Electro-Harmonix Micro Pog

I think this is the last entry in the EHX POG series that I haven't repaired. As usual, this came from eBay and doesn't work. No signs of life.

On first inspection, it looks like the diode in the switching power supply is trying to escape the PCB. I don't think that overheating could cause this without scorching the board, someone probably attempted to desolder this. My multimeter confirmed that this diode was a short circuit, so that's probably the fault. The diode is an SS14, which is also the same part used for input polarity protection. The switching IC is a CS51413 buck regulator.

I removed the diode and still measured a short across it's pads, so the switching chip is probably bad. That came off as well.

 

I ordered a replacement CS51413 but actually received a CS51414, which was annoying. However, the CS51313 and CS51414 are very similar, the '313 has an external sync pin where the '514 has an external bias pin. The Micro POG doesn't actually route pins 4 & 5 anywhere, so either chip can be used. I suspect that both are the actually the same die, with different pads broken out to external pins.

After replacing the controller IC and the switching diode, I still had a short from output voltage to ground. The output capacitor (C3) is also connected across these nodes, so I removed it and then the pedal worked. C3 is filtering the output of the 3.3V switching supply and doesn't seem to be always necessary. I don't know the value of C3, but the datasheet recommends 100uF. A 100uF ceramic capacitor in this package is actually quite expensive, so I used a 47uF.

The rest of the pedal is very similar to other Electro-Harmonix XO series units. There's a PIC18F2431 microcontroller, a DSP56364AF100 DSP and a PCM3052A audio codec.

When I had this working the "Octave Up" pot felt a bit weird. The shaft had actually broken away form the pot and had been re-inserted. I replaced it with a new 5Kohm part from Smallbear which is an almost perfect match.

 

That's it. Maybe someday I'll look at the HOG series as well.

Yamaha QY20

After fixing the MS-20 I was having a lot of fun with it, but really wanted some kind of sequencer. I would have preferred not to have to rely on a laptop and was looking for something small and cheap.

I found this Yamaha QY20 on sale from a Japanese eBay seller, listing as non-functional. It was a portable sequencing device from the early 90s, with some workstation-style sounds and MIDI I/O. The service manual was available online, it was very cheap and it looked like a decent form-factor so I took a chance.

Yamaha QY20, with carry case

I applied 12V from a bench power supply to the DC jack - it pulled around 135 mA but didn't show any sign of life. However, when I connected the headphone jack to a speaker and pushed some keys I could hear a piano. I could also connect up the MS-20 by MIDI and trigger notes using the QY20 keys. It looked like the there was just an issue with the display.

Usually LCD displays have a bias voltage supply that is much more positive or negative than the supply for the logic chips. I expected a bad switching power supply. The service manual is very comprehensive and has a complete schematic, it shows a MAX680 inverting charge pump generating a negative supply of -9.3 to -9.6V from the 5V rail. Either the charge pump has failed, or the contrast pot has gone open circuit.

LCD bias supply schematic, from service manual.

It's fairly easy to take apart the QY20, but there is a copper sheet for shielding that needs to be desoldered.

Shielding has to be desoldered at the MIDI and line output connectors.

Digital and audio PCB

The insides are fairly typical surface mount construction for early 90s, particularly for a portable unit.

Funnily enough, the microcontroler is a H8/520, the same family used in the MS-20 20+ years later. The main soundchip is a YMW-258-F "AWM & FM Tone Generator", which is likely to be some variant on the chips Yamaha were doing for synths and video games at the time.

I measured the Vee bias voltage for the LCD at only -5 V. It should be closer to -10V, as the MAX680 charge pump is an inverting and voltage doubling converter. I would guess that one switching stage inside the MAX680 is bad and it is only inverting.

I removed the MAX680 (looks like I forgot to take a picture of this, it's a SOIC 8 chip on the backside of the board) and the 4 22 uF switching capacitors (C26,C27,C28,C29).

C26-C29 removed

After installing new parts, Vee was now -9.2V - a little bit low, but certainly a change. When I re-attached the LCD board, the display worked again.

Reflective LCDs are surprisingly difficult to photograph,

I can change the cheesy electric piano to 100+ other throwback workstation sounds! Time to learn how to use this as a MIDI sequencer.

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