Theremax custom case and antennae concerns

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Theremax custom case and antennae concerns

Post by PAiA-Scott »

Jan Baumgart wrote:
> ok, i just realized that most of the paia talk forums are locked.
> I nearly went nuts searching for the "new topic" button ;))
> So here are my two questions:
> 1.
> When turning the timbre control fully clockwise, there's no more audio signal.
> The Volume seems to fade out within the last few millimeters of the turn.
> This is with 2 of 4 theremins. The other two are fine.
> Any idea ?
> 2.
> We are currently working on a custom case for a mobile theremax :)
> For checking the playability, we tried to connect the case-model's antennas to the theremax board
> with longer shielded cables. With no success.
> Are there any restrictions on the antenna cables ?
> Do we need to replace components on the board to compensate for the length/capacity of the cable ?
> Many thanks in advance !
> cheers,
> jan.

Hello Jan.

Yes, I wondered if I should mention that to you. The Jabber section is the only one open for posting. I'll copy this message over to the Service Q&A (Questions and Answers).

Take a look to see if the bare, solid wire for the point to point ground wiring is touching the lower terminal of the Timbre control. This would stop the wave at the cw end of the rotation (the control rotating clockwise, puts ground from the middle terminal to the top terminal where the sine is to let through only square, and vice versa, or with combinations at in between settings).

If this is not it, maybe L2 is stopped short of the real pitch tone and at a weaker harmonic. The method demonstrated at the YouTube link copied at the end of this message, and described in the tips/suggestions in the technical bulletins section of this forum, make this less likely as the volume cv isn't exaggerated as it can be using the alligator clip test lead as described in the manual. Try going more inwards with L2 for a louder Pitch Tone. If none encountered, try to find a more inward initial setting of the slug for L1 that will make a tone with the range with L2. Try 1/2 turn increments for L1. The more the slug is in, the stronger the oscillation from the oscillator circuit. A weak pitch heterodyne could be the reason for the loss of sound with cw adjustment of Timbre.

Wires laying against or too near the capacitors, resistors, and transistor in the oscillators for the pitch pair might be loading and weakening one of these which combine for the pitch tone (circuits including L1 and L2), or the pitch antenna cable ends. The pitch antenna should not be too near an appliance such as a lamp or bench equipment, or metal rod or pole of a shelf or table leg running down to the floor.

Check the Pitch Trim control connections on the panel. R79 should have the same code marked on back as R80 (the 1k marking is on the front of the brown layer towards the panel. The soldering for the red-vio-brown-gold 270ohm R85 must be solid and for wires B(upper terminal) and A (middle terminal) and the brown-green-red-gold 1500ohm that links down to the ground circuit at J5-S(leeve) below. Nudge each of the bare wire ground links from sleeve terminal to sleeve terminal to be sure one isn't loose.

V+ being 8.2v (within 0.1 v or so) is important too for the biasing of the transistors in the oscillator circuits and the Q8 and Q9 amplifier stages.

The next area for for a second look is area between the oscillators and the VCA/IC2amp. Inspect and confirm the printed-circuit side of the board to be sure the path from the pitch pair of oscillators through yellow-violet-black-gold 470ohm Rs 5, 6, and brown-black-orange-gold 10k R11 and on to Diodes D3 and D5 is correct. Yellow-violet-red-gold 4700ohm R26 and 221 marked 220pF C22 join the pitch heterodyne over to the Q8 amp stage through capacitor C23. Inspect the solders for this part and it's polarity (these taller parts can work like a lever against joint and printed-circuit. C29 is another, after the Q8 transistor amp stage for the pitch heterodyne signal. A yellow-violet-yellow-gold 470k at R29 and an orange-white-orange-gold 39k at R31 connect from the V+ supply to ground to bias the transistor so it's collector is a dc level in the range of about 2-6 volts. The voltage for the collector is from yellow-violet-orange-gold 47k R30. The audio signal into the base of Q8 runs towards orange-orange-brown-gold 330ohm R32 and 100uF C42 and the transistor amplifies this to be a stronger audio voltage on the collector. Frequencies from the high audio range and beyond go back through C27 and cancel but the desired audio goes on through C29 towards the Timbre control which is really just a variable shunt. It steers either the near-sine Q8 signal or the pulse train from IC1A away from the vca/audio-output-amp section that follows.

I have done limited experimentation with using longer wires attached to the existing antennae and learned that it was necessary to keep it supported up off the floor (I used cardboard boxes). Like this, movement within the small room caused changes in pitch. Be sure your antennae mounts aren't too near the floor or conductors to the floor, but also don't overlook the previously mentioned possibilities as to reasons for less than optimum oscillation amplitude. Look for signs of the shield wire having melted through the insulation of the internal wire where they separate and solder at the board. This (or it being smashed) could exaggerate the loading of the cable. The specifications for the RG-174u (8216) we provide for the antennae cables is linked below. In general, it is 50ohm impedance and about 30pF per foot, about 0.1 inches in diameter. It has an attenuation rating of just over 1dB for 100 feet at 1MHz so connected to Theremax there shouldn't be any problem with lengths of 10 to 20 ft or so. If you find that your case/antennae/cabling does adversely load the attached oscillator, then the 56k resistor at the emitter of the oscillator circuit transistor can be substituted with one of a lower value to present more energy to the oscillator tap (ie 47k, 33k, 22k, etc...). Before making any changes here though, try tuning and operating with the antenna cable detached (and moving the hand towards and away from the first (pitch) or fourth (volume) oscillator parts, then with the cable and no antenna, then, with the antenna. ... ut=english

YouTube video of Theremax QuickTuneMethod (QTM):

Example of the Theremax tone: <>

Tips/suggestions: <>
Scott Lee