Arpeggo" (oops!), which amuses me.  But, since we're talking about labels on PCBs, why are the labels in English?  Wasn't this synth designed in Japan?  Why aren't the labels in Japanese?  I've never understood why there's so much English on circuit boards made throughout the world.  

Here's the back of the Arpeggio button.  De-solder all six solder points.

(2) De-Solder the Old Button and LED Leads:  To de-solder the button, you'll note that there are six solder points.  Four are for the legs of the push-button and two are for the LED that's built into the button cover.  You'll need to de-solder all six.  You can do this using solder wick and/or a solder pump.  I used the pump for the bulk of the work (see my earlier post for a bit more description) and I used the solder wick to wipe up (soak) up the little bit of solder that remained.

First, apply heat until the solder melts.

Use the solder pump to suck out the molten solder.

After a final touch-up with the solder wick, here's what it looked like. 

(3) Unscrew the PCB:  To get the button off the PCB, you'll need to detach the PCB from the synth so that you can get access to the top of the synth.  The PCB that holds the Arpeggio button has lots of screws.  You need to remove them all.  This particular PCB is also connected to two daughter PCBs.  You'll need to unscrew those as well.

There are lots of screws to remove on this PCB ad its daughter boards.

One of the daughter PCBs holds the potentiometer Arpeggiator Speed control.  To free the PCB from the synth, you need to remove the plastic knob...just work your finger tips under the lip of the knob and pull it off.  Then unscrew the nut that holds the potentiometer to the case.  Easy.

For this PCB, you also need to remove the knob and the nut on the potentiometer.

You'll notice that you've collected quite a few bits and pieces from your synth at this point.  You've got screws from the synth's case, you've got screws from the PCB(s), and you've got the knob and nut from the potentiometer.  Don't loose those bits!  I have a set of little bowls that I use anytime that I remove items from my synth.  If the bits go into the bowl, then I know where to find them when I go to re-assemble the synth.  Any bits that remain in the bowl when I'm  "done" are a reminder that I must have missed something.  The bowl is critical!

I keep all of my screws and knobs in a little bowl while I work.

And now the PCB is no longer secured.  This particular PCB requires a little twisting and sliding, but eventually it comes from from the panel.

The PCB is free!

(4) Remove the Button Cover:  These push-buttons are composed a two main parts: the button itself and a button cover [*see comment section at bottom].  The button itself is what we need to replace.  It lies under the button cover.  The button cover is the colored plastic piece that you actually touch with your finger.  The button cover also holds the red LED that lights up when you press the button.  The button cover is what we need to remove first.

The button cover is held on to the PCB once the legs of the LEDs are soldered to the board.  The button cover is also held on through friction with the button itself.  To free the button cover, I generally use my soldering iron on the bottom of the PCB to push the leds of the LED up into the board (the soldering iron also melts any residual solder that might be holding the LED legs).  At this point, I can generally wiggle or pry the button cover off the button.  Once you do one, you'll have more confidence when you do others.

The button cover has been removed from the button.  Notice the two legs of the LED sticking out of the button cover.

(5) Remove the Button:  The button itself is held to the PCB by its four legs.  The legs are both soldered in place and they have a certain bend in their shape that helps them grab the board.  You've already removed most of the solder, but there might be a little left in there.  So, I again take my soldering iron and, from the bottom of the board, try to push each leg back into the PCB.  I'm trying to push the button up off the board through pushing on its legs.  I also use my needle-nose pliers to try to pry under the button while I'm applying the heat.  Surely, there is a better way of doing this, but eventually I'm able to get the buttons off. 

Using a soldering iron and pliers, I eventually get the button off the PCB.

(6) Attach the New Button:  Before you solder the new button, you need to buy a new button.  Via a post by Anon on the Polysix Yahoo Group, I was pointed to some Omron 12mm x 12mm tactile switches that did the job.  From Mouser, I ordered some Omron B3F-4000 and some B3F-4005 -- the only difference between the two being the amount of force necessary to activate the switch.  Once installed in the synth, I couldn't really tell the difference between the two, so either is fine.  The B3F-4005 is shown in the picture below.

An Omrom B3F-4005 P

To mount the switch to the PCB, line of the legs of the button with the holes in the PCB.  At this point, I usually get out the soldering iron (again) and heat the hole from the underside so that I melt any solder that might still be in the hole.  I then push one of the button's legs part-way (or fully) into the hole.  I then repeat for the other legs.  Once all four legs are in the holes, I heat and solder from the bottom of the PCB like normal.

Lining up the legs of the button with the holes in the PCB.

Unfortunately, because I'm no expert at doing this, I damaged the PCB when I pushed the legs through the hole. As can be seen in the picture below, I leg pushed one of the solder rings off the PCB.  This is definitely the mark of an amateur.  Lucky for me, all four legs of the switch are soldered into the circuit when only two are really needed....which means that there is redundancy!   I can lose one of the connections (due to the lifted solder ring) and still have a good solid button.  Lucky for me!

I lifted up one of the solder rings.  Dang!

(7) Re-Attach the Button Cover:  Once the button is soldered in, we can now re-attach the button cover. First, I line up the button cover with the button, including the legs of the LED with the holes in the PCB.  I then push the legs of the LED through the PCB holes, usually with some heat from the soldering iron to melt any left-over solder that's in the hole.  Once the legs are through the hole, I make sure that the button cover is fully-seated around the button itself.  Then, on the bottom of the PCB, I solder the legs of the LED like usual.

Seating the Button Cover on the Button.

(8) Re-Fasten the PCB and Close the Synth:  The work with the button is now done.  To finish, simply, re-fasten the PCB to the inside of the synth (do you still have all the screws?) and close panel of the synth (do you still have those screws, too?).  Turn on the synth and enjoy your new button!

The Button Now Works Great!

Killing and Reviving my Polysix (Replacing a Dead IC)

So far, I've been pretty successfull with hacking aftertouch, portamento, and detuning into my Korg Polysix .  My next big goal is the addition of velocity sensitivity, which requires me to better understand the VCF control circuitry on KLM-366.  Well, in probing the clock signals for the VCF multiplexing, I killed my Polysix!  This is the story of how I brought her back to the land of the living.

Probing IC12 on KLM-366.  I think that I've found the problem.

How Did I Kill It?  This all started when I was probing around the part of the KLM-366 board that processes the VCF EG signal for each voice.  Specifically, I was exploring the time-division multiplexing of the VCF EG through IC24 and IC23 .  This multiplexing is all kept in sync via clock signals "A", "B", "C", and "INH" that are generated from IC11 and IC12 (schematic below).  In probing IC12, I accidentally shorted one of the pins on IC12 (likely pin 11), which apparently killed it.  As you can see in the pictures above, the "B" signal is clearly bad (it is supposed to be a square wave pulse).

How Do I Fix It?  Replacing IC12 seemed like the only reasonable answer.  The chip itself is really cheap (68 cents).  The hardest part in replacing the IC is disassembling the synth so that I can get good access to the circuit board.  I don't like taking it apart because it gives me too many opportunities to loose screws and to re-connect connectors in the wrong place and stupid stuff like that.  But, in this case, I think that it needs to be done.

Shopping List:  The only part that I really needed was a replacement 14024 chip.  Like usual, I got it from Digikey.  The specific part that I bought was MC14024BCPGOS-ND.  I chose this part because (I think) it is the only through-hole version of the 14024 that they sell.  At 68 cents, I bought 2, just in case I damaged one during my installation.  Also, following the advice from The Old Crow, I decided to install an IC socket at IC12 in order to ease any future replacement of this chip. There are tons of choices for sockets.  Never buy the cheapest ones.  I liked the look of A32869-ND.  Even though it was only 78 cents, it seemed to be one of the higher end models.  Cool.

Removing the Dead IC.  The replacement process starts by removing the dead chip.  Some helpful folks at my workplace suggested that removing an IC is best done by first snipping off each of its legs so that the body of the IC just falls free of the PCB.  Then, as long as you snipped the legs high enough away from the board, there's plenty of leg left to grab with your pliers so that you can apply a little heat and lift each leg out one-by-one.  Pretty easy.

Snip Each Leg of the IC

All Legs Are Snipped, The Body is Loose, and the Legs are Still in the PCB.

Removing Each Leg from the PCB.

Removing the Old Solder:  Even after removing the IC's legs, there was lots of old solder that is plugging the holes in the PCB.  In order to get the new IC socket into all those holes, it's usually best to remove that old solder.  There are a few ways that you can do this -- solder wick and a solder pump being the two most common.  Clearing plugged through-holes is a perfect task for a solder pump.  With a solder pump, you first apply your soldering iron to the hole until the solder melts.  Then, you quickly put the spring-loaded solder pump over the hole and, while the solder is still melted, you hit the release button.  BANG!  The pump pops open and (hopefully) sucks out the liquid solder.  Because I'm new to this, I usually had to try 3-4 times before I got the solder out.  As you can see below, the holes looked fairly clear when I was done.

Using a solder pump to suck out the solder from the holes.

The holes are now fairly clean and ready for the new IC (or socket).

Installing the IC Socket:  At this point, I was able to insert my new IC socket into the holes in the PCB (see below).  It fit pretty nicely.  Now I just needed to solder it in.  Unfortunately, one really needs access to the bottom of the PCB to do this, which means the synth needs to be partly disassembled.  So, I removed the keybed, I removed the rail holding down the PCBs, and I unscrewed all the screws holding down the PCB.  I then removed a few (but not all!) of the multi-pin connectors so that I could tip up the PCB and get access to the bottom (see pic below...the power drill is holding the PCB upright).   Once I was able to access the bottom of the PCB, I was soldered the legs of the IC socket to the PCB.  We're cooking right along now!  Smell the solder!

Inserting the IC Socket.  It fits!

Getting access to the bottom of the PCB.

Soldering the legs of the IC Socket.  Heat the site and THEN apply the solder.

Finishing Up:  Once the socket was soldered in, I layed the PCB back down and I inserted the replacement 14024 IC into the new socket (see pic below).  I then re-attached all the connectors that I had undone, I double checked that I re-connected everything correctly, and I turned on the power.  With the oscilloscope, I checked the "A", "B", and "C" clock signals being generated by the new 14024 (see pic below).  Everything seems to work!

The replacement 14024 IC is nestled into its new home on my KLM-366.

After Replacing IC12, the clock signals all look good.

Oh, The Joy:  With the circuit looking like it's working again, I re-attached the keybed, closed the lid, and fired her up.  Here's my little jam of joy...the playing is crappy, but it's so good to hear her voice again.

Next Step: Tipping over the capacitors so that the keybed fits properly

Buying Parts and Using Digikey

Before I dive into my circuit modifications, I'd like to talk about how to buy parts.  As a non-professional electronics person, it can be hard to know what specific items to buy.  I mean, if someone says that you need a 10K resistor, where do you get such a thing?

Well, it's great if you can find what you need at hobbyist places like Sparkfun or Adafruit.  They really limit the choices to just the core essentials.  Having limited choices sounds like a bad thing, but it really is a blessing.  They have done all the work of sifting through the thousands of choices available and have narrowed it down to just the few choices that will likely fit what the hobbyist needs.  If you can find the part that you need at one of these places, it'll probably be the right one for you and you should buy it.

If they don't have what you need, you can go to a place like Jameco.  They've got more choices, which can be daunting, but they show a lot of pictures, so you can often shop by the pictures.

If you still can't find what you want, it's time to put on your big-boy pants and step up to "real" stores like Mouser and Digikey.  They're the places that professionals go to buy their electrical parts.  Their stores are ridiculously deep.  They're scary places to go at first, if you don't know what you want with an engineer's precision.  But, with a little practice, they get more comfortable, and then the whole world of electronics is open to you.

Let's go through an example of shopping at Digikey.  Nearly any time that you work with integrated circuits (aka "ICs" or "chips"), you'll have to use 0.1 uF capacitors to provide filtering ("decoupling") of high frequency transients on the power input line.  On most designs (even hobbyiest designs) 0.1 uF caps are everywhere.  Let's say you need to buy some.  In this case, they're so common that Sparkfun does carry them, so you should just buy them there.

Buying a 0.1 uF Cap at Sparkfun

But shopping at Sparkfun is not the point of this exercise.  The point is to try Digikey.  So go to Digikey and search for "capacitor".  You get 275,000 options.  Umm.  OK.  Now what?

Shopping for Capacitors at Digikey.  Lots of choices!

Well, for caps, you need to know what type (composition) of capacitor you want.  How do you know that?  Well, sometimes the schematic tells you ("electrolytic" or "polypropylene").  But, usually, it won't say.  What you need to know is that, for a given cap size, or for a given application, everyone seems to use the same type of cap.  It's tradition.  If you search around the web enough for people using a cap in a similar way as you, you'll be able to find out what everyone uses.

For synth hacking, most caps will be ceramic caps because they're small and cheap.  The biggest exception is for high capacitance caps (1 uF and bigger).  These caps are almost always electrolytic caps.

Returning to our example case of finding a 0.1uF cap for use around ICs, everyone seems to use "cheap" caps, which definitely means "ceramic" caps.  So, on the Digikey page, click on "Ceramic Capacitors".  This gets you down to 126,000 choices.  Note that the webpage has changed...

Digikey's page for filtering through all the choices for "Ceramic Capacitor"

This new page is giving you all sorts of filtering options.  Now's when we really start cooking:

1. First, click on the checkbox for "In Stock".  Now we've got only 41,000 choices.
2. Under "Capacitance", scroll down and select "0.1uF".  2,000 choices.
3. Under "Voltage - Rated", use Ctrl-click to select "25V", "35V", and "50V".  944 choices.
4. Under "Mounting Type", choose "Through Hole".  159 choices.

At this point, you've narrowed it down to a few pages worth of choices.  If the part you needed was a little less generic than simply a "0.1 uF capacitor", you'd probably only have a handful of choices now, instead of 159 choices.  So, you'd look at the pictures, maybe look at a datasheet or two, and then just pick one and go.

In the case of these caps, you probably want to down-select a little more.  Look under "Tolerance".  We probably don't want to pay for the best caps (smallest tolerance) and we never want the worst (biggest tolerance), so select "10%" and "20%".  That got us down to 128 choices.

Looking down through the first page of choices, the pictures all look like parts that I could work with.  Any of these would probably be fine.  Because I'm likely to be hand-soldering on a crappy proto-board, I think that I prefer the look of the caps with the long leads.  So, I'd probably buy some of Digikey Part BC2665CT-ND.  They're 37 cents individually.

Hopefully, a decent choice for a 0.1 uF capacitor from Digikey.

For cheap general-purpose components like caps, you should always buy more than you need right now.  I always buy extras.  That way, I might not need to order more when I go to my next project.  Having the parts on hand means that you can dive right in when the inspiration strikes and not have to wait for the shipping.  It's so much more fun when you can dive right in.

For something super general-purpose like 0.1 uF caps (or 10K resistors), I might buy 10 or more.  For these caps, you'll see that the price drops to 25 cents each when you buy 10.  Or, if you're really bold, you could buy 100 for only 10 cents each!  I'm not that bold....I'd probably just buy 10.

So, that's how one buys parts.  It's a very important skill to have when you're going to be hacking synths.  Anyone have any sites that they really like to use for parts?

Dead Keys on an Old Korg? Try CaiKote 44!

My first synth was a Korg Mono/Poly and I still love it. I've had it for 4-5 years. I got it off eBay and half the keys didn't work. Luckily, since it was cheap, I didn't have too many qualms about diving in to try to "fix" (or possibly make more broken) all its broken parts. I didn't have much to lose!  And, once I did actually fix the broken parts, the success inspired me to keep going into the endless world of synth mods. So fun!

But that's not the point of today's post. Let's start simple. Let's fix the dead keys...

Out on the web, one can find a bunch of approaches to fixing dead keys on old keyboards. The best description of how to work specifically with these old Korgs is from Old Crow...his site is really good...you should check it out...

http://www.oldcrows.net/~oldcrow/synth/korg/polysix/keyclean.html

His text and his pictures give you the confidence to dive right in, even if you (like me) might not have ever done anything like this before. Very inspiring.

Unfortunately, following his directions for simply cleaning the key contacts didn't fix my dead keys. Bummer.  After a bunch more internet searching, I came across a reference to this paint-on electrically conducting stuff...CaiKote 44. Oh man, this is where it's at...

http://store.caig.com/s.nl/sc.2/category.181/.f

A little of this stuff goes a *long* way, so all you need is the tiny little package shown on the left (not the bigger tub on the right). To use this stuff, simply: (1) follow Old Crow's description to get at the keyboard's rubber contact strips, (2) swab a very thin layer of this CaiKote stuff onto each rubber contact, (3) let it dry, (4) put your keyboard back together. Voila!

After applying this stuff, all the keys on my Mono/Poly worked. Outstanding.  After I got my Korg Polysix off eBay, I found that it had a few dead keys, too. I dug out my several-year-old tube of CaiKote 44, put a little bit on, et Viola encore! This stuff is great.

Oh yeah, for those of you on Muff Wiggler (an excellent discussion board), I discussed this fix in this thread:  http://www.muffwiggler.com/forum/viewtopic.php?t=71861

And I just found this...another guy who used CaiKote on his PolySix.  He's got great pics:
http://m.matrixsynth.com/2010/03/korg-polysix-keyboard-repair.html

Update: I just learned that LASynthCo is selling newly-manufactured key contacts for the Polysix and Mono/Poly.  They're not super-cheap, but it's a great option to have.  http://shop.lasynthco.com/product/key-contacts-for-korg-poly-oberheim-sequential-prophet-600-kawai-and-others

Update: As some of the keys on my Mono/Poly have stopped worked again (I guess the CaiKote is only good for a few years), I bought some new key contacts from LA Synth Co.  They work great!  You can check it out here.