Theremax Tuning Revised

A quick tuning method (no test lead jump used): Back all four tuning slugs fully out (ccw).  Turn L1 and L4 one turn in.  Connect the audio output to an amp/speaker and set the Volume control R83 to about 3:00.  Set the Velocity to max.  Slowly adjust L3 inwards through it’s range listening for a faint wheep-wheep as it goes through its audio heterodyne frequencies.  Now, as you go back and forth through this point, the Gate/Trigger LED should be lighting indicating the Volume CV is increasing and decreasing and the velocity cv is causing a trigger.  For Traditional volume control response, leave L3 set just before the onset of the first audio band encountered with the inward adjustment.  This causes a volume CV output and audio output that decreases with inward hand movement.  Adjust L2 inwards for the pitch heterodyne tone (the 3:00 R83 Volume setting may be a bit high now as the pitch tone will come through loud and clear in comparison to the volume het).
For controller response (opposite of traditional), adjust L3 to null, or, to keep the volume heterodyne at a post-audio frequency (for no faint background volume ‘pitch’) adjust it further inwards to be just beyond the second volume (volume cv) peak.
Note the front panel Volume Trim control is optimized for tweaking the volume pair to be at null.  For these post audio heterodyne settings, start with it centered and then you’ll notice it will work to bring the pair into or out of audio range.  Also it won’t make very much change in volume in comparison to the way it did in varying the pair about null.
Note the unit can be tuned/tested with the antennae detached. I often work on board and panel assemblies just spread out on a piece of wood or cardboard.  It can sometimes make the difference having the antennae or their cables not attached if there is trouble with the proximity of the antenna (or its mount) to a ground or a conductor to the floor.  A near short between the shield and the internal wire can adversely load the attached oscillators one or four.
Moving the initial setting of L4 more inwards than the specified 1 turn in can provide more output if there is something loading it more than usual.  The f-v circuits must have a normal strength heterodyne (no adverse loading of either oscillator in the pair) to produce the volume CV which varies the amplitude of ‘tone’ to the output; whereas, the ‘tone’ simply must be there–it being weak or not isn’t doesn’t have the consequences the volume pair does.
— SL  11/08

First time tuning can be tricky because both the Pitch pair and Volume pair of oscillators must be close to a null in order to hear anything at all.  If the Pitch pair are close, but the Volume pair are far apart you won’t hear anything because the Volume doesn’t come on.  If the Volume pair are close but the Pitch pair aren’t, you won’t hear anything because of no pitch signal.  In either case, there’s no clear indication of what needs to be adjusted to hear a sound.  You can eliminate this complication by temporarily disabling the heterodyne part of the Volume control circuitry and forcing the VCA “on” while the Pitch pair of oscillators is nulled.

Use the clip lead supplied to temporarily connect the V+ line (the banded end of the zener D1) to the “high” side of the volume control (lug 3 of R83).  When using this trick, it is very important to set the front panel Volume Control to mid-range or slightly less.  The V+ voltage is almost double the normal maximum CV and too high a Control Voltage causes the VCA to saturate and effectively turns it off.  If you forget, don’t worry, nothing will be damaged by having the Volume set too high, you just won’t hear anything until you turn it down.

Set the Pitch Trim and Volume Trim controls on the panel to mid-range and the Pitch CV, Timbre and Velocity controls fully Counter-Clockwise.  Turn the slugs of all the coils L1-L4 fully “out” (Counter-Clockwise) just until you feel resistance – the coils can be damaged by applying too much force while turning the slugs.  From the fully “out” position, turn the slugs of the two coils L1 and L4 one full turn “in” (CW).  These coils will not be adjusted again.

Plug the Tmax output into an amp and turn both on.  During initial setup you may need to adjust the volume for comfortable listening levels or to prevent overload and distortion.  Make these changes with the amplifier’s volume control only.  Do not change Tmax’s Volume Control from it’s mid-range setting until told to do so later is these procedures.

Nulling Pitch

Slowly adjust the slug of L2 “in” (Clockwise) while listening for the heterodyne tones that slide down form a very high pitch then go through zero frequency and begin to climb again.  If you have the gain of the amplifier that you’re using set high, you may find a couple of places where tones can be heard, but as you adjust the slug continuously from one end to the other you will hear one in particular that is very much louder than the 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 interaction 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 “compressed” so that all the change in Volume happens over only a few inches of hand motion, try turning L3’s slug just slightly CCW, about 1/16 turn.  This will produce a slightly lower overall output, which should be compensated by turning up the amplifier’s gain control.  If you want still more hand motion range, turn the slug out a little more and adjust the amplifier gain accordingly.  You will know you have carried things too far when you still hear a tone with your hand on the antenna, but you should find a satisfactory range of control well before that.

With a tone audible, rotate the Timbre control Clockwise and observe that the tone gets considerably more “reed like” as you fade from sine wave to square wave output.  Return the Timbre control to it’s fully CCW position.  Advance the Velocity control fully Clockwise and observe that as your hand is rapidly removed from the Volume antenna the Gate/Trig LED comes on.  Also notice that the character of the sound now changes as your hand is withdrawn, getting “fuller” when your hand is moved rapidly and settling to a purer tone when you slow down or stop.