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Midification of Conn 642 PART 2

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Dave’s Conn 642 MIDIfication PART 2

While awaiting delivery of parts, I decided to firm up my plan of attack. I already had an Asus barebones computer into which I had installed a 3 Ghz P4 processor, overclocked to 3.3 Ghz.  I had been using it as a home theater computer, but the addition of a DVR pretty well put it out of business.  It was suggested that a fast single core processor might be better than a slower multi-core anyway.  That certainly was the case, as this computer worked flawlessly using the Miditzer fluidsynth and the onboard AC97 audio. 

I thought I might use the organ amplifier and speakers for the Miditzer, but a temporary hookup proved very unsatisfactory.  I had an old stereo system, consisting of a Heathkit AR1500 receiver, and the Heath version of the Acoustic Research AR3 speaker systems.  A quick trial was beyond success.  The Miditzer sounded beautiful!  My jubilance was short-lived however.  Upon hearing some strange sounds, I removed the grill cloth and found that the foam rings surrounding the woofer speaker cones was disintegrating.  I remembered an outfit in nearby Cleveland, Ohio that did speaker repairs.  Empirical Sound did a top-notch repair job for $30 a speaker.  Happiness returned!

I have a maintenance manual for the Conn 642, so that made things a little easier. It was made in the mid to late sixties and is fully transistorized, but a little too early for integrated circuits.  The top lifts up and the keyboards flip up, exposing a wonderland of electronic construction.  It’s easy to understand why such an organ cost so much.  I would try to keep everything intact.

There are ten contacts under each key in the solo manual (fewer in the accompaniment), one for each of the keyed tibia voices, with the remaining contacts being used to mix various audio waveforms for all the other voices.  There is a +30V buss that is switched by the keyboard to turn on various tibia notes through a keying transistor - one transistor for each note. The +30V is fed to a rotatable rod that runs the length of the keyboard, and there is a rod for each of the tibia tabs.  Only one side of the rod is conductive, and each tibia tab (16', 8', etc.) rotates the rod through a control cable like those used to operate bicycle hand brakes.  When a tab is down, the conductive side of the rod can make contact with one of the contacts under any key which is pressed.  Thus, when a key is pressed AND a particular tab is activated, the appropriate note will sound  What an awesome system!

It is that keying voltage that I will use to feed the DIN shift registers. +30V would quickly wipe out the shift registers, so I added a 62Kohm resistor in series with each input to form a voltage divider with the 10Kohm pullDOWN resistor (my modification as mentioned in part 1) on the DIN board.  This gave me a little over 4 volts at the chip.  The chip specs state anything over 5 volts will destroy the chip, and that anything over 3 volts will be seen as a high.  The 4 volts was an attempt to strike a happy medium.  I had to add an isolation diode at each keyer as well so that the tibia sustain would work.  If you are considering a similar modification to a Conn organ, I can provide additional details.

Two DIN modules were used for each 61 key keyboard, so one CORE module and four modified DIN modules were used for the two keyboards.  The pedalboard did not lend itself to any such technique, so a standard DIN module was used for the pedals, and another for the combination pistons I decided to add.  These two modules were used with the second CORE module, which also accommodated a single potentiometer for the expression pedal.  More about this later. I also ordered a 15" Hyvision touchscreen monitor from Newegg for the stop tabs.

The board kits arrived, so it was time to heat up the soldering iron, and get busy.  There are excellent step-by-step instructions by Jim Henry at, and there is additional information at and  If you haven’t soldered before, take the time to learn, as well as recognize a poor or cold solder joint.  A poorly constructed module will surely insure failure.  Look around the forum, and you’ll find plenty of help.  You’ll need a 15 or 25 watt soldering iron, and I highly recommend a soldering stand with a sponge for cleaning the iron.  Both are inexpensive at Radio Shack.  You’ll also need some rosin core solder, wire cutters, and needle nose plyers.

The ICs are the only components that are really sensitive to electrostatic discharge, so leave them in their protective wrapping until the bitter end.  Since you probably don’t have anti-static workstation, you must be extra careful when installing the ICs.  Get into position, sit perfectly still, and touch a grounded object - something metallic that is likely to be grounded through a grounding electrical plug (computer case?). While you’re handling the IC, do not move your feet or squirm in your chair.  So far, this has worked for me.

As mentioned various places, the use of a display is optional, but I sure got a nice warm fuzzy feeling when it lit up and started telling me things were OK.  Jim Henry includes a link to general information on displays in his CORE assembly article and also wrote a nice tutorial on wiring a display at  A data sheet came with the display I ordered, and the connections did indeed match Jim’s.  Be sure to orient the connector properly when plugging it into the CORE board.  I built only one of the two CORE modules, and tested it before proceeding.  I next built only one of the DIN boards as an initial test.  That way, if I were doing something wrong, I would discover it before building any additional modules.

To test the DIN board, I’ll have to load some software (firmware?).  While I have been fiddling with computers for some years now, this process was still the most challenging.  We’ll talk about that in part 3.


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Way to go dave! This kind of thing can be really helpful. Not to mention just fun to see what others are up to ;)

May I suggest that, if you want to separate the posts into individual threads, you link to your other threads, so that if someone stumbles across part 2, they can find part 1?

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