Korg MS20 Mini

Korg MS-20 Mini Repair:

I have wanted one of these since they were first released, but ended up waiting until a broken one came along. I probably overpaid a little for this, but the fixes weren't too bad.

This came from eBay, the seller had bought it as faulty with a non-functioning headphone output. When they received it they found it wouldn't power up, so they sold it on again. I am expecting some kind of power fault and something else wrong with the headphone output.

Despite the popularity of this synth and the number of mods documented online, there isn't a lot of information on some of the parts used, so hopefully this will be useful to others.

There are 3 main PCBs, all mounted to the sheet metal case and connected together with cables.

1. Digital board - power entry and power supplies, the microcontroller, MIDI and USB ports.
2. Analog board - all synthesizer circuits, knobs, switches and jacks.
3. Keyboard PCB - breaks out the keys to a connector. I didn't look at this.

Digital Board (KLM-3163C):

The digital board is the largest difference between the Mini and the original MS-20, and unfortunately there is no available manual or schematic.

KLM-3163C

There is a switching power supply on the left hand side of the image, the large transformer/dual inductor is a give-away. The microcontroller (IC3) is right next to the keyboard connector and all the pins are routed right to it, so it is doing keyboard-scanning as well as USB and MIDI.

I applied power and found that there was no voltage at the switch mode power supply inputs, and therefore nothing powering the analog board.

There is a component marked "F2" that is connected to both the SMPS input and the incoming 9V. Despite the silkscreen, I was pretty sure this was a P-channel FET and not a fuse. IC50 is a CD4011 Quad NAND which is powered by 9V - I am fairly certain this uses some surrounding resistors, capacitors and transistors as a timing circuit to turn on the P-FET gate a short amount of time after power is applied. This works as a "soft-start" and limits the inrush current.

Temporary fix to get things working

To test this, I just shorted across the FET and connected power to the switching power supply chip. This worked, I now had +14.5V and -14.5V rails, and when I connected everything back together the synth worked when using the main output jack, but not the headphone jack.

I later confirmed the part number of F2 and replaced it.

Here are part numbers for all the ICs and transistors, from my notes:

F2 (?) - RRL025P03 - Pch -30V -2.5A Power MOSFET - used for power supply polarity protection. Amusing numerated "F2".
http://www.farnell.com/datasheets/2706699.pdf

IC1 - R1154H036B - 3.6V voltage regulator. MCU (IC3) runs at 3.6V.
https://www.mouser.com/datasheet/2/792/r1154-ea-923729.pdf

DT1 - DT4 - 2DTC114 digital transistors, marked "24" - used in other Korgs.
https://www.rohm.com/datasheet/DTC114ECA/dtc114ecat116-e
http://pdf1.alldatasheet.com/datasheet-pdf/view/201787/RICOH/R1154H036B.html

IC2 - BU4227 - marked YU, used in other Korgs. This is an under-voltage detector, it resets the MCU if the supply voltage drops below 2.7V.
https://www.rohm.com.tw/datasheet/BU4325G/bu42xxg-e
http://rohmfs.rohm.com/en/products/databook/datasheet/ic/power/voltage_detector/bu42xxg-e.pdf

IC3 - H8S/2210C - main microcontroller, covers keyboard scanning, USB & MIDI.
http://pdf.datasheetz.com/data/Integrated%20Circuits%20(ICs)/Microcontrollers/HD6473042F16-datasheetz.pdf

IC9 - JRC 4558 - opamp, not sure what this is doing.

IC44 - 74LVC1G126DCKRG4 - Single Bus Buffer Gate, marked CN5, also used in Kronos.
http://www.ti.com/lit/ds/symlink/sn74lvc1g126.pdf

IC47 - TPS54240 3.5-V to 42-V Step-Down DC - DC Converter With Eco-Mode™ - uses an onboard transformer to generator the analog voltage rails, which are regulated down further by IC49 and IC53, probably to clean up the switching noise.
http://www.ti.com/lit/ds/symlink/tps54240.pdf

IC49 - TPS73801 1.0-A Low-Noise Fast-Transient-Response Low-Dropout Regulator - regulates the +14.5V rail.
http://www.ti.com/lit/ds/symlink/tps73801.pdf

IC50 - CD4011 - Quad 2 Input NAND gate
https://www.ti.com/lit/ds/symlink/cd4011b.pdf

IC 53 - TPS7A340 1–20-V, –200-mA,Low-Noise Negative Voltage Regulator - regulates the -14.5V rail.
http://www.ti.com/lit/ds/symlink/tps7a3401.pdf

DT1 - DT4 - 2DTC114 digital transistors, marked "24" - used in other Korgs.
https://www.rohm.com/datasheet/DTC114ECA/dtc114ecat116-e

PC1 - Toshiba TLP2368 Optoisolator - isolates the MIDI input.
https://www.kynix.com/Detail/697092/TLP285GB.html

Analog Board (KLM-3162C):

I also wanted to fix the headphone output, if possible.

The analog guts are all on one large PCB that has all the pots and jacks mounted. The jacks and pots are not actually panel mounted with nuts, they just poke through the panel. This does not give the greatest tactile response - everything wobbles a little bit - but it probably explains how Korg managed to keep the price so low. There is at least a large sheet metal cover to stiffen the PCB.
 

KLM-3162C with shield

KLM-3162C uncovered

The headphone output is in the upper right. The original MS-20 has a very simple headphone amplifier, just an opamp driving each side. The Mini is pretty different, and seems to add transistors to buffer the opamp outputs.

Headphone output circuit. C251 was removed for testing and later repopulated.

I held down some keys and probed around for a signal. The headphone output seemed to disappear at one side of a 10 ohm resistor (R314 & R315) for both left and right outputs. I desoldered these and they confirmed they were open circuits. I replaced them with new 0603 10 ohm parts and the headphone worked. It's possible these were killed by someone patching the headphone jack into somewhere strange and pulling too much current (?)

Offending 10 ohm resistors

Korg Service Manuals:

There are a lot of Korg schematics and service manuals online. Many are publically accessible on the Korgusa.com portal, even though the page requires a dealer or repair center account. I used some of the following as reference material:

MS20 Original service Manual
http://www.synfo.nl/servicemanuals/Korg/MS-20_SERVICE_MANUAL.pdf

MicroKorg Service Manual
https://elektrotanya.com/korg_microkorg_x-1110.pdf/download.html

MicroKorg XL Service Manual
http://dealers.korgusa.com/svcfiles/MKXL_SManual.pdf

Korg Kronos Service Manual
http://www.markpenny.ie/wp-content/uploads/2017/03/273600278-Korg-KRONOS-Service-Manual-v2-0-2012.pdf

Korg Volca Bass Service Manual:
http://www.ksadhu.niezba.org/sajty/korg_volca_bass_sm.pdf

Monotribe schematic:
https://www.korg.com/download/global/monotribe_schematic/monotribe_sch.pdf

Monotron delay schematic:
https://www.korg.com/download/global/monotron_delay_schematic/monotron_DELAY_sch.pdf