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.|
|VR2 let out the magic smoke. It is probably dead.|
|Close Inspection Reveals Some Charring (Black Carbon)|
|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.|
|Replacement 100 kOhm trimer potentiometer. The pot is small, so you have to bend the outward.|
|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|
|Adjusting the new pot to achieve the desired voltage on VCO1.|
It's good to have the old girl back working again.