Converting a Hammond M2 Organ into an M3
by
Jim Ormond
jameseo@aol.com

When one is considering whether converting their M2 to an M3 is worth the trouble, or even if the conversion is something they could do themselves, it helps to begin with a few questions to clarify things.

1. Why convert my M2 organ to an M3?

The functional difference between an M2 and an M3 is the percussion circuit. If your playing style would benefit from the addition of percussion, then you may want to consider modifying your M2 into an M3. Since there is no other substantive difference between the two models, then the desire for percussion would be the driving force in your decision.

2. What are the percussion features I would be gaining?

The M3 percussion features selected by the percussion control switch panel (installed as part of the conversion) are 1) percussion on/off, 2) selection between 2nd and 3rd harmonic percussion, 3) selection of "percussion volume" normal/soft, and 4) percussion decay fast/slow.

3. What hardware changes are involved in the conversion?

The conversion consists of replacing the M2 amplifier with an M3 amplifier, replacing the M2 matching transformer with an M3 matching transformer, and the installation of the percussion control switch panel in the left cheek block of the lower manual. The bulk of the job is moving connections from the old amp and transformer to the new ones. Other wiring involves the new percussion switch wiring to the new amp and three of the upper manual draw bars.

4. What skills are required for the conversion?

As with most electronics projects, the most important skills will be the ability to read and interpret schematics and wiring diagrams, and good soldering technique. Most of the job consists of removing the wires connected to the old M2 amp and reconnecting them to the new M3 amp. Additional wiring includes connecting the new percussion control switch assembly wires to the amp, and wiring in the new matching transformer. Good schematic reading skills are required to assure the signals are connected to the correct places, and good soldering skills are required to assure quality electrical connections (i.e., no "cold" solder joints, etc.) and that the matching transformer is not damaged by overheating or other soldering accidents.

5. What is the general level of difficulty of this task?

The conversion is not hard, per se, just tedious. Anyone with a hobbyist level of interest and skill can perform the conversion without difficulty. It is mainly time-consumming because you will want to take your time interpreting the schematic to understand the relationships between the wires and the schematic, and because there are about 50 connections involved (a few more if you have Leslie connections, too). It took me two 4-hour sessions to complete the conversion because I didn't want to rush through it and make mistakes.

6. Would you do it again, knowing what is involved compared to the value of the added feature?

Sure would. In my estimation it was well worth it because the style I play (R&B and Gospel) is enhanced by the ability to dial in percussion where desired. Of course, the best thing would have been to get an M3 to begin with, but since I had the M2, that was my starting place. (Actually, the best would have been to get a B3, but that's another subject. :-)

OK, so you know you want to do this. What's Next? First, you will need to locate parts. The best source for that (or at least a good starting place) is on the Hammond mailing list. Here's what you will need for the conversion:

1. M3 schematics and wiring diagrams. You may optionally want a set of M2 wiring diagrams and schematics to help correlate old M2 connections to new M3 connections, but this is not a necessary thing if you are reasonably good at interpreting diagrams.
2. An M3 amplifier. A new amp is necessary because it contains the percussion amplifier circuitry.
3. An M3 matching transformer. This unit has extra connections to handle the percussion signals that your M2 didn't have.
4. Percussion switch assembly. You may find one already installed in a cheek block, or not. The assembly is such that installing it in your own cheek block is just a matter of cutting a rectangular hole in the block and mounting the whole assembly as a unit, so it's no big deal if the switches don't come in a cheek block already. It's not like you have to cut out individual holes for each switch, or anything.
5. Tubes. Don't assume the amplifier will come to you populated with tubes. You will most likely be buying your amp from another hobbyist on the list, who has tucked them away for future use in his or her own equipment. Be sure to know explicitly (by asking, of course) whether the amp will be coming with or without tubes so you will know to order some or not. By the way, your old M2 amp will have only three tubes you can use.
6. Five-pin speaker plug. The speaker connections to your old amp are via the terminal strip on the back of the amplifier. Your new M3 amp will expect those connections to be made via a 5-pin plug on the top of the amp. Be sure to ask whoever you buy the amp from to include this plug in the package, if possible. Also, determine whether the M3 amp came out of a model that used a field coil type speaker or a permanent magnet type. The M3 came in both flavors, and it is important to know if you will have to replace your speaker type to match.
7. Stranded 18 gauge (or thereabouts) wires in various insulation colors (green, black, gray, brown) for extending certain connections. The M3 amp is of a different physical size than the M2 amp, and a few of the connections, most notably the pedal amplifier and speaker connections, will require splicing on extra lengths of wire to reach their new connection points.
8. You will need the usual tools associated with working on electronic equipment: slotted screwdriver, needle-nose pliers, cutters, wire stripper, soldering iron, and solder. You will also need a drill to create a couple of new mounting screw holes for the amp.

Once you have obtained the necessary supplies, above, you are ready for the conversion procedure.

1. Study the current connections with regard to correlating the bundles with their location on the wiring diagram. It may help to mark bundles with masking tape and identifying text, such as "vibrato scanner", "pedal amplifier", and so forth (or simply, "A", "B", etc.). You needn't bother with marking individual connections. The M3 wiring diagram is well drawn, with bundles easily identifiable, but they are not labeled, so you may want to mark the wiring diagram the same as you mark the bundles. If you mark a certain bundle "A", mark its representation on the diagram as "A". Your main task here is simply to understand how each bundle relates to the wiring diagram. When it comes to making the new connections, it will become obvious that "pedal amp" gray (or bundle "A" gray) goes to terminal "R", and so forth. Also, if you have a Leslie connected, be sure to mark those wires with regard to organ wires, such as "to speaker brown", "to power black", and so forth.
2. Disconnect all wires from the amp.
3. Disconnect the expression pedal lever from the expression control shaft on the "tower". You may want to hook the end of the expression pedal spring into the end of the expression pedal lever to hold it up out of the way.
4. Remove the four screws holding the amp to the bottom of the cabinet and lift out the amplifier.
5. Place the M3 amp in the cabinet, lining up the left-hand mounting holes on the amp with the left-hand mounting screw holes in the cabinet. Using a "Sharpie" or other dark marking pen, mark the locations of new right-hand mounting holes by poking the pen through the right-hand mounting holes on the amp. Remove amp.
6. Using a screwdriver, or other shaft-like object, push down through the old right-hand mounting holes to force the threaded inserts out through the bottom of the cabinet. Match a drill bit to an old mounting hole, and drill new mounting holes marked in step 5. Install the threaded inserts into the new mounting holes and reinstall the new amp and mounting screws.
7. Reinstall the expression control lever onto the expression control shaft, taking care that the set screw on the lever makes contact with the flat side of the shaft.
8. Install percussion switch assembly:
   1. If you will be mounting the switch assembly in your old cheek block, first mark the new location of the inside cheek block mounting screw:

      Look underneath the left end of the lower manual and observe the under side of the cheek block. Observe the outer mounting screw on an "L" bracket against the left side of the cabinet. Observe the inner mounting screw that goes through a rail located approximately 2/3 of the way along the cheek block and to the immediate left of the metal keyboard bottom cover. Now continue looking to the right of the inner mounting screw and observe a hole in the metal keyboard bottom cover located underneath the right end of the cheek block. That hole will be the new location of the inner mounting screw, so use a scribe to mark the new location on the cheek block by poking up through that hole and into the cheek block.

   2. Remove the two mounting screws observed in 8a, and remove the cheek block.
   3. If you are mounting the switch assembly in your old cheek block, cut the required rectangular hole in the block and mount the switch assembly using standard woodworking skills and tools. Also, drill a pilot hole for the inside mounting screw as marked in step 8a.
   4. Install cheek block with percussion switch assembly, routing wire bundle to the back of the organ through opening between the upper manual and the tone generator. The main (longest) bundle will be routed down the back to the amplifier. The shorter bundle will remain in the top portion of the console for connection to the matching transformer and the draw bars.
   5. Fasten cheek block in place with mounting screws as located in 8a.
9. Make connections to the terminal strip of the new amplifier.
   1. Using the wiring diagram and bundles observed and marked in step 1, make the connections to the M3 amp.
   2. Some wires, such as the pedal amp connections will need to be lengthened with extra wire. Observe good splicing practices, and be sure spliced connection is insulated.
   3. Since a few of the terminals have more than one wire going to them, it is suggested that you simply crimp all connections onto their respective terminals, carefully checking your work as you go. After all connections are crimped on, then go along the terminal strip soldering the connections in one operation.
   4. The speaker connections are via the five-pin plug. Connections are:
      1. Green
      2. Brown
      3. Black
      4. Jumper from 2
      5. Gray
10. Wire in the new matching transformer. The best way to do this is to unfasten the old matching transformer from the mounting rail, take off its cover, place it side-by-side with the new one, and one-by-one remove wires from the old one and solder them to the new one. There is one new connection, which is a shielded cable from the percussion switch assembly. When done, mount new transformer on rail.
11. Rewire upper draw bars 4, 5, and 9.
    1. Locate the bundle of six wires coming from the percussion switch assembly containing 2 white, 2 yellow, and 2 green wires.
    2. Remove the white wire from draw bar 9 (the left-most looking from the back of the organ).
    3. Solder one of the white wires from the bundle to draw bar 9 (it doesn't matter which one).
    4. Connect the other white wire from the bundle to the original white draw bar wire and insulate the splice with electrical tape.

       NOTE: The M2 and M3 upper manuals differ in that the key contacts for drawbar nine go all the way to the top note of the keyboard in the M3 (for triggering percussion over the whole range), but the key contacts for drawbar nine on the M2 (i.e., yours) go only to the F# second from the top. This means that you will loose triggering at that point. In order to attain triggering over the whole range (or as near to it as you can get), you may want to use drawbar six instead of nine for triggering. In that case, follow the above directions for the white wires, but rewiring drawbar six instead of drawbar nine. The down side is that you will not have drawbar 6 available while percussion is turned on.

       An alternative is to supply the missing key contacts by either replacing the upper manual with an M3 manual, or modifying your M2 manual. Other possibilities include coming up with your own circuit for obtaining the trigger signal in a manner that doesn't take away any drawbars (e.g., a single-shot flip flop circuit, etc.). Unfortunately, instructions on any of these options is beyond my current expertise to offer advice on.

    5. Repeat steps 11b through 11d for Green (draw bar 5) and yellow (draw bar 4).

At this point you have completed the conversion and are ready to install the tubes, plug it in, and test it. I would suggest that if you are running your organ through a Leslie or other external tone cabinet, you begin with the external system not connected and just check things out via the internal speaker. If there's no smoke and things seem to work as expected, connect the auxilliary stuff and see that it is also working properly.

Have Fun

- Jim