From jlcason@yahoo.com Sat Sep 20 05:58:46 2014
Subject:Re: Mojo matching transformer settings



I have done some further examination of the wording and schematics in the B3
service manual:

Quoted from section 1-20, page 1-9 of the manual.

Each playing key carries the fundamental note and its eight harmonics. When
a playing key is depressed, its 9 frequencies are impressed on the 9 bus
bars of the manual. A preset key, when depressed, completes the circuit to
the mixing transformer. The preset keys enable a series of previously set-up
tonalities and tonal combinations to be available immediately on the two
manuals withouth the use of the drawbars. The preset keys are wired to the
preset panel. Flexible leads carrying the varrious harmonics from the preset
keys are connected to the desired taps on the mixing transformers. The
adjust keys connect the manuals to the harmonic drawbars, which are then
wired to the same input taps of the mixing transformers as the preset panel.
The harmonic drawbars provide a means for combining the simple tones which
add up to the complex tones (mixtures of fundamentals and harmonics). These
drawbars enable the organist to select and regulate the desired harmonics of
certain strengths to achieve varioius tones and shades of tones.

End of quoted material.



Looking carefully at the schematic diagram, it appears that there are
separate connections from the Drawbar Groups and the Preset Panel to the
primary of the Matching Transformer(s). One connection is connected to
ground, and there are eight connections, which seem to directly follow the
eight shown bus bars. These eight steps correspond NOT to the drawbars
themselves, but to LEVELS zero through eight (like the zero through eight
registration numbers). Level zero buss connects to ground, level one
connects to the next closest tap, level two connects to the next closest
tap, ... and level eight connects to the entire primary of the mixing
transformer.





My conclusion is that the eight taps on the matching transformer indicate
the eight VOLUME LEVELS dependent on which contact buss a particular drawbar
is pulled out to engage. Therefore, a different voltage step-up amount
occurs depending on the numeric amount of the drawbar registration. Really a
clever way of accomplishing the job back in the analog day, although there
is a great deal of wiring needed. So - the matching transformer taps are
NOT changed to make a particular drawbar higher or lower, the taps are
selected based on just how far out a drawbar is pulled.



This is in no way a criticism of the Mojo's designers using the words
"transformer" and "sliders." Those words do give some idea to the average
user what effect occurs in making the changes. However, a real B3 did not
make changes in that manner (for that matter, a real B3 did not give one the
choice of different "generators" or different "Leslies," either). It is
often possible to add features in software that could not be done on a
single original analog instrument.



BTW - I was a Hammond Technician (along with Baldwin, Conn, Lowrey, Rodgers,
Gulbransen, Kimball, and others) earning my livilhood for 27 years servicing
those instruments along with other musical electronics like keyboards,
amplifiers, etc.



Jim

Moody Blues Keys