Jaycar kit - pitch circuit affecting volume circuit voltage

Posted: 11/26/2012 12:17:24 AM

Joined: 1/14/2011

Hi all, when tuning a Jaycar kit, is it normal that adjusting the volume circuit (T1 & T2) affects the volume circuit (T3, T4 & VR2)? I think the answer is "no", but I'd like to check. Here's what I've found so far...

Kit: Jaycar MkI, with Max Baars' mods (http://thereminworld.com/article/14272/silicon-chip-theremin-modifications)

Tuning procedure: Started with the slugs of all coils at the top. Following Max's instructions, I turned the slug in T4 until the voltage at pin 3 of IC2 was at maximum voltage (about 2.9 volts) and adjusted VR2 until the voltage at pin 1 of IC2 read 3.5 volts. Moving my hand closer to the volume antenna made the voltage drop off to zero. No sound from the speaker so far.

I then turned the slug in T2 until a sound came from the speaker. While doing this, the voltage at pin 1 of IC2 dropped and the sound became choppy, like a motorcycle idling.

I think there's something wrong with the pitch circuit, does that sound reasonable? If I adjust T4 or VR2 so that the voltage at pin 1 of IC2 reads about 5 volts, I can get the pitch circuit working, but then the volume circuit either doesn't work at all or moving my hand near the volume antenna makes the sound "choppy" again.

I applied all the modifications at once, so I can't give a good before/after comparison, sorry. The only test equipment I have is a multimeter, too, so I'll be limited as to what I can test.

Thanks in advance!

Posted: 11/26/2012 9:53:06 AM

From: Colmar, France

Joined: 12/31/2007

The "search" function would have guided you to this posting. :-)

It is very important that the volume oscillator's free-run frequency is at least 7kHz lower than the one of both pitch oscillators. This will prevent both circuit parts (pitch and volume) from interfering.

Posted: 11/26/2012 9:27:36 PM

Joined: 1/14/2011

Hi Thierry,

Thanks very much - I've got that post saved, but most of the steps use an oscilloscope. I think it's time to get myself some proper test equipment!

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