Korg Mono/Poly Analog User Modifications

Do It Yourself Special:

FREE plans to add MIDI capability

to the Korg Mono/Poly with the

Synhouse Original MIDIJACK!

The following plans describe a method of adding MIDI capability to the Korg Mono/Poly analog synthesizer with the Synhouse Original MIDIJACK. This will allow the Korg Mono/Poly to be used as a MIDI analog monosynth. Some drilling is required. There is plenty of space on the Mono/Poly to mount the MIDIJACK with the provided hardware. The micro size and black color of the MIDI button are such a perfect match for the Mono/Poly that it makes it look as if it came from the factory with the MIDI interface. The installation of the MIDIJACK in an Mono/Poly is one of the easiest modifications of all. In fact, with the MIDIJACK, it is possible to convert the Mono/Poly to MIDI without even removing a single circuit board from the instrument! The correct analog I/O signals are easy to find and connect to the MIDIJACK. Like many older American synthesizers, the Korg Mono/Poly is of the volt/octave type because it uses SSM (Solid State Music) synthesizer chips. Another similarity is that it has panel-mounted CV/gate jacks with solder lug terminals and the MIDIJACK wires can simply be soldered right inside the panel and the whole MIDI conversion job can be done in one hour. The 1/4" jacks do not even need to be unscrewed from the back panel. They may be left in place and soldered right on the spot. This modification reroutes the local keyboard signals through the computer-controlled analog switching matrix of the MIDIJACK by extracting the signal and inserting the users' choice of local keyboard control or MIDI. You can do this yourself if you have a little experience with electronic repair and the soldering of wires and circuit boards. If not, it is recommended that you send the instrument to Synhouse L.A. for a quick, low-cost Factory Installation. This document is based on Korg Mono/Poly serial #371 450, other revisions may be different. It is best to download these notes and photos and print them out on paper to look at while working on the instrument and make notes and check off the steps as you go. As with any project, you should completely read and understand each step of the instructions before starting. All repairs and modifications made to your instruments will be done at your own risk and Synhouse Multimedia Corporation assumes no liability for personal injury caused or damage to equipment or loss of use caused directly or indirectly by the use of these plans. If in doubt, don't do it!

Instructions:

1) Be sure to have the correct tools and supplies for for the job. If you do not have them, get them. You will need a regular size Phillips screwdriver, a smaller size Phillips screwdriver, needlenose pliers, wire cutters or other flush cut nippers, a hobby knife such as an X-Acto, scissors, a soldering iron, solder, electrical insulating tape, and a black Sharpie permanent ink marking pen. If you intend to mount the DIN jack on the back panel with the rest of the jacks (highly recommended), you will need to use a chassis punch (a small hand tool that safely cuts a clean hole in a metal panel) to make the hole for the DIN jack, and an electric drill with a 1/8" or similar size drill bit to drill holes for the 4-40 hardware used to mount the DIN jack, and also a 1/4" or 5/16" drill bit to make a pilot hole to start the chassis punch. The correct size for mounting a MIDI DIN jack is 14.5 mm metric or 5/8" SAE (.62"/15.9 mm) in American sizes. A chassis punch may be purchased from any good tool or hardware store. If it is more convenient, a punch may be mail ordered via internet or telephone from Mouser Electronics at http://www.mouser.com or (800) 346-6873. The Mouser part number is 586-3803 for the name-brand Greenlee 730-5/8 (about $30). The cheaper house brand part number is 380-0145 (less than $20). The service from Mouser is unpredictable and the house brand ordered by Synhouse for the test installation took three months to be delivered, while the Greenlee part was delivered in one week. Mouser refused to give even a small discount to customers of Synhouse, so no recommendation is deserved or being made here, and any other source you know of to buy this type of tool is highly recommended and certainly a better place to buy from for all of your needs now and in the future. You will also need an 11 mm wrench (for Greenlee) or 1/2" wrench (for the Mouser house brand punch) or adjustable wrench to turn the chassis punch while cutting the hole. An automatic center punch would also be useful. This is an inexpensive spring-loaded pointed punch that can mark your drilling spot without the use of a hammer. Marking the holes with this small indentation will allow you to drill cleanly without slipping and scratching the synthesizer or drilling through your knee.

2) Fully test the Korg Mono/Poly to be converted to MIDI. Be sure that all functions such as the envelope generators work and that the instrument plays in tune while playing along with a known well-tuned instrument such as a newer digital synthesizer or sampler keyboard. A Mono/Poly often requires 30 minutes or more to warm up and play in tune. If it doesn't work properly without MIDI, it certainly won't work with it.

3) Extreme caution should be taken while working on the Korg Mono/Poly. The unit should be unplugged while open and even then, the power supply may pose some electric shock hazard due to residual voltage in the power supply.

4) Remove the seven Phillips screws that hold the top panel in place. Two are on the front right side, two are on the front left side, and three are under the back edge of the synth, going through the crumbly old particle board. Lift the top panel and set it back on its' rear, being careful not to stress the wires that are still connecting the two.

5) Turn the synthesizer to the side or upside down and shake out any dust and debris that may have accumulated inside the instrument over the years.

6) Determine the place where the MIDIJACK circuit board will be mounted and test fit the board into its' correct place inside the case. Be careful to avoid putting the board or wires near the high voltage connections in the right rear of the instrument. Mark the correct mounting holes on the panel with a pencil, marker, or needle using the paper drilling template provided with the MIDIJACK hardware packet. In the example photographed here by Synhouse, the Original MIDIJACK circuit board is mounted inside the plastic box which house the modulation and pitch wheels. A photo called MonoPoly-pic1 shows the paper drilling template in place:

The perfect size drill bit for the switch stem and two screw holes is 9/64", and the perfect size for the scale adjust trimpot is 3/16". Drill the holes. A photo called MonoPoly-pic2 shows the location of the newly cut mounting holes:

7) Mount the MIDIJACK board in place. When mounting the MIDIJACK board, the switch should be fitted so well in the panel that the switch stem will not wiggle at all once in place. It should not have any free play but also should not be so tight that it binds. When the switch is pressed, it should have a definitive "click" and bounce back like the button on a new VCR. You will never regret spending too much time on this and good attention to detail will make the perfect MIDIJACK installation. The hole in the panel that is over the MIDIJACK scale adjust trimpot should be large enough so a Synhouse Pocket Screwdriver can fit through the panel for periodic adjustment. The perfect MIDI control panel installation is shown in MonoPoly-pic3:

Such an installation will be nearly invisible, yet put the MIDI function button at the players' fingertips. For the serious Analog User and synthesizer collector, an ultra-clean installation pays off.

8) Determine the place that the MIDI input DIN jack will be mounted. A good place to mount it is on the back panel to the lower right of the OUTPUT jack, as photographed in this example. There is a lot of room on the rear panel of the Mono/Poly, so you may mount the DIN jack wherever it suits you best. Be sure not to mount it too close to the crumbly particle board sides of the instrument, which have rear supports which would obstruct the rear of the DIN jack. The Korg factory finish is a hardened coating that may well chip and separate after cutting the hole for the DIN jack, so it may be best to mount the DIN jack from the outside of the panel instead of inside the panel as in a normal MIDIJACK installation. This will conceal the somewhat jagged edge of the finish. To do this, mark one of the two wires (#9 orange or #10 gray) at the terminals of the DIN jack with a marking pen or tape to remember the polarity then desolder both wires. Use the paper template to mark the correct spots to drill and cut as shown in MonoPoly-pic4:

It is advisable to use a chassis punch to make the hole for the DIN jack. Remember that the DIN jack is to be mounted with the smaller 4-40 hardware size rather than the larger 6-32 size that secures the main board. Drill two holes for the screws then drill a slightly larger hole in the center to act as a pilot hole for the chassis punch. Use the chassis punch to cut the hole and be sure that the wrench is turning the tool from inside the Mono/Poly, not outside, so the cutting edge is coming from the outside. This will ensure that the outer edge is perfectly smooth. The MIDIJACK hardware packet contains both long and short 4-40 screws for the DIN jack. Use the two short ones for installation on a thin steel panel such as the Mono/Poly. When all three holes are perfect, put the DIN jack in place outside the chassis and secure with the two screws from the outside, and two split washers and two 4-40 nuts on the inside and tighten with a small Phillips screwdriver from the outside and needlenose pliers from the inside. These nuts should be very tight as they are going onto the metal surface of the panel. By using the Synhouse paper drilling template, a center punch to start the drill, and a chassis punch, your DIN jack mounting can look perfect like the one shown in MonoPoly-pic5:

If done cleanly and correctly, the Mono/Poly will look like it had MIDI when it came from the factory. The wires to the DIN jack may be reconnected by soldering at this time as seen in MonoPoly-pic6:

9) The wonderful thing about installing the MIDIJACK in an Korg Mono/Poly is that all six necessary wiring connections can be made to the back of the existing analog interface jacks and a single circuit board in the Mono/Poly (on the top side that is easily accessible) and the entire modification can be performed without removing any boards from the instrument. The Mono/Poly has panel-mounted 1/4" jacks.

10) The MIDIJACK #1 black and #2 red wires must be soldered in place to get the ground and power for the MIDIJACK. There is a ground terminal on each of the 1/4'' jacks as well as a piece of buss bar wire which all connect to the sleeve of the jacks and is grounded directly to the chassis. Solder the MIDIJACK #1 black wire to any one of the ground points, such as to the buss bar wire as shown in MonoPoly-pic7:

The correct regulated +15v power is available in a few places on the small power supply board in the center of the crumbly particle board base of the instrument. It may be most convenient to find a resistor lead which connects directly to the +15v power and solder the MIDIJACK #2 red wire there. Such a point is resistor R13, the front lead pointing toward the keyboard, as shown in this example photographed in MonoPoly-pic8:

Solder the MIDIJACK #2 red wire to this point.

11) Locate the factory Korg brown wire which is soldered to the back of the CV IN jack. It is the terminal oriented to the right hand side and conducts directly to the tip of an inserted plug. Remove the wire from the solder terminal by desoldering it. Solder the MIDIJACK #3 blue wire to the now-empty isolated CV terminal.

12) Solder the MIDIJACK #4 white wire to the now-disconnected factory Korg brown wire. Carefully wrap the solder joint with electrical insulating tape.

13) Locate the factory Korg red wire which is soldered to the back of the TRIG IN jack. It is the terminal oriented in the upper direction and conducts directly to the tip of an inserted plug. Remove the wire from the solder terminal by desoldering it. Solder the MIDIJACK #5 yellow wire to the now-empty isolated TRIG terminal.

14) Solder the MIDIJACK #6 green wire to the now-disconnected factory Korg red wire. Carefully wrap the solder joint with electrical insulating tape.

15) Photo MonoPoly-pic9 shows the inside of the rear jackpanel containing the analog interface jacks and TRIG POLARITY slide switch with the MIDIJACK wires already connected to the proper points. A look at this will give some idea of what it will look like when done correctly:

Here in MonoPoly-pic10, it appears as a direct top view above the terminals which may help clarify where the connections are to be made:

Here in MonoPoly-pic11, the connected wires are shown:

16) The MIDIJACK #7 brown wire and #8 violet wire (unless it is used for a special function as described in the Advanced Installation Manual) are not required for adding MIDI to the Korg Mono/Poly, but it is a good idea not to permanently cut these wires off, as an alternate installation method may become useful later. It is best to wrap the ends of these unused wires with electrical insulating tape and bundle them with the other wires when finishing the installation.

17) The MIDIJACK hardware packet contains nylon cable ties which should be used to tie the MIDIJACK wires into little bundles and to attach them to the factory wires inside the Mono/Poly now that all connections have been made. This will secure the MIDIJACK wires to the inside of the chassis so they will not rattle and break loose inside the case once the instrument is returned to service.

18) Carefully examine all soldered connections for possible short circuits before closing the instrument. Before closing the instrument, it may be a good idea to use epoxy glue to reattach the loose crumbly particle board sides to the crumbly particle board base to help reinforce the instrument once it is put back into service. You may also use a bit of epoxy to reattach the loose end bits of fake wood veneer strips, but beware, once reattached, they will eventually pull loose again because crumbly particle board cannot be effectively glued to anything, especially even itself due to perpetual crumbling, which is why it is incorrect to say "pull loose again" because it was probably never pulled on in the first place. Who pulls on veneer? It falls off all by itself. It is difficult to decide which is worse about the Korg Mono/Poly: A) The unreliable "conductive" elastomer keyboard mechanism which needs to be cleaned repeatedly because it gets dirty without use or dirt, or B) The crumbly particle board construction of the base and sides. With the installation of the Original MIDIJACK and some epoxy glue reinforcement, both of those 20 year-old problems can be circumvented in most cases.

19) Close the instrument and secure it with the seven Phillips screws.

20) Test and calibrate using the procedures described in the Original MIDIJACK Quick Installation Manual.

21) This installation can be completed in approximately one hour.

Performance Tips:

The Korg Mono/Poly has a trigger circuit separate from the gate circuit to provide multiple note envelope triggering. This is not required for MIDI operation because the Original MIDIJACK has special software to do multiple note triggering like an ARP or single note triggering like a Moog. The user can select either mode at any time. For this reason, set the TRIGGER SINGLE / MULTIPLE switch to MULTIPLE. Since you are using the V-trigger output from the MIDIJACK, set the TRIG POLARITY slide switch to +15V.