Tuesday, 14 July 2020

DOD American Metal

Another quick one. This American Metal would power up, pass a clean signal but wouldn't pass an effected signal.

Schematics are older DOD pedals aren't too hard to come by (I believe some of them were supplied in the original boxes). I suspected a bad switching JFET was blocking signal, but they all measured fine on the oscilloscope.

I found that two of the transistors near the distortion control had very little voltage on the collectors. The schematic shows a series 1k resistor and a 47uF capacitor from the 9V supply, so there's a good chance that capacitor is shorted.

The 47uF is right in the middle of the PCB. Pulling out brought back the effected signal, so I replaced it and put things back together.

This is a very cool pedal, definitely in the HM-2 family. Maybe not as much gain and not as much EQ control, but it certainly does the buzzsaw sound when turned up.

Monday, 13 July 2020

Digitech PDS 8000

I got this one for completeness sake after looking at the other PDS series.

The insides are very similar to the PDS 2000, with 4 times the memory. The PDS 8000 schematic is online, but the quality of the scan is so poor that the 2000 schematic is probably more useful.

As arrived, no activity. The reverse polarity diode was shorted, after clipping it out I had 5V at the digital chips but no 9V at any of the analog stuff.

I knew that the 9V supply has a soft-start circuit (as in the 2000) - the JFET was also dead. With a repaired soft-start, the 9V rail was shorted to ground. There is only one 470 uF electrolytic capacitor from 9V to ground, replacing this fixed the pedal. It also got new footswitches as the old ones were prettyy unreliable.

Thursday, 28 March 2019

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.

Friday, 15 March 2019

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.

Tuesday, 12 March 2019

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.


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".

IC1 - R1154H036B - 3.6V voltage regulator. MCU (IC3) runs at 3.6V.

DT1 - DT4 - 2DTC114 digital transistors, marked "24" - used in other Korgs.

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.

IC3 - H8S/2210C - main microcontroller, covers keyboard scanning, USB & MIDI.

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

IC44 - 74LVC1G126DCKRG4 - Single Bus Buffer Gate, marked CN5, also used in Kronos.

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.

IC49 - TPS73801 1.0-A Low-Noise Fast-Transient-Response Low-Dropout Regulator - regulates the +14.5V rail.

IC50 - CD4011 - Quad 2 Input NAND gate

IC 53 - TPS7A340 1–20-V, –200-mA,Low-Noise Negative Voltage Regulator - regulates the -14.5V rail.

DT1 - DT4 - 2DTC114 digital transistors, marked "24" - used in other Korgs.

PC1 - Toshiba TLP2368 Optoisolator - isolates the MIDI input.

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

MicroKorg Service Manual

MicroKorg XL Service Manual

Korg Kronos Service Manual

Korg Volca Bass Service Manual:

Monotribe schematic:

Monotron delay schematic:

Thursday, 14 February 2019

Another Ibanez ES2 Echo Shifter repair

I took another look at an Ibanez Echo Shifter that I had previously repaired, but had come back to me. This time it wouldn't light up, but did pass a clean signal. I suspected a bad power supply for the digital section.

I opened it up, and I could see ~8.5V volts powering the opamps but no voltage at any of the digital parts, where I would expect 3.3V.

Way back in July 2017 a commenter asked if I knew what the part number for U14 was, likely to be a buck-converter, and that they suspected it was a Texas Instruments TPS62056. The package, function and chips marking all looked like a perfect match so I ordered a few this week to see if it was right.


Desoldering was straight-forward, I used hot air and kapton for protecting parts I didn't want to overheat.


The new chip has near identical markings to the old one. Re-soldering was a little harder as there isn't a lot of space, I ended up removing C73 temporarily to get better access. The bridged pins are fine, all those pins are connected together at the PCB.


Everything works again with the new chip. Thanks to Shane Bussiere for doing the research and sharing the part number, sorry I didn't help out.

All working again.

Looking at the TPS62056 datasheet, I can guess why this failed. The buck converter chip has a maximum Vin of 10V, the Echo Shifter runs the power jack through a series Schottky diode for polarity protection and then to the TPS. If you use a 9V power supply the chip gets around 8.6V, which is fine, but using a 12V power supply or higher will probably kill it. I couldn't find a compatible chip from TI with the same footprint and pinout but higher maximum input voltage, let me know if one exists.

I would really like to modify this to add a modulation rate control. I hunted around for modulation signals and unfortunately there doesn't seem to be a LFO onboard, it looks like the modulation is done in software in the ADAU1701 DSP. A modulated square wave is run out of the first audio DAC on pin 46.

While I had this open I desoldered the 24AA128 serial EEPROM and dumped the contents, it can be downloaded here. Afaik the ADAU1701 instruction set is not publicly documented, so I don't think the firmware can be easily modified.

Friday, 8 February 2019

2 70s Electro-Harmonic Small Stone phasers

2 vintage Electro-Harmonix Small stone phasers. The artwork for both of these is almost identical to the 90s reissue (which uses LM13700 OTAs) but the lack of any LED indicators gives away that these are older - probably late 70s or 80s.

Small Stone #1

I think this is the pedal I have owned the longest with repairing. I started buying broken stuff  to repair from eBay in 2011, and picked up a couple of vintage Big Muffs. One was a lot that came with this pedal, a Small Stone, pretty much as pictured - no knob, no pot, no switch and a lot of broken wiring.

The Small Stone has 4 phase shifting stages, using 1 OTA as an LFO and 4 more as variable RC filters that give varying phase shifts. Early versions of the Small Stone used CA3094 OTAs branded as "EH1048", a house-marking for Electro-Harmonix. This version is an "Issue J" and has the 5 EH1048 chips, dated to 1977.

Issue J

EH1048 - 1977, week 32.

I bought a new 24mm reverse log pot, installed a 3PDT switch and re-wired the pedal (true bypass, why not). It didn't pass an effected signal. I think I put it aside at this point, I suspected that the OTAs might be bad and didn't have any replacements.

I came back to this recently. The oscilloscope showed that there was no LFO signal anywhere on the board, even though most of the voltages on oscillator OTA looked reasonable. I bought an RCA metal can CA3094 and replaced it - now I had an LFO, but still no wet signal. Looking at the input and output pins of each OTA, I could see that the first stage was phase shifting, but the second had no output. I shorted together the input and outputs of the second stage, and now I had a phase-shifted signal. It wasn't quite as deep as it should be, with only 3 or 4 stages active, but it verified that the other two ICs were good. I ordered one more CA3094 to replace the dead EH1048, and complete things.

Before re-housing
I sourced a new hockey put knob that fit the new 24mm pot. Originals are hard to come by, but it fits the right aesthetic.

This was inspected by... Elsa? Cheers, Elsa.

There is usually some foam behind the PCB on the back panel of the housing. The PCBs just hang off the back of the rate pot, the foam is to prevent it from shorting out on the back panel. This foam had perished, so I taped some card down to insulate the PCB instead.

Small Stone #2

This second unit is a similar vintage, I picked it up hoping it might help repair the first one.

This is a slightly different PCB (with a phenolic substrate instead of fibreglass?), but looks to be the same circuit more-or-less. It also has 5 EH1048s, dated to 1979.

This one actually worked despite being sold as faulty (this is not that unusual). It just had a couple of quirks. It was a big help in verifying the switch wiring on the other unit, and for taking reference voltage readings off the OTA chips.

Phenolic PCB, instead of fibreglass?
The first was that the rate pot had some odd damage, the casing was partially open. I guess this could have been caused by dropping the pedal onto the knob, or by pulling on the PCB while it was still attached to the enclosure. Or during factory assembly, this was EHX after all. This was easy to close and re-crimp with pliers.

Opened pot housing...

...closed again

The second was that it would start to oscillate with the "color" switch in the up position and no input. It works fine with a guitar connected, or a buffered pedal in front of it, but with no cable or dangling unconnected cable it will start to ring at the top of the phaser sweep. As far as I can tell this is just something that this revision does, to fix it I would have to switch to a shorting input jack or modify the pedal to reduce the positive feedback when the color switch is in the up position, neither of which I really want to do.

These turned out really well. Feels good to have them done. They sound almost identical, the only change that jumps out is that the two different brands of pots don't quite match up - the plastic shaft CTS pot physically rotates further than the new Alpha pot, so the rates are slightly different when the pointers are matched by eye.
I don't have a modern Small Stone, or a Sovtek, to compare with. They definitely sound smoother than JFET phasers I'm used to.

One or both of these will probably hit Reverb in the next couple of days, get in touch if interested.