Diagnosing Jaycar/EPE/Silicon Chip/Kees Enkelaar Theremins

Posted: 3/8/2010 8:39:19 AM

From: Colmar, France

Joined: 12/31/2007

After I'd been contacted by Email by someone who has a non-working Enkelaar Theremin, I wrote a reply which allows diagnosing systematically all Theremins mentioned above since they work all based on the same circuit.

For me as a non-English native speaker it was a big task to write down all that and I think I should share that here so that others will also make use of it:

[i]diagnosing theremin problems is like diagnosing old analog RF circuits. That means that you have to verify the operation of each stage and tune it. So an oscilloscope with a minimum bandwidth of 5MHz, a frequency counter and a multimeter are required. As soon as you find that a signal is missing or tuning is not possible by going through the instructions below, stop and repair the circuit part on which you are actually working or come back to me with more detailed information, telling me at which point you do not get the wanted signal or cannot do the needed adjustment.

a) Turn all control knobs on the front panel into mid position (12 o'clock), make sure to work on a wooden table and that all metal parts (tools, cables etc.) are as far away as possible from the antennas.

b) Check if all three oscillators (fixed pitch, variable pitch, volume) are working.
- fixed pitch oscillator around Q1: Check with an oscilloscope that there is a signal of about 460kHz and at least 0.5Vpp at Pin 1 of the demodulator IC1 (MC1496)
- variable pitch oscillator around Q2: identical check at Pin 10 of the demodulator IC
- volume oscillator around Q3: identical check at the base of Q4

c) If all oscillators are working, make sure that the beat frequency is in the audible range.
- Check the frequency at Pin 1 of the demodulator with a precision frequency counter and write it down
- Check the frequency at Pin 10 of the demodulator with a precision frequency counter and adjust the white coil T2 near Q2 until f(Pin10)=f(Pin1)-800Hz

d) Then make sure that the volume CV circuit is working.
- Check the frequency at the base of Q4 with a precision frequency counter and adjust the white coil T3 near Q3 until f(bQ4)<=f(Pin1)-7kHz
- Adjust the black coil T4 near Q4 until you get 1.7V at Pin3 of IC2 (LM358) while all metallic parts, tools, hands, multimeter cables etc. are far away from the volume loop!
- Make sure that the voltage at Pin3 of IC2 decreases when you approach your hand at less than 10cm / 4 inches from the volume loop
- Adjust the nearby potentiometer to get 4.3V at Pin1 of IC2 while the voltage at Pin3 is 1.4V

e) Check the AF section.
- Recheck the frequency at Pins 1 and 10 of IC1 and make sure that there is a difference of about 500Hz - 1kHz
- Check with an oscilloscope at Pin12 of IC1 if this difference frequency of about 500Hz - 1kHz appears with at least 250mVpp
- Same check at Pins 5,6 and 7 of IC2 and at the audio output.

f) Fine-Tune the white coil T2 and eventually the "environment" knob on the back side in order to obtain an optimal pitch response.[/i]

I added in my reply that as soon as there would be a problem when going systematically through these instructions, one should stop and recontact me (or post here) telling what is not possible at which point, allowing a deeper analysis of the problem.

Happy diagnosing!
Posted: 3/8/2010 9:23:23 AM
Jeff S

From: N.E. Ohio

Joined: 2/14/2005

If it's not making any sound at all, the first place to look is the JFET transistors. They have been known to be a problem and susceptible to static electricity.

"The Transistor that would be causing the problem would be a JFET 2N5485 there are three of these so it would be best to replace all of them" - Kees Enkelaar

Posted: 3/1/2012 1:14:30 AM

From: Florianópolis, Brazil

Joined: 2/1/2012

Hi, Mr Thierry!

I am theremin enthusiast from Brazil, already have a Etherwave clone known as the RDS, and just bought my first Jaycar MKII kit ... A few hours ago I finished soldering and started testing ... My electronic skills are in a very low level and I do not have an oscilloscope, but my MKII is already making sound ... But it seems to sing with two voices, one voice is the normal theremin, the second is a very high tone, seems to change in pich while my hand gets closer to the volume antenna. I have noticed that this second (and annoying) voice stops whenever I touch VR2 ... Grounding? Any clue how to solve it?

Thank you ... And sorry for any bad english ... I read better than read ...

Posted: 3/1/2012 1:20:58 AM

From: Florianópolis, Brazil

Joined: 2/1/2012

I almost forgot... Is this theremin's pitch range area this short (about 6 inches) or have I forgot to tune something??

Thanks again!! 

Posted: 3/1/2012 11:36:00 AM

From: Colmar, France

Joined: 12/31/2007

If it sings with two voices, that means that the frequencies of the pitch oscillators and the volume oscillator are too close. In order to prevent this, you should follow step d) with the help of a precision frequency counter.

If you do not have such a device, you can proceed as follows:

- Pull the telescopic antenna fully out

- Turn both T1 and T2 half a turn counterclockwise. Fine-tune T2 so that you hear a sound, even when the pitch field seems too long, too short, inverted or whatever for the moment

- Turn T3 half a turn clockwise.

- Tune T4 for 1.7V at PIN3 of the LM358

- Shorten the length of the telescopic antenna by 10cm by pushing the upper (most finest) element downwards while all other elements should remain fully pulled out.

- Fine-tune T2 for a ~ 30cm/6inch pitch range.

Never again touch at T1 - T4. You may now fine-adjust the pitch field by pulling out or pushing in the top element of the telescopic antenna. Take care: a few millimeters will make a big difference.

Posted: 3/1/2012 2:34:51 PM

From: Florianópolis, Brazil

Joined: 2/1/2012

Thanks a lot, Mr Thierry!!! All worked perfectly!!!! Singing like a bird, and now with improved pitch range!!! My next steps are to finish a new housing for it, I'm using brazilian native woods (soon I post the pictures), install etherwave antennas and study Jason's mods to improve setings and dispose them on front pannel (add more pots to control brightness, waveform or whatever its possible)... I'm actualy accepting suggestions!

Thanks again!!! 


Posted: 3/2/2012 6:41:51 AM

From: Colmar, France

Joined: 12/31/2007

My recently updated suggestions for mods are here.

Sorry for the bad drawing quality - I tried to use a pen based drawing and annotation software on the iPad. The result is not (yet) very convincing...

Posted: 3/2/2012 8:26:33 PM

From: Florianópolis, Brazil

Joined: 2/1/2012

Woah!!!! You`re the man, Mr Thierry!! I was very worried on adapting Jason's mods (wich are made for the 2006 model) to 2009 model's schematics, these your suggestions are going to ease a lot my work!!!!

Question: Each page is a independent mod so that I can apply and test each one separately?

I'm asking help to a friend to make a PCB design to your waveform/symetry schematic... Don't you have a picture or drawing of it?

Thank you very much (again)!!!

Posted: 3/2/2012 8:35:11 PM

From: Florianópolis, Brazil

Joined: 2/1/2012

Question #2: Part 4 mods, on the mais circuit mods... Some resistors will have to be removed... Do I jump this contacts or only remove the component?

Posted: 3/3/2012 8:21:31 AM

From: Colmar, France

Joined: 12/31/2007

Answer 1: yes, these mods are independent. After choosing which of these you want to apply, you should proceed step by step. This will make diagnosing easier in case of problems.

In my eyes there is no need for a PCB. I built that waveform/symmetry circuit on a kind of Veroboard. That's a PCB with single copper spots in 0.1inch raster which allows to solder the components and to make the connections by hand.

Answer 2: the resistors have simply to be removed. Jumping them would cause the MC1496 to go up in smoke!

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