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Theremax 21
others. The point at which the frequency of these louder tones go
through zero is the ideal setting. But don’t confuse this null point with the
two super-audio regions when the slug is a little too far “out” or “in”.
You won’t be able to set exact null by tuning L2 unless it’s by random
chance. You’ll find that the pitch shifts slightly as you remove your hand
from tuning the slug, or as you remove the slight downward pressure of
the tuning tool. This is OK, the control panel’s trim knob is used to set
exact null.
When you have gotten close to null using L2, play with the Pitch Trim
control on the panel some. You should find that rotating this control has
about the same overall effect that turning the tuning slug did, except with
much greater resolution and control. Notice particularly that you can
easily null the Pitch exactly.
With the pitch nulled, try the antenna response. In a properly operating
Tmax you should find that pitch first responds when your hand is about
16 inches from the antenna and that pitch increases smoothly up to a
peak when your hand is about 2 inches away.
Calibrating Volume
When you’ve played with pitch for a while and have a feel for the interac-
tion of the tuning slug and Trim control, it’s time to move on to setting up
Volume. Leave the Pitch Trim set so that there is some easily audible
tone and remove the clip lead from D1 to R83. Turn Tmax’s front panel
Volume Control all the way to max.
Now slowly adjust the slug of L3 “in” while listening for the output tone.
While turning the slug from end to end you will hear the volume rise, then
fall back to quiet, then rise again. This corresponds to the way the pitch
fell from a high frequency to zero then rose again. When you have
observed this, return the slug to it’s fully “out” position and once again
turn it “in” slowly until the volume starts to increase. The setting of this
coil that yields that greatest sensitivity and range of control will be about
1/16 turn short of the first volume peak. At the optimum setting, the max
signal level (hand fully removed from antenna) will be about the same as
it was when the test jumper was in place.
Try the Volume Antenna response. You should hear the volume begin to
decrease as your hand gets closer than about 10" to 12" from the
antenna and should be completely off when your hand is closer than
about 1/4". Touching the Volume Antenna should not produce any audible
artifacts such as chirps or pops. If the range of control seems “com-
