Korg PolySix Tuning – The Long Story

The tuning of a Korg PolySix is mostly a tedious process. Let’s go in-depth to make it even worse. The tuning process described here is normally not necessary, but if you have replaced some components on the Voice PCB this might be your last resort.

Korg PolySix (with KiwiSix CPU board, your LED states may be different)

Sources of information

  • The PolySix Service Manual. There is some important information in there, like the fact that the PolySix is not speced to be able to tune in the highest octave. But generally the tuning instructions in the service manual should be read, understood and avoided.
  • The KiwiSix manual, section 5) “PolySix Adjustments”, paragraph 9) “Voice Pitch Tuning”. It has helped me out many times. (You don’t need to have the KiwiSix upgrade installed.)
  • This page. It’s for when you can’t get it right with the KiwiSix procedure.
The voice PCB, KLM-366

Step-by-Step Instructions

Front Panel Preparations

  • Set Waveform to Saw
  • Tune Knob = Center
  • Modulation Level = 0
  • Bend Intensity = 0
  • Move Modulation wheel to 0
  • Move Pitch Bend wheel to the Center detent

Voice Board Preparations

  • Everything will happen on the voice PCB (KLM-366). Pull the slide switch between VR1 and VR2 towards the rear of the PolySix (away from connector CN12). This is the ‘normal’ tuning position. The other position is for stretch tuning.

These “global” trimmers will affect all voices

  • VR1 (Adj Center)
  • VR2 (Tune High)
  • VR3 (Tune Low)
  • VR14 (D#4-E4 Adj)
  • VR15 (Tune Mid)

Per-voice trimmers

  • VR10 (Tune Low) * 6
  • VR11 (Tune High) * 6

Note: In early production models
VR14 is not present 
VR15 (Tune Mid) is called VR1 and sits on the KLM-396 daughter board

Trimmer positions

Tuning Procedure, Global + Voice 0

First adjust global trimmers together with Voice 0. Save Voices 1-5 for later. Use the LEDs on the Voice PCB to identify which voice is currently playing.

a) Set Octave to 4′ (on the front panel)

b) Hit C5 (second top C) until the LED for Voice 0 is lit. Adjust VR11 to obtain C7 0 cents.
(If VR11 is far off from center: adjust VR2 (Global) Tune High a bit and try again)

c) Play C2 (second lowest C) and adjust VR1 to obtain 0 cents for Voice 0.

Repeat (b – c) until C2 and C5 are in tune for Voice 0

d) Play C3 and adjust VR15 to obtain 0 cents (It might not be possible right now. If not, do your best)

e) Play D#4 and E4 and adjust VR14 until both are spot on. (It might not be possible right now.)

Repeat (d – e) to get C3 and D#4-E4 a bit closer to correct pitch

Repeat (a – e) until C5, C2 and C3 are good. Then go to (f).
If you can’t match C2 and C3, i.e. if step (c) and (d) seem to counteract each other, please also go to (f).

f) Set Octave to 16‘ and play C2. Adjust VR3 (Tune Low) for 0 cents. This can help with steps (c) to (d). Try to leave VR10 close to center position for Voice 0.

Repeat (a – f) until Voice 0 is in tune

Voices 1 – 5

Generally, from here on, you want to avoid changing the “global trimmers” VR1, VR2, VR3, VR14 and VR15.

h) Set Octave to 4′
Play C5 and adjust VR11 for Voice 1 – 5

i) Set Octave to 16’
Play C2 and adjust VR10 for Voice 1 – 5

Repeat (h – i) until Voice 1 – 5 are in perfect tune

Done!

Technical Notes

If you are unable to tune a PolySix, there are some components that fail more often than others in the “auto-tune” and expo converter section.

  • 4051 multiplexers are always suspects
  • The opto-coupler (aka “Vactrol”) HTV P1501, cannot be sourced nowadays, but might be replaced with VTL5C9 or NSL32-SR3. Due to differencies between specs, but also between individual optocouplers, other alterations may be needed in the PolySix “auto-tune” circuit. If VR15 (Tune Mid) and VR1 (Adj Center) counteract each other, so that C3 stays too sharp, and you finally hit bottom while turning VR15 clockwise, then you may need to decrease the value of R216 a bit.
  • Sometimes IC17 – 19 need to be replaced. If you change IC17 and IC18 to TL072, instead of the less common JRC072, there’s a resistor change you need to perform, specified in the KLM-266 schematic, in the service manual.

 

Ursa Major Space Station SST-282, 82S123 replacement

How to make a replacement for the 82S123 ROM chip (ADS-1 board – U19) in Ursa Major Space Station SST-282, using an 22v10 Programmable Logic chip and a cheap EPROM programmer.

I used Atmel ATF22v10CQZ-20PU and the common TL866iiPlus.

This is not a step-by-step guide, you are expected to know how to use an EPROM programmer.

Main SST-282 main image
Two Space Stations in for service, one with a problematic 82S123

What is the N82S123N chip

The Space Station doesn’t have a processor, but you can still select different reverb-, filter- and delay algorithms. This is achieved with hardware buttons that make the 82S123 IC output various control signals to the rest of the machine.

Two M82S123N chips from the SST-282
82S123 from the two Space Stations above

The 82S123 is not an ordinary EPROM, it’s a bipolar TTL PROM. It is way faster than EPROM, with response times in the nanoseconds. You can get them NOS on eBay. The problem is that an EPROM programmer that supports this type of chip is expensive.

I haven’t verified this, but these chips should be direct replacements for the 82S123:
7112, 27S19, 6331-1N, 74S288, 18S030, 7603

The ATF22v10C Programmable Logic IC

Atmel F22v10CQZ-20PU and little brother F16v8BQL-15PU

The ATF22v10 is a Programmable Logic chip, made by Atmel. Other brands are sometimes called PAL chips. ATF / PALs are much more flexible than EPROMS, mainly because you can run small programs on them, so they behave differently depending on signals present on the input pins. They are also very fast, and that’s why they are fit to replace an 82S123.

So in short, I needed the 22v10 to output one of 32 numbers, depending on the state of its input pins – just like a small EPROM.

Research

Luckily I found someone that made a 82S123 replacement using the GAL22v10D, which is “practically the same” chip: REPROGRAMMABLE 82S123. Beware though that not all EPROM programmers and ATF / PAL / GAL chips are compatible.

So I read a bit of documentation on 22v10, as well as the programming language (CUPL Compiler for Universal Programmable Logic) and tool (WinCUPL)

Hardware adapter

22v10 is a 24-pin chip, whereas 82S123 is only 16-pin, so an adapter is needed. Please refer to this site for detailed information: REPROGRAMMABLE 82S123

My version looks like this

82S123 socket to 22v10 IC adapter top
82S123 socket to 22v10 IC adapter bottom

Programming the Space Station data to 22v10

You need an EPROM programmer that is compatible with the 22v10 brand that you have

Done

Ursa Major Space Station with the new ATF22v10C added

Addendum

“Why not use the smaller 16v8 PAL instead? It’s smaller but it has enough pins…”

Well, if you try to compile any code that contains a 32 byte ROM table and the necessary pin bindings in WinCUPL it’s going to fail. WinCUPL even outputs a nice Word document that shows exactly where the resources are exhausted.

The differences between Roland Juno-106 and MKS-7

The Roland MKS-7, a “6 note polyphonic / mono bass / TR707 drum sound” module

The Roland MKS-7 was allegedly intended for MIDI-file playback and karaoke, thus being a consumer market product. The voicing hardware is similar, but not identical, to the Juno-106.

Roland Juno-106 patch-making controls

Background

A Roland MKS-7 could be considered a more affordable, yet in some respects more capable, version of the Juno-106.

Juno-106 advantages

  • Sliders and buttons for every patch parameter
  • User patch storage
  • No restrictions on how to use the 6 voices – simple

MKS-7 advantages

  • Velocity control of filter and VCA
  • Lower second hand price than Juno-106
  • More voices – 6 poly voices, plus one dedicated bass voice, plus drum sounds
  • Multitimbrality, to some extent

Here are the parameter differences between the two. The tables are from the MKS-7 service manual, with Juno-106 specifics noted in orange. My notes follow after the image.

Differences, not mentioning Juno-106 keys, knobs and sliders…

So what’s specific for the MKS-7?

  • Dynamic select (aka velocity control) – Yeah baby! This is a big thing. The Juno-106 lack of velocity sensitivity is possibly its biggest weakness.
  • Sub OSC level in 4 steps instead of slider – Seriously, this wouldn’t make much of a difference.
  • High Pass Filter is ON/OFF instead of a slider – Personally I don’t think this damages patch creation much.
  • Chorus, only one mode – This is a problem. A lot of the dreamy character of Juno-106 stems from Chorus II.
  • Noise – Noise is only available on 2 of the 6 voices, and even so it’s only available at full volume or nothing. Some people like to add grit to a pad sound with a little noise.

The Bass voice

The bass voice differs from the ordinary voices in many respects. It has no modulation at all except envelope. It cannot have saw and pulse waveforms at the same time. No noise, no inverse envelope, no HPF etc. On the plus side, the Bass has an analog envelope. This is important because it’s exponential, so has more punch with short attack- and decay settings. All things considered, this bass is a lot more trimmed-down than any bass synth, like SH-101 or MC-202. Since it lacks velocity sensitivity I dare say it’s even more basic than the TB-303 sound engine which sports accent and slide controls. Still, it’s a rock solid bass, and a lot of people like it a lot.

Creating patches and saving them

The MKS-7 comes with 100 chord / melody presets and 20 bass presets. A software SysEx editor, or DAW plugin, can be used to edit the MKS-7 parameters. But everybody ideally wants hardware sliders and buttons. You could get a Stereoping Juno-106 programmer or a Behringer BCR for that. But doing so, you still can’t save your patches in the MKS-7. So you will have to save your SysEx patch in your sequencer, in a SysEx librarian, or within your programmer if it allows that (Behringer BCR does not).

Conclusion

Using the MKS-7 to get a Juno-106 is just too much hassle. The only things making me consider it is the price difference and that the MKS is velocity sensitive. I’ll stay with the JU-06.

Arp Sequencer 1621 / 1601 Button Debounce

Arp Sequencer 1621 / 1601. The changes between versions are only cosmetic.

While servicing an Arp Sequencer 1621, I noticed that the three button switches were in a bad shape. One button press would produce anything from one to six signal changes.

These switches can’t be sourced nowadays. The switches are glued together from factory so they can’t be opened.

One possible fix would be to fit some type of modern (long slew) replacement switch inside. You would probably need to design an individual adapter PCB to make it fit the footprint on the 1621 PCB, and also 3D-print an adapter to stick the white button caps back on. However, there’s no guarantee that any other switch will work flawlessly even if it’s new. All electromechanical switches bounce.

I tried removing and washing the switches with ultrasound, warm water and dishwashing detergent. That helped to some extent, but they still exhibited double-triggering and trig on release.

Top traces show how a button press generates bounces

Using the original switches

I decided to hardware debounce the switches, using RC filters. I had the following design goals, in addition to making it work:

  • Don’t change any components on the 1621 PCB
  • Make it visually clear to anyone what my addition does
  • Stick my PCB to the main PCB, not the bottom of the unit, to keep it simple to open and service

The lower traces in the images above show the RC filtered signal, with the third image showing an extreme case, which I’ve not been able to replicate again. The RC filter will introduce a delay from button press to state change, but it can probably be kept under 10 ms. It depends on how badly degenerated the switches are, their original construction and possibly in what manner they will be hit in actual use: A more firm hit will probably make a shorter period of contact noise (?)

Here some Internet programmer’s well designed RC debounce circuit. Button closing will make pin 1 of U1 see a smooth signal falling towards LOW. Opening the button makes pin 1 see a signal rising to HIGH. The fall and rise rates are defined by R2 * C1 and R1 * C1 respectively.

The diode, while not necessary, makes the charge rate (going towards HIGH) of the capacitor depend only on R1, not R1 + R2.

U1 is an inverter, so the resulting voltage state at pin 2 will be the opposite of pin 1.

https://mayaposch.wordpress.com/2018/06/26/designing-an-rc-debounce-circuit/

CD40106B inverter

The 40106 is a 20 volt tolerant inverter with Schmitt-trigger. Logic levels in the 1621 are +15 and 0 volt. The Schmitt-trigger handles noise and slowly rising or falling voltages well and is a good companion to the RC filter for debouncing.

Schmitt-trigger (noisy) input and output

There is no technical advantage of inverting the output signal, but the inverter isolates the switch and RC network from from the rest of the circuitry. This can be important when fitting a debounce circuit to an existing system, otherwise you’re likely to unintentionally create voltage dividers that will affect the performance of the debounce circuit, and possibly offset the resulting signal.

Upper trace: RC filtered signal. Lower trace: Filtered signal after inverter + Schmitt-trigger

Special considerations and adjustments are needed when fitting something like debounce to an existing system. In the 1621 the signals from the switches are later combined with other signals from the input jacks. The original switch construction doesn’t have a HIGH and a LOW state, it has LOW state and UNCONNECTED (in case of the “Start / Stop” and “Reset” switches), and HIGH state and UNCONNECTED (for the Step switch).

Start/Stop Example

When “Start/Stop” switch is UNCONNECTED there’s a weak pull-up that keeps the signal line in HIGH state. This pull-up is weak enough for another connected signal (via External input jack) to pull the line down to LOW if needed.

1601 schematic, the “Start/Stop” input signal and switch

The RC scheme however, can’t have an UNCONNECTED state, only HIGH, LOW or intermediate values. When the debounced switch is off it could therefore affect the “Start/Stop” signal line, and hinder other signals from forcing it LOW. To remedy that, I added D1 and D2 in the following image.

Implementation

The schematic only shows two circuits. That’s because “Reset” and “Start/Stop” use the same top design (except for one resistor value). “Step” uses the bottom design.

When I tested everything out, I realized that I had created an unwanted behavior in the 1621. Every time I turned the power on, it jumped a number of steps into the sequence, as if the “Step” button had been pressed a few times after startup. That’s natural, since in practice I had delayed the switches by a couple of ms, not only when pressed, but also on startup. I changed R3 to 22k so it would appear to the 1621 as if “Reset” button was pressed a few ms after the other buttons, taking the sequencer back to step 1.

Breadboard test
Transferring
All paths to switches are cut on the PCB, wires connect them to the debounce board
There’s a piece of acrylic between the PCBs to keep the distance
Keeping related wires together

Conclusion

This Arp Sequencer 1621 works as it was intended again. The reaction to button-presses is fast enough, and feels solid and reliable.

I could have done this with a lot fewer wires if I had been more creative with where I cut the PCB paths, in combination with inverting signals once more to get back to uninverted state. But then it would have been much harder to follow the signal flow for the next service technician.

I taped a printout of the schematic, and added a link to this site on the inside of the unit.

Korg Lambda ES-50 Tuning Instructions

Disclaimer: There may be a better way to tune the Korg Lambda. My method is a bit “hit and miss”. Still it took less than five minutes to accomplish.

Korg Lambda ES-50 string machine

Background

The Lambda is not a VCO->Filter->VCA synthesizer, it’s constructed lika an organ / string machine. The tuning procedure is different and often simpler on an organ than on a synthesizer. Maybe that’s why, when searching the web for tuning info, I came across statements like these:

Actually, I don’t think the Lambda stringer has got any trimmers to do the tuning, except the little pot on the chorus circuit board, that tunes the LFO rate range of the effect.

Well, it’s very rare that such string machines would require tuning becuase they use different technology than synthesizers.

The knobs positions on the front

Tuning the Korg Lambda

Turn all instrument buttons off, except BRASS. Set Chorus Phase Off. Set all three TUNE knobs: Total Tune, Tune A and Tune B to center position. Vibrato Off. Get a very small screwdriver.

It’s likely that the Lambda sounds very dissonant in this state. If it does, adjust VR Tune A and VR Tune B until both LEDs are steadily lit. Now it doesn’t sound dissonant anymore, but it’s probably still out of tune.

The trimmers (VR). Their positions match the positions of the knobs on the front.

Now the guessing starts

  1. Adjust VR Total Tune right for sharper tuning, left for flatter. You will notice that the sound also gets dissonant, so you can’t really hear if you hav hit the right pitch.
  2. Adjust VR Tune A and VR Tune B until both LEDs are steadily lit again. Now you can hear the actual pitch of the instrument.
  3. It’s likely that you will realize you turned VR Total Tune too far, or not far enough. Repeat steps 1-2.

Soon you get familiar with how much you need to adjust VR Total Tune in order to raise or lower the pitch by a few cents.

That’s it

Korg Poly-61 – Changing the NiCd to a Litium Battery

The Korg Poly-61, DCO analog synthesizer

The Korg Poly-61 has the same type of battery as the Korg Polysix, the type that creates the infamous battery-leak disasters. Here’s how to modify the Poly-61 for a standard litium coin-cell battery. The coin-cell is seated in a battery holder, so can easily be changed by the owner, without soldering.

Background

The original batteries in Korg Poly-61 (and Polysix) are charged at any time the synth is on. The coin-cell litium battery is not rechargeable. If you would just insert a litium battery in the circuit, the battery would break and possibly catch fire or explode. So you need to stop the current flow from the PSU to the battery.

Also don’t replace the NiCd battery with a new rechargeable battery such as Lithium-Ion or NiMh. The Poly-61 charging method is not suited for those, and they will break.

Old Crow (RIP) has info on battery modification and battery leak repair for the Polysix. I Adapted it for Poly-61.

Changes

Remove this

  • Old battery
  • Resistor R70 (replace with diode)
  • Capacitor C29

Add this

  • Diode, eg 1N4148 (Please watch the direction, see schematic)
  • Coin cell battery holder
The changes
PCB positions of R70 and C29

PPG 1020 Tuning Instructions

Disclaimer: I have no idea. Maybe this can be done better. But here’s my best shot.

Background

There is not much technical information around about this very rare synth. In order to figure out how to tune it, I had to remove the front plate which is a lot of work. This article is an attempt to save you from that.

The specimen I got to the workshop was close to a prototype. I don’t if the numerous modifications and extra installments were factory-made, or done later. The PSU is part of the main PCB, except for the transformer, so it’s not immediately apparent what is what. There is trimmer for +12 volt, and it was easy to get it very exact. There’s no trimmer for -12 however, and it was around -10.5 volts, don’t know if that’s within specs, but it seems a bit far off.

Thanks to Stefan Huebner I knew that the oscillator control voltage is part digital and part analog: The octave is selected digitally, and the voltages within an octave are analog. This means that if you notice that F1 key, for example, is off by 20 cents, then every F is going to be off by 20 cents. But some other note, say all D keys, could be in tune.

The problem

The two oscillators had the same pitch on some notes, but where off by almost a quarter-note on others. The behavior repeated for each octave. Which exact notes where in tune could be adjusted with the OSC B PITCH knob.

Some tuning info

On an analog synthesizer, there’s traditionally one Offset trimmer (VR), and one Scale VR for the Lin-to-Exponential Converter for an oscillator. The circuit assumes that the input is a fairly exact linear 1 v / octave voltage. I found Scale VR and Offset VR for OSC A, and a Scale VR for OSC B.

Part of the PCB

It was not possible to adjust the OSC A Offset enough to get it in tune, probably due to one of these reasons:

  • The negative supply was only -10.5 v
  • Some modification to the PCB has broken the voltage supplied to the lin-exp converter
  • I misunderstood how the TUNE knob is supposed to work. Maybe it’s more like the Sequential Pro-One, where the oscillator is in tune when the knob is at zero. I assumed that the “neutral” setting would be in the middle of the knob range, which is 5. Otherwise it would not be possible for the user to tune the synth if it is too sharp.

Tuning the PPG 1020

Oscillator A

OSCILLATOR CONTROLLER / TUNE = 5, turn down OSC B, all modulation off, OUTPUT I LEVEL = 0, OUTPUT II LEVEL = 10

  1. Press any G key, for example G2
  2. Use OSC A Offset VR to get correct pitch for G2. If not possible, adjust the OSCILLATOR CONTROLLER / TUNE knob
  3. Press G#2
  4. Use OSC A Scale VR to get correct pitch for G#2
  5. Repeat 1-4 until both G2 and G#2 are in tune

This is the way to get the best pitch overall. On my synth about half of the keys within the octave were in perfect pitch, while others were flat or sharp by up to 10 cents.

Oscillator B

Turn up OSB B in the mixer. You’ll hear some beating between OSC A and OSC B when playing notes.

  1. Press G2
  2. Use OSCILLATOR B; PITCH knob to get zero beating
  3. Press G#2
  4. Use OSC B Scale VR to get zero beating
  5. Repeat 1-4 until there’s no beating

You will notice that even if one key, say D#, is off by +8 cents, it’s exactly the same for both oscillators, so at least there’s no beating.

That’s it! Thanks for listening

Roland TR-505 – A Better Mod for Individual Outputs

Descriptions of Individual Audio Outputs modifications for the Roland TR-505 have been around for a while. I searched the web for sources that describe how to do the mod:

There are many sources talking about having it done, without explaining how:

The problem

However, the modifications that are described share one problem: They don’t handle Cymbals and Open HiHat very well. That is because the signal is tapped too early in the output summing circuitry, before the volume envelope is applied. Citing circuit benders’ description of their mod:

“Longer sounds such as the crash and hats consist of a compressed sample, with a primitive envelope applied to it so it sounds vaguely realistic. As the outputs have to be sourced in the circuit before the envelope is applied, some sounds will not be identical to the main mix outputs.”

Image from masutopedals.wordpress.com/

How to do it better

To sort it out, you need to tap audio from (LT MT HT TB) (CC RC) and (CH OH) after transistors Q6, Q7 and Q8 respectively, because the transistors apply the volume envelope to the audio signals. However the signal can’t be taken directly from the emitter of the transistor, because it’s is very weak at this point, and you will introduce noise, hum and instability. Instead, we take it after the buffers / amplifiers IC16 and IC15 respectively.

Red shows the standard 505 mods, green shows my modification spots

You will use the standard mod spots for (HCP RIM) (LCB HCB) (SD) (LCG HCG) and (BD), and the green ones for (LT MT HT TB) (CC RC) and (CH OH). I used breaking 3.5 mm jacks, so the sound is removed from the R / L mix when a cable is inserted in the individual output jack.

What you lose

You are going to lose the predefined, fixed panning that some of the sounds in the TR-505 have. But you will have all sounds on individual mixer channels to pan, equalize and add effects.

Also, the volume levels of the individual outputs may not be exactly the same as on the R / L outs.

Roland CR-8000 BD / SD Mod

This drum machine from 1981 is a close relative of Roland CR-78 and TR-808. It has a couple shortcomings, such as no user editable sounds, a Snare Drum with a lot of tone (what is called “Snappy” on 808) but a lack of noise / high frequency content. It sounds more like a tom. Also the Hand Clap is low volume, at least on this specimen.

This CR-8000 was modified for individual audio outputs for each instrument group, a louder Hand Clap, as well as couple of potentiometers for Bass Drum and Snare Drum tuning. The BD now has two controls: Pitch and Decay, the SD has two controls: Pitch of the “snappy” portion, and Noise level.



The BD and Clap modifications are taken directly from gumielectronic. The exact changes with component values are listed further down in this article.

When servicing or modifing synths I try to conform to Hippocrates’ oath for medical doctors: “First, do no harm”. So I made a break-out box, instead of drilling holes in the CR-8000 exterior. Also I left the original parts in place on the PCB where possible, should someone want to reverse this mod in the future.

The break-out box sticks to the metal back plate of  the CR-8000 with neodyme magnets. The wires are routed through existing vent holes in the back.

The following list is an excerpt from gumielectronics’ extensive CR-8000 modifications. The red markings are the ones I chose to use (since there was limited space on the break-out box) along with my component selections. The Snare Drum are my own changes, as some of gumielectronics’ suggested changes didn’t make that much difference. C47 is Snappy Pitch, the changes after Q7 is the SD Noise portion boost.

Below are Snare Drum specifics. The exact original SD sound isn’t available anymore, it now has a bit more noise content.

Here are a couple of images of the PCB and break-out box internals:

Bass Drum (green wire and some resistors) and Snare Drum Snappy (yellow wire bridging R34):

Snare Drum Noise Boost (red wires) and Snappy pitch (blue wires):

Individual outputs:

Tidying up a bit:


Caveats

Individual Outputs: The CR-8000 was not designed to have separate outs to begin with. Some things are not possible to do the way you’d want because of this. The Clap for example, is lower at the individual output than it is in the main mix. (Just mix it up a bit with the knob, or on your mixer). Also the accent effects are not applied to the individual outs, so in some cases the Main out instruments will sound beefier than the indy versions. This is often not a problem, since you can either do a lot to beef the sound up on an individual mixer strip, or you can leave that particular instrument on the main mix out and direct everything else to individual outputs.

BD Decay: The Bass Drum will not have the oomph of the TR-808, maybe because of a shorter trig pulse or some other volume envelope characteristic that might be different.

SD Decay: Decay of the Snare Drum is hard to modify, unlike the Bass Drum. The mod point called “decay” in gumi’s description doesn’t change the overall decay of the instrument, only the length of the Snappy portion within the (very short) total length of the Snare Drum, and this change is hard to hear to be honest. I think it would need a longer trig pulse to make the Snare longer, but I haven’t tested this idea.

Conclusion

I believe the changes described here are a significant improvement to the CR-8000.

 

Doepfer MCV4 Mod for Roland SH5


It seems Doepfer cut some corners when designing the MCV4 MIDI to CV interface. Firstly the MCV4’s gate voltage is to low to drive the Roland SH5 gate input. This can be remedied by opening up the interface to move a jumper, and then replacing the 9v power supply with a 12 volt one. After that the MCV4 controlled the SH5 gate and Pitch CV as expected. But it had no effect on the filter at all. That is because the output from the DAC isn’t buffered, and the DAC itself cannot drive the load of the Filter CV input. An MCP6002 OpAmp was connected as a basic voltage follower to buffer the Note Velocity output.

Beware that the image shows an unsuccessful experiment with a TL072, that has a different pin configuration.

Don’t use this image as a guide for connecting the MCP6002!

Kawai R-100 MIDI-Controlled Pitch and Circuit Bend Mod

A mid-80’s drum machine, modified for MIDI-controlled sound chip selection, circuit bending and pitch. Any MIDI sequencer or keyboard can be used. No changes are made to the exterior of the machine.

The Kawai R-100 drum machine classic was modified using an Arduino Nano, the circuitbenders.co.uk “R-ROM Switcher” and a HC-SR08 board. The Nano reads incoming note data on MIDI channel 2 to control the HC-SR08 and ROM Switcher board. Incoming MIDI data can

  • Control the playback pitch of the entire machine
  • Select sound ROM chip
  • Control a couple of the circuitbend connection points

The pitch mod: The HC-SR08 is a development board for the Analog Devices AD9850 Direct Digital Synthesis chip. The output DDS frequency is controlled by The Nano, reading MIDI notes and pitch bend. The output square wave is used as a clock signal for the R-100’s Address Generation Unit. This means that when the Kawai R-100 tries to play any sound, the rate at which the sound sample data bytes are found in memory is set by the 9850 output.

Actually this can be done without the HC-SR08, only using a pwm output on the Nano. This is how I originally tested the idea. But while this would work fine for very low pitches, the closer you’d get to the R-100’s original pitch, the less pitch resolution you would get. Since I wanted to be able to play it in a resolution of semi-tones (or even cents, using pitch bend) I had to use a higher resolution clock generator :)

The circuit bend: There are pre-made circuit bend points in the ROM Switcher pcb. It’s sufficient to have the Nano connecting these points to ground, they don’t need to be interconnected. To get back to non-circuit bent sound, the Nano pins are left in a floating state. The sound quality of the bends are much like a comb filter or phaser.

The ROM chip selection: To enable one ROM chip, the original ROM-Switcher connects the Output Enable (or possibly Chip Enable) pin of that chip to ground. Others are in HIGH state. A very simple task for a microcontroller.