Korg KR-55 Recap Guide

I'm repairing my second Korg KR-55 drum machine. They both had a ton of failing electrolytic capacitors that needed replacing. In the first machine, the capacitors had leaked and damaged the board. In the second, one had failed and shorted, causing the snare, hihats, and cowbell to not work.

Recommending "recaping" as a general fix, is something of meme now. It's no joke on the Korgs though. I advise replacing all the electrolytics in them, so I made a the following images and lists to help.

The logic board only has a few capacitors. The 470u are the large, axial type, and don't seem to be an issue. They're listed for completeness.

Name	Value	Voltage
C21	.33u	50V
C1	10u	16V
C22	470u	25V
C23	470u	25V

Here are the relevant capacitors on the sound board. They're color coded by value.
Red = 1u / 50V
Org = 10u / 16V
Ylw = 100u / 16V
Grn = other

Here are the footprints on the bottom, to help with desoldering. These are not coded, because they should all be removed.

Here are the types and totals of capacitors:

Value	Voltage	Total
.15u	50V	2
.22u	50V	2
100u	16V	5
10u	16V	14
1u	50V	14
47u	16V	1

Here are the individual capacitors, and their function in the circuit. This can be helpful for debugging.

Name	Value	Voltage	Role
C59	.15u	50V	Low Conga
C60	.15u	50V	Hi Conga
C49	.22u	50V	HiHat
C61	.22u	50V	Tom Noise Audio
C50	100u	16V	Cymbal Power
C52	100u	16V	Hat Power
C106	100u	16V	Power
C107	100u	16V	Power
C123	100u	16V	Metal Power
C20	10u	16V	Bd, Sn Power
C21	10u	16V	Bd, Sn Power
C51	10u	16V	Cymbal Power
C71	10u	16V	Tom/Conga Audio
C72	10u	16V	Tom/Conga Power
C73	10u	16V	Tom/Conga Power
C98	10u	16V	RS/Clav/CB Audio
C99	10u	16V	RS/Clav/CB Power
C100	10u	16V	RS/Clav/CB Power
C101	10u	16V	RS/Clav/CB Power
C103	10u	16V	Mix Audio
C105 (C05)	10u	16V	Power
C112	10u	16V	Noise
C114	10u	16V	Noise Power
C2	1u	50v	Bd Trigger
C5	1u	50v	Bd Audio
C10	1u	50V	Sn
C13	1u	50V	Sn Audio
C16	1u	50V	Sn Body Audio
C18	1u	50V	Sn Audio
C54	1u	50V	Low Conga
C55	1u	50V	Low Conga
C64	1u	50V	Hi Conga
C65	1u	50V	Hi Conga
C90	1u	50V	Claves Audio
C95	1u	50V	Cowbell
C108	1u	50V	Noise
C111	1u	50V	Noise
C19	47u	16V	Bd, Sn Power

CZ 1000 Noise Fix (DAC Adjustment)

So I picked up an as-is CZ 1000. It started off terribly distorted, but after jumping a few corroded traces it started to sound more normal. Still not right though. Some presets had an odd harshness and some a "zipper" sound as they decayed.

It might not come across on soundcloud, but here's an example.



After some searching I found several people discussing similar problems. Each time the solution was to adjust the DAC's offset voltage. The schematics can be found easily and they depict the DAC portion like this:



VR2 is the obvious candidate, but turning it had no effect. I started looking at the board and it turns out that there is another, less visible trim pot (VR4) up near the MIDI jacks. I can't even find it in the schematics, but once you get it in range (12 o'clock in my case), VR2 starts being helpful.

If VR2 is too far clockwise, you get more of the aliased "zipper" sound and too far counter-clockwise you get a more hissy/breathy sound. I can't find a specified voltage nor a test point for calibration, but I found it easy enough to calibrate by ear.

I had to take the CZ completely apart for other repairs. The screen had missing rows, but almost any HD44780 based LCD works as a replacement. Some buttons were unresponsive, but contact cleaner made them operable.
While I had it disassembled I took some photos of the trimmer locations. They're on the main board appropriately marked "M4152-MAIN".
To be clear, you only need to remove the bottom of the case to adjust things. Casio was nice enough to provide some holes so you can access the trimmers from the underside of the board.