The Korg Delta (or DL-50) is a string synth/polysynth hybrid from the late 1970s to early 80s, from the time when every synth company was focusing on those. I’ve got an Arp Omni 2 and Roland RS-09 of the same era here in the workshop right now and the Delta is my favorite of all three, even just based on its string sounds (to be fair, the RS doesn’t have a “synth” section). It’s got a thicker, more dirty sound (in a good way), the chorus has a sort of rumbly, “lumpy” quality that is really unique, and there’s a wildness about the modulations that is really distinctive, and really contrasts with the reserved quality of the other two string synths. You can pitch modulate the string section which sounds delightfully sinister and twisted, and there’s an EQ section which really expands your timbral range. Size-wise, it’s right in between the huge Omni and the little RS-09. If I wanted to buy a string synth, I’d consider this one for sure.
Darian handled the repair on this one which was pretty simple. The client had already done some digging and guessed that the LM13600 op amps in the filter section might be to blame, and even bought some to give us. I think he was half right… there is a very common issue with these where the LM13600 and the four surrounding 1000 pf polypropylene capacitors both fail, making the filter start acting up after the synth heats up. After we replaced those it was fixed and able to run for a whole night.
Service tips: LM13700s could certainly be put in place of the LM13600s for greater long-term reliability. We used the 13600s because the client had given them to us. The polypropylene capacitors in the filter section are being used to tune the tracking VCF and they have to be polypropylene because of their distinctive temperature behavior. They are bent over with their “heads” pressed against the board to help regulate their temperature too.
Work done: replaced (2) LM13600s and (4) 1000 pf polypropylene capacitors in filter section.