# Checking EM Thermin Circuit and Tuning/ An Induction Question

Posted: 2/9/2008 11:06:08 PM

Joined: 9/25/2007

Hi,

I've just finished assembling the EM Theremin described by Moog in the DIY article. I used my meter to check for everything in the "checking it out section", and I observed everything except for -.6VDC at the emitters of Q6 and Q7 (I did however read 12VDC at the collector of Q6 and Q7). Also, I read 7VAC across L11 instead of "about" 10VAC. Is the 7VAC alright (at L5 and L6, I read 10VAC)? I've checked over the section where Q6 and Q7 are (volume oscillator), and I haven't found any mistakes. Does anyone have any idea what could be going on? One thing I've been wondering about is that I mixed up the 5mH and 2.5mH inductors for the volume antenna circuit. I think I have them in the correct order, but I'm not sure. Would those 5mH (Bourns #6304) appear to have a larger diameter of windings than the 2.5mH. That's what I based my guess on, so I used the ones with a larger winding diameter for the 5mH inductors. Also, L11 is a Coilcraft inductor that's a substitute for the one in the plans. I think the Q-min is much lower: 30 instead of 60. Could that have anything to do with my lack of reading at Q6 and Q7?

Thanks,
Dan
Posted: 2/10/2008 7:46:51 PM

From: Colmar, France

Joined: 12/31/2007

At the emitters of Q6 and Q7 you should have between -0.6 and -0.8 V, if not there is something wrong and the volume oscillator is probably not working correctly. But the most important is that you may be able to adjust L11 so that you get a voltage between 0 and -12V on pin 12 of U3 when you approach your hand to the volume antenna as described in the article.

Concerning the coils L7 to L10: The only important is that L7 is a 2.5 mH coil. The other three coils are in series, so their order does not matter.
Posted: 2/12/2008 7:10:18 PM

Joined: 9/25/2007

I have observed that when I touch the volume antenna, the pitch goes down about 200Hz, with no effect on volume. Does this mean anything significant?
Posted: 2/12/2008 7:22:05 PM

From: Colmar, France

Joined: 12/31/2007

The pitch variation is only a side effect. In the meantime I'm almost sure that your volume osc is not working or at least totally besides normal parameters. Could you please measure the DC voltages (against ground) on both leads of L11 and on both bases of Q6 and Q7 and tell us the results here?
Posted: 2/12/2008 7:49:02 PM

Joined: 9/25/2007

My cheap meter reads about +12.8VDC on both leads of L11. This is the same voltage distributed throughout the circuit as +12V, and it works for the pitch components, so that seems right.

On the collectors of Q6 and Q7, I read the same +12.8VDC.

On the emitters of Q6 and Q7, I read 0VDC, and they both had continuity to ground.

On the bases of Q6 and Q7, I read 0VDC.

Thanks,
Dan
Posted: 2/13/2008 10:59:04 AM

Joined: 9/25/2007

So after I got up this morning, I tried the theremin and it worked! I didn't do anything to it last night that I think would have changed the volume response, but who knows?!? Now I'm just waiting to get an amp so I can really try it out :) (the whole thing produces no sound if I don't have anything hooked up to pin 12 of U3 when I play it through my laptop or regular speakers.)

Should the volume drop all the way to zero when I touch or go near the antenna? I sacrifice a fair amount of maximum volume if I tune it this way. Technique wise, does one touch the volume antenna during performance. If I touch it, I get a slightly audible tone when playing lower notes.

One thing I discovered is PC based oscilloscope called Zelscope that works very well for tuning- it can be set to display the frequency. It was very interesting watching the wave shape while changing the timbre controls.

Thanks,
Dan
Posted: 2/13/2008 3:59:14 PM

Joined: 9/25/2007

After using it with the Moog TB15, there is still a slight problem: I can only get any audio if pin 12 of U3 is connected to ground, and in that case, the volume can't change. This is how I had it set up earlier, but with regular speakers. I can however change the volume if I have pin 12 of U3 connected to one lead on my multimeter when it's set to read DC voltage. The volume also works when I have the multimeter hooked up from pin 12 to ground. I expected that the signal for the amp would work, and I'd get audio without any connections around U3. The plans stated how to test it with regular speakers, you need to ground U3, which I was doing. That made me think that if I use it with an amp, I could just use it normally without those test connections. Does anyone have any idea what's going on here?
Posted: 2/13/2008 6:05:09 PM

From: Colmar, France

Joined: 12/31/2007

Yes, your volume oscillator doesn't work, that's all. In the moment, my work doesn't let me think a lot about this, but I'll come back with a systematic way to check this out.
Posted: 2/13/2008 7:00:33 PM

From: Colmar, France

Joined: 12/31/2007

OK, here in France it's now 1 AM in the morning and I'll take some time to help you finding the volume osc. problem.

At first: Pin 12 of U3 is the VCA processor output. That means that If it is at 0V, the theremin will play with max output volume, at -12V, it will be almost muted. So you may (in the meantime) shorten it to ground to get the maximum output without volume control which will allow you to play (adjusting the volume at the amp) until we found the mistake.

Now, let's see for the volume osc. circuit: Q6 and Q7 are the "real" oscillator transistors in a pseudo-differential-amplifier circuit where one transistor helps stabilizing the other. The circuit around Q8 is for tuning.

At first I would unsolder Q7 and Q8 which will prevent the oscillator from working and allow to verify the DC setting of Q6. Without Q7 and Q8 you should get around 12V DC at the collector of Q6, 0V DC at the base and -0.7V DC at the emitter.
Now, if the values above are correct (if not, post the values you measured here and do not go further) unsolder Q6 and resolder Q7 (still without Q8) and you should get the same voltages on c, b and e of Q7. Let us know the results and we'll go a systematic step further.
Posted: 2/13/2008 7:15:53 PM

Joined: 9/25/2007

I'll definitely do that and let you know the results. However, I was able to adjust L11 so that the DC voltage at pin 12 changed from -12 to 0. I just have no control over the volume without one lead of the meter hooked up to pin 12 and the other ground and on the meter must be set to reads volts. Does this really mean that the volume oscillator is not working?