Converting a Hammond M2 Organ into an M3
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
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
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
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:
- 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.
- An M3 amplifier. A new amp is necessary because
it contains the percussion amplifier circuitry.
- An M3 matching transformer. This unit has extra
connections to handle the percussion signals that your M2
- 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.
- 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.
- 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
- 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.
- 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.
- 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.
- Disconnect all wires from the amp.
- 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.
- Remove the four screws holding the amp to the
bottom of the cabinet and lift out the amplifier.
- 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.
- 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.
- 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
- Install percussion switch assembly:
- If you will be mounting the switch
assembly in your old cheek block, first mark the
new location of the inside cheek block mounting
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.
- Remove the two mounting screws observed
in 8a, and remove the cheek block.
- 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.
- 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.
- Fasten cheek block in place with
mounting screws as located in 8a.
- Make connections to the terminal strip of the
- Using the wiring diagram and bundles
observed and marked in step 1, make the
connections to the M3 amp.
- 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.
- 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.
- The speaker connections are via the
five-pin plug. Connections are:
- Jumper from 2
- 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.
- Rewire upper draw bars 4, 5, and 9.
- Locate the bundle of six wires coming
from the percussion switch assembly containing 2
white, 2 yellow, and 2 green wires.
- Remove the white wire from draw bar 9
(the left-most looking from the back of the
- Solder one of the white wires from the
bundle to draw bar 9 (it doesn't matter which
- 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
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
- 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.