I worked on this Korg Trident shortly after it had been serviced by a well respected tech, but it had been shipped home to Philadelphia and was unfortunately damaged in transit… a tragic event considering it was in great condition and was apparently working perfectly. The owner repaired the physical damage to the wood end pieces but took it to us because the manual controls and the piano / clavinet presets for the synthesizer section were inoperable. Since it had been recently serviced, I didn’t suspect a failed component and decided to first inspect the boards for physical damage and look around for loose connections or broken solder joints. Unfortunately that wound up being a fruitless pursuit and I ended up having to dig into the schematic and trace the path of the cv voltages from the panel to the programmer board, which is where the voltages were getting lost.
I eventually traced the problem to a single 100K resistor which had failed and essentially created an open circuit. It’s really a mystery as to why it failed… it could have been corroded by a past battery leak but just hadn’t fully failed until it underwent a large shock while being damaged by the mail carrier… though it didn’t look corroded. It also could’ve just broken internally. Regardless, it was measuring in the megaohms. The worst part about it and what made it take longer to figure out is that the version of the service manual that we got was mislabled and showed this resistor connecting to -15V rather than +15V. For those who have the schematics and are curious, this is R37, which biases the output of the comparator IC20. It raises the output above 0V, which allows it to correctly clock the 4174 flip flop IC22. Without this resistor, that flip flop doesn’t work and the voltages won’t pass through to the buffers that follow it. When I tested the flip flop’s clock input, it did seem low, but I also saw it biased to -15V so I figured there was no way it could be any higher… except the flip flop’s clock in this circuit should be expected to oscillate above and below 0V so there has to be a positive voltage at some point! It didn’t make any sense, but I wound up replacing the flip flop and the comparator before noticing on the actual circuit board that that resistor was connected to +15V, confirming my suspicions that that clock should’ve been higher. Once I replaced that resistor, everything worked perfectly again!
I’ll also say, this is a really beautiful sounding synth and was a lot of fun to play with while it was here. I’m glad I was able to get it back on its feet after its unfortunate accident.