Hammond Model A, Руководство пользователя, Страница: 154 / 298

All pedals play the wrong pitch (or do not play at all) on one or more low pitched
stop tablets. One frequency divider is not operating correctly, in which case all
dividers below it will also fall. A cathode-ray oscilloscope connected from ground
to the plate of any divider tube should show a rectangular wave, while the plate
of any divider driver tube should show a very sharp and narrow negative pulse. If
electrolytic capacitor C78 ss open or very low in capacity, all the dividers may
fail to operate.
Key thumps or clicks: If capacitor C81 is open, there will be a loud thump each
time a pedal is played.
Hum: An excessive 120 cycle hum in the output will result from failure of one of the
filter capacitors C75, C71,and C78.
Tuning of individual notes: The individual note tuning system consists of 32 small
inductance coils, each of which is adjustable by moving the coll on its iron core.
This tuning system is very stable because it has practically no aging effect and is
very insensitive to ordinary humidity and temperature changes. However, after long
use under adverse climatic conditions it is possible that some pedal solo notes may
not be exactly in tune with each other.
Always tune first with the tuning knobs as indicated above. Keep In mind the fact
that it is generally desirable to have the pedal solo unit slightly out-of-tune with
the organ. If you are sure some notes actually require tuning, proceed as follows:
(a) Disconnect the two cable leads from the G-G terminals on the preamplifier and
ground the two wires. Connect one set of oscilloscope plates (either horizontal
or vertical) to one G terminal and ground.
(b) Connect the other set of oscilloscope plates to ground and to pin 3 of V6
through a blocking condenser.
(c) Remove the cover of the tuning coil box at the rear of the console, exposing the
numbered tuning coils. The wiring diagram shows the location of these coils. Set the
fine and rough tuning knobs to their center positions.
(d) Push in the pedal drawbars, turn the vibrato off, and turn all pedal solo
tablets off. Using only the first white drawbar on either manual, hold down the
second key G key from the top. Hold down the highest pedal.
(e) Loosen the clamping screw on coil 32 and slide the coil carefully forward or
backward until the note is in tune as indicated by the oscilloscope wave pattern
standing still or moving no more than one cycle in two seconds. Tighten the clamping
screw.
(f) Release key and pedal and press adjacent F# key and pedal. Adjust coil 31 in
same way. Repeat for all other pedals and coils in chromatic order downward.
It is important to start with the highest pedal and progress downward one pedal at
a time because the tuning of the lower notes is dependent upon all of the higher
coils. Each pedal adds an increment of inductance in series with all coils above
it, and adjusting any single note wlll detune all those below it.
9