MIDIJACK II Advanced Installation Techniques
Most advanced installation techniques will void the Synhouse factory warranty for the MIDIJACK II, but some installers will choose to venture outside this product protection due to their experience in such matters. The MIDIJACK II is a high-technology, microelectronic board with all surface-mount electronic components that is entirely machine-made because it cannot possibly be hand-soldered reliably due to the small size of the parts involved and the PC board cleanliness required by the high-speed circuitry. As a general rule, soldering on the top of the MIDIJACK II board will void the warranty. There are a few solder pads that are drilled and can be soldered by hand from the lower side of the board. There are many possibilities for customization not described in the MIDIJACK II Quick Installation manual. A few of them are listed here:
1) The MIDIJACK #8 violet wire (auxilliary MIDI function input) is not an analog input or output. It allows control of the MIDI function button by momentarily connecting this wire to ground. The MIDI function button is a normally-open, momentary contact SPST (single-pole, single-throw) switch that has one end of the switch connected to the computer and the other end connected to ground, so that when the button is pressed, the computer input is grounded, causing the use of the button to be detected. This is known as an active-low switch configuration. The MIDIJACK #8 violet wire is connected in parallel with the computer input line, so that touching this wire to ground is the same as pressing the MIDI function button. This wire can be soldered to a new jack (or existing jack rewired) on the panel, so that a standard normally-open footswitch may be plugged in and used onstage to access the functions of MIDI channel selection, MIDI panic button, MIDI Off mode, MIDI On mode, single note triggering, and multiple note triggering. The wire could also be connected to a switch the synthesizer already has to make for a more discreet MIDI installation. If the installer preferred the look of some switch other than the factory-installed part, such as an older, more retro-looking panel-mount switch, it may also be wired to the MIDIJACK #8 violet wire (auxilliary MIDI function input) to perform the same function while the stem of the factory switch is cut short enough to fit beneath the panel. Do not put any voltage into this wire. Leave this wire open (unconnected) or momentarily connect to ground only.
2) The MIDIJACK II 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 some cases, there is no regulated voltage +12 volts or greater and the unregulated voltage inside the synthesizer must be used. When installing a MIDIJACK II in such a synthesizer, connect the MIDIJACK #2 red wire to the unregulated supply (which may be fluctuating somewhere around +15-25v DC). 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. Such a capacitor is readily available from any electronic supply store, even Radio Shack with their part number 272-1028. This extra capacitor may be added in any other MIDIJACK installation as well, but extensive testing has proven this to be completely unnecessary.
3) Sometimes an MIDIJACK II installation may be performed in conjunction with a total change in the configuration of the instrument. Consider an analog synthesizer in poor condition with broken keys: Searching for replacement keys for a tiny and unreliable 2-3 octave keyboard may seem pointless to some users when the MIDIJACK II will give it a 49-note range through MIDI. One popular solution is to cut off the keyboard and put it in a rack or configure it as a free-standing tabletop synthesizer module. When this is done, there will be no local keyboard signal to bypass with the computer-controlled analog switching matrix on the MIDIJACK board. For this reason, there are two solder pads on the lower side of the board that have unswitched CV and gate outputs. They are marked on the lower silkscreen. The gate output is marked SP1 and the CV output is marked SP2. The MIDIJACK #3 blue, #4 white, #5 yellow, and #6 green wires would then be unused. In the case of an S-Trigger installation, the MIDIJACK #7 brown wire (S-Trigger output, unswitched) would still be used because the S-Trigger wire is always unswitched. This installation method utilizing the unswitched CV/gate outputs may also be used for a synthesizer that did not have a keyboard to begin with, such as a Korg MS-50, because there is no keyboard to bypass.
4) With the full heavy-duty hardware installation kit, the MIDIJACK II is very small, only extending about 17 mm below the panel on which it is mounted. This allows it to fit most places, but some users may prefer an ultra-low profile custom fitting which will require removal of the stock factory hardware. If the two aluminum mounting brackets are removed, the board can be configured to be only 7 mm thick. In such a case, it could be mounted with double-sided foam tape. Such tape is sold in hobby shops as servo tape for radio-controlled (R/C) vehicles. It could also be mounted with lower profile hardware, and the tall plastic switch could be trimmed to be shorter. To do this, you must hold the switch by the stem (not the PC board) in a pair of pliers, and then cut it shorter. A good way would be to cut it with nippers and then file it smooth with sandpaper or an emory board.
5) On many synthesizers, the
portamento is disabled and doesn't function while the synthesizer is under
external CV/gate control. This has always been the case with many
synthesizers, even though glide is generally thought to be an oscillator
function, not a keyboard function. The portamento circuit is
placed between the local keyboard voltage and the CV/gate input jacks,
so when the jacks are used, the keyboard voltage is cut off and so
is the portamento circuit. The portamento circuit needs to be rewired
to function correctly. On most analog synthesizers of this type,
the original circuit may be isolated and inserted where it belongs,
between the CV/gate input jacks and the VCOs. Not for neophytes,
this is an advanced modification for real synthesizer technicians only.
The MIDIJACK II is exactly the same as using external CV/gate control,
so if the synthesizer has no glide while under CV/gate control, it
won't with the MIDIJACK, either. The modification suggested
here is not only a good solution for use with MIDI, it is a good
and meaningful improvement to the instrument overall. Generating
glide is so trivial that it may be slightly simpler to build a new exponential
glide circuit. This can be done quite simply by taking an audio-taper
potentiometer of at least 500K or 1 meg Ohms wired as a straight variable
resistor (so the resistance goes up when turned clockwise) and connecting
one end of it to the MIDIJACK #4 white wire (MIDIJACK CV output) and the
other end to a 10 uF (or greater) capacitor which has the negative end
to ground. The junction of the cap and pot should be connected to
an op amp configured as a buffer-follower. This buffered output goes
to the VCOs. This puts a lag generator between the MIDIJACK and the
VCOs, and you need to find a place to mount the new knob you have
created. If the op amp used is of the standard older variety like
the ones used in the synthesizers themselves, the negative power
pin should be connected to the negative supply in the synthesizer and not
the ground. If this is not done, the op amp will be unable
to reach ground, causing a tuning problem that will be accentuated
when the synthesizer is playing notes that are near 0 volts. A cross
between these two methods that would preserve the original look of the
instrument would be to use the original glide pot on the panel, rewired
to the new glide circuit. Before any work is done, check the
synthesizer portamento first. Some synthesizers already have the
glide circuit correctly wired in the post-jack position. If any of
this terminology is not immediately clear to you, you should not
even try this modification!
The MIDIJACK II
Quick Installation Manual must
be read fully and understood completely to qualify for warranty coverage
and technical support. The MIDIJACK is warranted against defects
in parts and manufacture for a period of 90 days from the date of purchase.
Warranty becomes void if in the opinion of Synhouse Multimedia Corporation
the MIDIJACK has been subjected to unauthorized service, modification,
or unintended installation or usage. No liability is assumed by Synhouse
Multimedia Corporation for any loss or damage, direct or indirect,
resulting from the use of or inability to use the MIDIJACK.
Copyright © 1/2/2002 Synhouse Multimedia