Misbehaving Jaycar KIt

Posted: 8/13/2009 7:54:47 AM

From: UK

Joined: 8/8/2009

Ok I'm diving into theremin world with a cry for help! I've recently completed a Jaycar Theremin kit, not sure which version it is, is there an easy way to tell?

Anyway, I have some success and some hopefully almost theremin sounding noises from it. I have been unable to tune it according to the instructions and currently have both volume and pitch antennas effecting pitch. However I'm ignoring tuning issues for the moment as I have an obvious problem with my voltages - better get that sorted first I think!

Symptoms are as follows:

7805 output is 5.7v
IC2 pin 1 is 4.46v (closest I can get by adjusting VR2)
IC2 pin 8 is 2.73v <-Problem
IC3 pin 6 is 2.89v <-Problem
D1 cathode is 1.79v

As I mentioned the theremin is partially working but output levels on the speaker and line output are VERY low and it needs to be almost full volume to hear anything.

I've checked and double checked for solder bridges, dry joints and component orientation and all seems fine, although obviously something is wrong. I'm now getting pretty stuck.

Has anybody seen this kind of problem before with the jaycar kit? Any suggestion to possible areas for the fault would be most welcomed!
Posted: 8/14/2009 3:04:43 AM

From: Los Angeles

Joined: 3/8/2005

I have a Jaycar kit that does the exact same thing: both the volume and the pitch control the pitch. I have never been able to find anything wrong with my assembly, so I have always assumed a defective bit somehwere, but I would not know where to begin figuring out what is bad, so it sits in a box.
Posted: 8/14/2009 1:26:20 PM

From: London, UK

Joined: 6/5/2007

Can you take out IC2 and measure the voltage on pin 8? This pin should be 5.7V so if its still low, check this PCB track for shorts.

Please post back your results.


Posted: 8/15/2009 6:27:33 AM

From: UK

Joined: 8/8/2009

Hi MarkT, thanks for the input.

I've been tinkering around and have now managed to get 5.7v on pin 8 of ic2 and pin 6 of ic3 as expected.

However the output volume is still pretty low. I think I may also have a grounding issue as I'm getting a fair bit of radio interference noise on the circuit. This gets worse and the volume dramatically increases if I touch the metal of VR1 in any way. I'm going to change all the wiring to VR1 to see if different type / length makes any difference.

Stay tuned.
Posted: 8/16/2009 8:29:53 PM

From: Colmar, France

Joined: 12/31/2007

Did you make the wired connection between the grounding point of the circuit board and the housing of VR1?

Did you try to make a good grounding connection between the theremin and something which is really good grounded, i.e. a heater?

Output volume will also be low when you operate this theremin on a metal workbench or a table with metal frame and legs. Try using it in a different, metal free place.
Posted: 8/17/2009 9:41:04 AM

From: UK

Joined: 8/8/2009

Yes the case of VR1 was is grounded and I've been testing the unit on a wooden table well away from anything hugely conductive.

I've replaced the wiring to VR1 and there was a great improvement, volume greatly increased and noise removed. Connecting the line out to an external amplifier has eliminated any remaining noticeable circuit noise.

So now I have everything mounted in the case and I've had a chance to try again with the tuning with surprising success!

I've got the volume antenna now just controlling volume, although the range from full volume to silent is probably only around 3 - 4 cm.

I've also got a good pitch range (high to low at antenna) but again the range is compressed with the full range covering only about 30cm from the antenna.

I now need to expand the fields to make it more playable although I'm a little reluctant to start fiddling with the tuning again now it's "working"

Is there an easy way to expand my fields a bit that isn't going to mess up the tuning?

On a slightly unrelated note, the tuning instructions tell you to tune it with low at the antenna and high away which seems the opposite to what I'd expect. Can these kits be tuned the opposite way round or is there a technical reason for this oddity?
Posted: 8/17/2009 11:12:27 AM

From: Colmar, France

Joined: 12/31/2007

If you do not want to add another pot for pitch tuning, you may do so by modifying slightly the pitch antenna length: Tune the theremin's coil with the last antenna element only half pulled out. Then you may do fine tuning by pushing/pulling it by little steps of 1/10"...

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