Korg Mini pops 120, Question about capacitors.

[Avraxas]

Hello everyone,

I have a Korg Mini Pops 120 that I want to recap completely, since all of the electrolytic capacitors are leaking.

Since this is an analog drum machine from the 1970s, I have read that the original capacitors from that era were generally not low-ESR, and that it might be a good idea to preserve that characteristic rather than replacing everything with modern ultra-low-ESR parts.

However, I have also read that at least in the power supply section, it would be a worthwhile upgrade to use low-ESR capacitors in order to reduce output voltage ripple. For that reason, I am considering using Panasonic FC series capacitors there.

I was also thinking of using Panasonic FC series capacitors in the ROM IC & Buffer section as well, hoping for better timing accuracy or stability.

For all the other sections, I am considering using Nichicon FG (Fine Gold) audio-grade capacitors, since their ESR is not especially low, and I assume this would help preserve the original sonic character of the machine.

My biggest question concerns the 1µF / 50V capacitor located in the BD.LC (Bass Drum / Large Conga) section. I was advised not to replace the original polarized electrolytic capacitor with another polarized type, but instead to use a non-polar electrolytic, specifically the Nichicon Muse ES series.

How exactly would this change be considered an upgrade?

I would really appreciate your thoughts on this—both regarding the 1µF capacitor in particular, and the capacitor choices for the other sections of the schematic as well.

Below, you will find a photo of the schematic showing all the relevant sections.

Thank you very much!

[acheld]

Personally, I would not overthink this.   No capacitor that you put in the device will match what was there when it was new. 

My criteria would be:
1)  Match the capacitance.  Don't worry about the ESR, and forget "audio grade" for this drum machine.
2)  Increasing the voltage spec of the caps in the power section where you can, not so much in the signal path.
3)  Do try to match lead spacing where you can.  This is more important than you might think.
4)  I do like to use film caps where possible, but it's not always feasible due to space limitations (film caps are always larger than electrolytic caps).  Film caps last a very long time, and seem to be robust even when I make mistakes.

While I can't make any sense of the layout you provided, I personally would use a film cap 1uF @ 50 or 63V in your conga section.   If the drum machine was designed for a polarized cap, it is unlikely that it would benefit from a bipolar cap, or even a film cap (these provide better linearity IF the signal crosses below zero).   

Does the drum machine work now?  If not, pay attention to other components that may have failed along with the capacitors.

These are my opinions only, based on repairing a lot of stuff... but I am not a pro.

[Avraxas]

Thank you very much for your reply.

The machine has not been tested at all due to the condition of the capacitors. I have already removed them from the board and, out of curiosity, I even measured them. Most of them are completely dead, without any measurable result at all.

I know I may be overthinking a simple capacitor replacement, but I feel there are paths and details I might be overlooking where the finer points could actually matter.

Given that this is a machine from 1979, I would like to stay as close as possible to its original era. Not because I owe it to my ears… (I honestly doubt I could even hear the difference), but out of respect for the machine itself.