Diagnosing Jaycar/EPE/Silicon Chip/Kees Enkelaar Theremins

Hi, Thierry! 

I've checked the PCB and found very easily all changes from your schematics... Applying them seems prety quick!! And about the ICs on mod 4... They are both LM358, right? This week I'm finishing the cabinet, so soonly I'll show it here... As said before, I've used a redish  brazilian native wood but with some pine wood details, inspired on Moog EW design... As I'm ordering as soon as possible the components (and they may take up to 2 weeks to be delivered where I live), I should post a preview picture before installing pots and knobs as well as the PCB and wiring...

Thank you again for all your help!!

The IC in mod 4 is a LM358. This IC has 2 OPamps inside. Thus you will only need one IC (see also the pin numbers in my drawing).

Uhm! Understood... Thanks again! I decided to make a post listed as "Another Jaycar Mod ".. Check it out!

Hi, Thierry, I know this topic may not be the right one for this question, but where is the best place on MKII's schematics to place a (on/off) led?

Ps. Cabinet is almost ready! Pics soon!!!

Connect a 390 Ohm resistor to TP1 and the LED from the other end of this resistor to ground.

Hi, Thierry! Hope you're following the changes on my theremin! I'm just about to apply mod 4 of your schematics (other 3 were succefuly applyed!) and reading it more carefully, I've noticed that it has no indication of where connect the 1,5k resistor of symetry schematics... It goes from 5k pot to +9v, right? Thanks again!

Yes that's right, I forgot to write +9V to that end, but it's like in the schematics above.

@Thierry: "For me as a non-English native speaker it was a big task to write down all that"

And what a great job indeed you've done of it! Votre anglais est bien meilleur que mon français!

:-)

Hello,

First, sorry to dig out this post which was old, but very useful ;)
My jaycar theremin doesn't make sound, I checked multiple times and every components are in the right place.
I tested all three oscillator, and the volume one seems to be faulty, I don't have any signal at Q3 and Q4 base, the problem comes from the IF transformers, am I right? No signal on T3 and T4 pins either.
So should I replace both IF transformers?

Thanks for your attention,

Bastien

First of all, I'd check the DC voltages around FET transistor Q3: ca. 0.1V at the source pin, 0V at the gate pin, and ca. 5.6V at the drain pin. Please write back what you found.

Then, I'd check the white coil assembly T3 with a multimeter for resistance. You should find a low (<50 Ohms) resistance between pins 4 and 6, between pins 1 and 2, and between pins 2 and 3. You should find an infinite resistance (open circuit) between either pin 4 or 6 and either pin 1,2, or 3. Please write back what you found.

The coil assemblies normally do not break if they aren't overheated during soldering. If the volume oscillator works, you should find a ca. 460kHz sine wave with an amplitude of 300mVpp on pin 6 with an oscilloscope. As long as you do not have that, there is no need for further investigation around Q4/T4.