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Post coils???.....Q6 capacitor
Maybus Doomsday wrote:
> I finally got around to putting together my theremin and it
> worked fine, But a few days later it when out, Looking at
> all the things that could be wrong with it, I'm pretty sure
> that I got my theremin a little to close to a magnetic field
> and the oscillators when funny. How can I order 4 new oscillator coils?

Hello Maybus.

I have included a pdf file of tips and suggestions that might help you get your kit going again. Maybe the shield wire is touching the antenna mount, or some other connection is making/breaking when it hadn't before.
The antenna of an am radio can be used to 'hear' the oscillation of each to confirm if they are on the similar tuning ranges. If the oscillators in a pair can be tuned to the same frequency, they should work to produce the beat or heterodyne frequencies for the pitch tone and volume CV (control voltage) to vary the volume of the pitch tone.

Please let me know what you find.
Thank you.

Sincerely, Scott Lee


Maybus Doomsday wrote:
> I took your advice.
> I stared by looking at all the wires and connections. And they seemed fine, But I replaced or took the ends off of some of them and soldered them back together just in case there was something that I was not seeing.
> Still could not get the theremin to make any sounds at all. The only thing I can hear is a faint hum and some radio station that I can't make out. When I place the jumper wire from D1 to lug 3 of R83
> I used the radio trick, and have found that I can make the AM radio go from a sort of high pitch to no sound at all by turning the L2 oscillator, And the L3 oscillator makes the same kind of sound but it doesn't go silent. Both at a little under 800 kHz. But the L1 and L4 oscillators don't seem to want to make any sounds at all.
> i must be doing something wrong or over looking something. I really don't know what to do next.
> I also found that if the velocity is turned up all the way. The gate/trig light turns on when I first power up the theremin then it goes off after a short time. I'm unsure if that happened before, or if there's something wrong with that too?

Hello Maybus.

Can you send a photo viewing the top and bottom sides of the board, like linked below? I might be able to spot trouble and offer advice as to such.
Here is the soldering and then a back view of the same Kit/FPA accessory that was through here for service: ... dering.JPG ... Repair.JPG


Here is a link to the PAiA Talk site and if you scroll down to the second message there are photos of the top and bottom of a an assembled circuit board:


Thanks. --Scott


Maybus Doomsday wrote:
> I took some pictures of my theremin and was going to send them to you. But when I was looking at the pictures you sent me, i saw that my board did not look the same as the normal one. I found that the antenna mount and some other wires ends came off the board somehow. just enough where i could not detect them; until i enlarged, by taking a picture. So i made some jumpers and got power to them that way. my theremin seems to work fine now.
> The only thing is. is that I can hear a very faint radio station coming out of my speaker, and some times the tuning seems to shift a little. i wonder if that's normal or if there is something i can do to lessen that?
> In any event, thanks for all your help i really appreciate it.

Hello Maybus.


Do you operate Theremax with the Pitch Pair tuned to null, so there is no sound until moving the hand to the Pitch antenna? If so, try it with the pitch pair set for a lowest anticipated frequency, with the hand to the volume antenna keeping things quiet until needed. When the pitch pair is teetering on the brink of null, noise such as interference from a radio broadast can push the pair in and out of null according to the program, exaggerating the noise. If you prefer having the pitch tuned at null, the gimmick capacitor (or just a very low pF one, ie 5pF) soldered to link the pitch pair emitter resistors can help to minimize the in/out of null noise. Following are some checks for possible trouble.

Maybe L2 is stopped short of the real pitch tone and at a weaker harmonic. The method in the tips/suggestions make this less likely as the volume cv isn't exaggerated as it can be with the alligator-clip test-lead. 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 can make background noise more apparent.
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.

If you visit the PAiA Talk forum, there is an example of the Theremax tone:

viewtopic.php?f=9&t=37 <>

Thank you.

Sincerely, Scott Lee


Maybus Doomsday wrote:
> I made the changes that you talked about in your last e-mail and my theremin started sounding great. really smooth and clear.
> So I took it down to the local community college and the music professor there, had me introduce the theremin to his class. and he said that he had not seen one since 1989.
> it was a really big hit, and a lot of students got to play around with it.
> But for some reason if i turn the timbre or volume up to much, or after about 20 minutes of playing or so, There seems to be some kind distortion, or feedback of some kind; like static. and I'm not sure what that is. or how I can fix that? (i been turning my theremin off then turning it back on again, and it seems to go away, but it always comes back.)
> By the way what is the pitch CV and Velocity nobs all about? When I turn them nothing really seems to happen? but when I turn up the timbre there is a big change and it sounds really cool.

Hello Maybus.

Great! I'm glad it's working for you now.

There's a lot of energy in the pulse trains that are output from the comparator stages in the f-v (frequency to voltage) conversion circuit sections and when the panel Volume (audio output level) control is fully advanced a feedback condition can sometimes creep in where the pulse train gets into the audio output. Keeping the Volume control at about 3:00 or less makes this condition less likely to occur, but a capacitor can be tack-soldered to the Q6 transistor to help prevent it too. If you want to try this, use a 0.1uF and trim and form its leads to fit on the bottom of the board from the Q6-collector solder joint (transistor pin-out is emitter-base-collector left to right viewing the flat face, legs down) to a nearby ground solder joint: There's a string of ground joints for nearby parts R27, D14, C25, D13, etc...

The Pitch CV and Velocity CV controls adjust amounts of these control voltages that appear at the 1/4" output connectors. When interfaced as a controller for devices with cv inputs, the controls work to vary the range of the volume cv from none to a maximum of a 4 volt range according to the Theremax pitch and velocity parameters. The panel Volume control adjusts an amount of volume cv that goes to the volume cv output, AND, the theremax vca circuit section which does the actual variation in audio-output level. The Velocity CV is also introduced to the vca section and works to throw it out of balance which distorts the sine wave at the ccw Timbre setting--rapid volume changes make velocity and this velocity can then change the timbre as can occur with rapid or more intense playing of a traditional instrument.

Here is an image of the above-mentioned, added capacitor:

Q6_0.1uF.png [ 454.97 KiB | Viewed 9285 times ]

Let me know if you need me to mail one of these capacitors.

Thanks again. --Scott

Scott Lee

Tue Mar 02, 2010 11:57 am
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