Minimoog Analog User Modifications

The following plans describe a method of adding MIDI capability to the Moog Minimoog analog synthesizer with the Synhouse Original
MIDIJACK. This will allow the Moog Minimoog to be used as a MIDI analog monosynth. Some drilling is required. The installation of the Original MIDIJACK in a Minimoog is one of the easiest modifications of all. All of the electronic work can be done on a single board.

This simple modification not only adds internal MIDI to the Minimoog, but it also solves an age-old problem for Minimoog users:

The poor sample and hold circuit used by the Minimoog has always been unstable and caused many players problems in many situations. Many users have tried to control the Minimoog by external CV and S-trigger, only to find that the instrument constantly goes out of tune. This is due to the leakage of the sample and hold circuit in the Minimoog. The Original MIDIJACK and this installation method engineered for it incorporate a keyboard bypass modification to solve this problem. There is no need to constantly hit a key before each take to charge up the sample and hold again, because the MIDIJACK properly bypasses the keyboard the way that any analog synthesizer should.

The correct analog I/O signals are easy to find and connect to the MIDIJACK, and all connections are made on a single board. The entire MIDI conversion job can be done easily in one hour or less. 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, you can ship the unit to Synhouse L.A. for a quick, low cost Factory Installation.

For self-installation, it is best to print these notes 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, it is best to completely read and understand each step of the instructions before starting. The particular installation on which this document is based was on Moog Minimoog serial #10796, other revisions may be different.

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!

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, and some heat shrink tubing or electrical insulating tape. 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) or step drill (a cone-shaped drill bit with many steps on it from small to large) to make the hole for the DIN jack, and an electric drill with a 3/32" 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). 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. If you choose to mount the DIN jack in the soft wooden portion of the Minimoog chassis, ordinary household drill bits for wood will make the hole quite easily, and will also make the holes for the screws as well, so no chassis punch, electric drill, or drill bits are needed for this alternate quick mounting method. 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.

Always wear eye protection such as safety glasses when working with power tools, cutting tools, or punching tools.

The Minimoog has no regulated power supply of +12v DC or greater as required by the MIDIJACK, so the unregulated DC power supply must be used. To stabilize this power, it is recommended that a 100 uF/25v (or greater) radial-lead electrolytic capacitor be added to the MIDIJACK board. Such a capacitor is readily available from any electronic supply store, even Radio Shack with their part number 272-1028.

2) Fully test the Moog Minimoog to be converted to MIDI. Be sure that all functions such as the envelope generator 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. 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 Moog Minimoog. 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) To open the case, lift the control panel section of the Minimoog up and support it by inserting a yardstick or similar item under it. Remove the screws holding the rear aluminum panel in place and remove the panel. Remove any dust and debris that may have accumulated inside the instrument over the years. Removing this panel will expose the Mini Moog Board No. 2 where all of the electronic work will take place. A photo called Minimoog-pic1 shows this:

5) Remove Mini Moog Board No. 2 by removing the two retaining screws and gently pulling the board out of the two edge card sockets holding it in place. Once removed, the synth will appear as in Minimoog-pic2:

6) Determine the place where the MIDIJACK circuit board will be mounted and test fit the board into its correct place inside the
case. There is plenty of room inside the Minimoog, so the mounting location is up to you. The installation done for this article shows the board and DIN jack mounted on the rear connector panel with all the other jacks, but this is merely a suggestion. 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. A photo called Minimoog-pic3 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. Before mounting the MIDIJACK board, you must make a modification to accommodate the unusual power supply of the Minimoog. The Original MIDIJACK gets power to operate from the synthesizer in which it is installed. Although the MIDIJACK has a very high-quality regulated supply of it's own which allows it to operate with an unregulated input, it is preferable to use the regulated supply inside the synthesizer when available, and it almost always is. In the Minimoog, there is no regulated voltage +12 volts or greater. It has only a +10v regulated supply. To install a Original MIDIJACK in a Minimoog, we will later connect the MIDIJACK #2 red wire to the unregulated DC power supply (which will be fluctuating somewhere around +20v). As an extra precaution to help stabilize any voltage ripple which may become audible in the form of unwanted vibrato in the CV, two solder pads have been placed on the lower side of the MIDIJACK board to solder a 100 uF/25v (or greater) radial-lead electrolytic capacitor in place, upside-down, under the board. The pads are on the end of the board nearest the header for the wiring harness and the positive side of the capacitor should go in the square solder pad marked with a + on the silkscreen. Carefully solder the capacitor in place.

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 small pocket screwdriver can fit through the panel for periodic adjustment. A photo called Minimoog-pic4 shows the board mounted in place with the capacitor hanging from the underside, outlined in red:

The photo Minimoog-pic5 shows the new MIDI control panel:

8) Determine the place that the MIDI input DIN jack will be mounted. The location used by Synhouse in this example was on the rear jack panel 1.5" below the LOUDNESS input. This location requires notching the rear panel before it is replaced, using a hacksaw or other cutting tool. An easier place to mount the DIN jack might be right next to the MIDIJACK board itself, inline with the other holes. The way to make this look like original Moog factory equipment is to mount the DIN jack outside the metal panel instead of inside the panel as in a normal MIDIJACK installation. 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. Tape the Synhouse paper drilling template in place, as shown in Minimoog-pic6:

9) It is advisable to use a chassis punch or step drill (a cone-shaped drill bit with many steps on it from small to large) 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. If using a chassis punch, 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 Minimoog, not outside, so the cutting edge is coming from the outside. This will ensure that the outer edge is perfectly smooth. The properly cut mounting holes may be seen in Minimoog-pic7:

10) The MIDIJACK hardware packet contains both long and short 4-40 screws for the DIN jack. Use the two long ones for installation on a thick aluminum panel such as the Minimoog. 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 the four split washers and two 4-40 nuts on the inside against the back of the DIN jack and tighten with a small Phillips screwdriver from the outside and the needlenose pliers from the inside. These should be very tight as they are going onto the metal surface of the DIN jack. If done cleanly and correctly by using the Synhouse paper drilling template, a center punch to start the drill, and a chassis punch, your Minimoog will look like it had MIDI when it came from the factory, perfect like the one shown in Minimoog-pic8:

Resolder the wires to the DIN jack at this time.

11) All of the electrical connections can be made on the single board that has been removed, so put it on a soft work surface. For a quick preview to show what it will look like when done, see Minimoog-pic9:

Note that there are two edge card connectors on this one board. Looking at the bottom foil side of the board, the one on the left is A and the one on the right is B. The pins of each of the edge card connectors are numbered starting with 1 from the left, looking at the bottom foil side of the board. We will be taking a closer look at each connection in the steps ahead.

12) The MIDIJACK #1 black and #2 red wires must be soldered in place to get the ground and power for the MIDIJACK. The MIDIJACK #1 black wire must be connected to pin 2A on the left, the ground pin, and a convenient place is on the protruding lead of a component less than an inch from the edge. Solder the MIDIJACK #1 black wire as shown in Minimoog-pic10:

The MIDIJACK #2 red wire must be soldered to the pin 4B on the right, the unregulated positive DC voltage pin, and a convenient place is on the protruding lead of a component less than half an inch from the edge. Solder the MIDIJACK #2 red wire as shown in Minimoog-pic11:

13) The CV connections must be made next. Locate the connection point for the MIDIJACK #3 blue wire. It is near the upper left corner of the board, right next to an "S" etched into the board. Solder the MIDIJACK #3 blue wire to this point as shown in Minimoog-pic12:

14) Locate pin 4A on the left, and note that 4A and 5A are shorted together by a trace in the PC board. You must cut the trace going to pin 4A and 5A. One good way to do this is to score two parallel grooves with the X-Acto knife and then lay a very hot soldering iron down on one side of the tip between the two cuts. This heat will ruin the glue holding the center patch and delaminate it leaving the two sides isolated. This is often a lot less effort than digging and digging a groove wide enough to break a circuit trace. Use the X-Acto knife to scratch the green solder mask off a small area of the trace from pin 4A. This will allow you to solder the wire to it. Solder the MIDIJACK #4 white wire to this bare trace. The photo Minimoog-pic13 shows the wire soldered in place and the cut trace next to it:

15) Moog synthesizers generally do not disconnect the keyboard from the trigger circuit with a switched jack as they should. For this reason, the trigger jacks will still trigger the envelope generator even with the MIDI activated. The MIDIJACK S-trigger wire must then be connected in parallel with the existing trigger wire. The MIDIJACK #7 brown wire (S-trigger output, unswitched) must then be connected in parallel with the existing trigger wire. Locate pin 19B on the right side of the board. A convenient place to connect the MIDIJACK #7 brown wire is several inches up the trace on the protruding lead of a component coming through the trace. Solder the MIDIJACK #7 brown wire to the trace as shown in Minimoog-pic14:

16) The MIDIJACK #5 yellow wire and #6 green wire are not required for for adding MIDI to an S-Trigger Moog such as a Minimoog, 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 heat shrink tubing or electrical insulating tape and bundle them with the other wires when finishing the installation.

17) Now that all connections have been made, carefully check over your work and examine all soldered connections for possible short circuits before closing the instrument. The MIDIJACK hardware packet contains nylon cable ties which can be used to tie the MIDIJACK wires to the original Minimoog wire bundles to prevent them from moving around inside the case. Secure the wires, then remount the board in the edge card sockets and secure with the two original screws.

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

20) It is probably a good idea not to use the Minimoog built-in local keyboard or oscillator CV and S-Trigger input jacks while MIDI is activated or vice versa.

21) This installation can be completed in as little as one hour, depending on how you mount the MIDIJACK board and DIN jack, but extra time spent making a perfect installation is time well spent.