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.