Debugging an EM theremin

Posted: 9/7/2011 1:39:05 AM
Rimantas B.

From: Vilnius, Lithuania

Joined: 8/15/2010

Hello again everyone,

as Fred suggested on the forum about a year ago, I stopped trying to build the theremin from and am now constructing the EM theremin.
As of now, I am pretty much there - I have the cabinet, the antennae, the front panel and the components are soldered in the circuit boards (I am using universal general purpose boards for the antennae circuits and a home-made PCB for the main board because I had difficulties finding suitable universal boards in my area).

On the article, it says that the collectors of all transistors should be at about +12V.
I get that on most of my transistors, but not on Q6 and Q8 - they get to only about 7.4V.
I also get the suggested readings on the emitters of all transistors except these two and Q7. Q6E and Q7E go to 0.04V, Q8E is at 0.95.
May anybody suggest anything to check?

I have checked and re-checked the board, soldering, etc. Of course, I might still have missed something, but maybe there is something more to check than simply whether the components are in their places?
Posted: 9/7/2011 2:17:20 AM

From: Colmar, France

Joined: 12/31/2007

Multimeters tend to show false values on oscillator stages when you try to measure DC which is combined with RF signals. What says the oscilloscope at the collector of Q6? Is the oscillator working (you should see a sine wave with about 22Vpp)? At which frequency (should be about 500kHz)?

If not:

Either there is a problem with the biasing of Q6, Q7 and/or Q8, so that there is ways too much DC current through L11

Or L11 has a problem itself with a too high resistance.

Disconnect the collector of Q8 temporarily. The volume oscillator around Q6 and Q7 must work without the external reactance stage around Q8 which is only needed for tuning. Tell us the voltages around Q6 and Q7 now. Check with the oscilloscope. Is the oscillator now working?

If they aren't still ok, take L11 off and check its DC resistance, it should be below 8 Ohms.

Let us know all this, so that we can go further in diagnostics.
Posted: 9/7/2011 3:53:11 AM
Rimantas B.

From: Vilnius, Lithuania

Joined: 8/15/2010

Thanks a lot for the quick reply!

I measured the DC resistance of L11, it was about 1 ohm, so I don't think this was the problem.

Will check the other things when I get back from university... And if I manage to get a 'scope (there are a few rarely used scopes where I work, hopefully no one will mind if I borrow one for the evening...)
Posted: 9/7/2011 12:24:27 PM
Rimantas B.

From: Vilnius, Lithuania

Joined: 8/15/2010

I managed to take a scope home from the laser lab I work in, it will probably make things easier.

I removed Q8 for now.
It turns out all the oscillators are working at full blast (26Vpp sine waves, pitch oscillators at 250kHz, volume oscillator ar 500kHz), however, the volume oscillator transistors are still at different voltages than recommended:
Q6C ~7V
Q7C ~12V
Q6E and Q7E ~0.07V.

What could I do next?

After soldering Q8 back in, the volume oscillator gets some nasty modulation in its waveform (the scope shows something like 2 closely, but not exactly matched sine waves)

Edit 2:
What a fool I was!
It appears most or all of the problems were caused by a pin numbering mismatch on the connector and the ribbon cable leading to the front panel.
As of now, the scope shows the oscillators nicely responding to the tuning controls and the audio output changes according to the waveform and brightness controls.
Will report when I finish tuning up (or reach another obstacle)!

Edit 3: So, it works! Or something like that.
Is it normal to hear many "voices" when tuning? (i. e. sometimes like 3 or 4 sine/quasi sine waves of different frequencies at once, some rising, some falling when I turn the slugs of the coils). I can find the "main one" which is the loudest of them, but the tone is very harsh - I do not have the scope with me anymore, but it sounds much more like a square wave than a sine wave, and turning the waveform control does not help much.
Any further advice?

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