Debugging My Circuit Board

I completed building my theremin from jaycar today.

Unfortunately it's just not working. With the multimeter I'm seeing that power is flowing, good first sign, but that's about it. I tried the two things the instructions suggest checking, the transformer casing and two of the chips, and nothing.

When I poke around randomly with the multimeter everything seems to match up (I'm going to do a systematic one by one tomorrow if I get no better ideas). but what I noticed is when checking between the power and the positive wire going to the speaker, every once and a while I'll get static coming from the speaker. Quite, in fact, at first I thought it was just the sound of the meter tip scratching the metal on the pin, but realized it's actually from the speaker.

Any idea what to do in this case? (I now understand why people get them prebuilt. I'll stick with my Rubik's Cubes, they're easy, heh.)

Now a "confession" I did have the speaker humming earlier when I jiggled "VREG 7805" in the top left corner (looking at the printed sign, orienting up by the Jaycar title), but it stopped doing that, but I've check that out, juice is still flowing, I didn't break anything with my jiggling, but don't know how that is, or isn't involved.

Any ideas?

Hi, do not worry, you will be up and running!

Check all polarised components are the right way round.

Check all soldered joints, check them again.

Check the voltages at the test points as mentioned in the instructions.

Post the results back here.

Regards

MarkT

I'm no expert, so take this for what it's worth.

I believe simply building a theremin is only the first half of the battle. The solder joints may be good and juice may be flowing throughout the circuit. However, the next and trickiest part is tuning the circuits to create a functioning theremin. Since you have not said you've done this, I have to go on the assumption you haven't. I cannot tell you the best way to go about it. Perhaps someone else can.

The fixed and variable oscillators of the pitch circuit will need to be tuned to each other. The volume oscillator will need to be tuned so that it does not lock to or interfere with the pitch oscillators.

From what I've seen, there is at least one pot/trimmer and four variable inductors to accomplish the circuit tunings. The front panel knobs should be set to the middle position for the initial tuning.

Hopefully, someone else will expand on this. Good luck!

Normally with these, when first completed you get some noise (other than static) through the speaker, the tuning part comes later on.

Maybe building one is only a quarter if the battle? Get it working is next, then tuning then learning to play?

But really check the components are the right way round first.

Regards to all

MarkT

Hi MarkT, thanks for the advice and encouragement.

I've checked that the polarized components are facing the right way, as far as I can tell.

I checked that all points are soldered, and that when the theremin is on, there is a complete circuit from the power to every point but one.

If you look at the board so the printed side is up and Jaycar is upright, the far right upper corner has 1N4004, the bottom (toward the wires) joint there doesn't register as having any charge.

I tried the test points in the instructions, and got absolutely no result.

I do not want to offend anyone, but I think that building and debugging a theremin without enough knowledge in RF electronics and without an oscilloscope is like eating a schnitzel with the fingers instead of a knife and a fork...

A unadjusted theremin may for example be working well with the only default that the output frequency is outside the audible frequency range. You might think that it isn't working at all...

So my suggestion for Gesi: Contact me by Email theremin(at)tfrenkel(dot)com in order to get my address and send me your circuit board to France for checking/debugging and pre-adjusting. After you get it back you will only have to put it into the case, solder the peripheral components and do some fine tuning.

I will not charge you anything for this service, except for eventual spare parts which you might have grilled and shipping fees for sending it back to Canada.

BTW: Today I took the time to finish my Jaycar kit.

Balance:
Circuit board: 75 min.
Wiring external components, screwing all in the housing, testing: 60 min. (Worked from scratch)
Tuning: 10 min.
Fun factor: for ever...

Dear Thierry,

I have no knife and fork. Please will you eat my schnitzel.




:-)

[i]Please will you eat my schnitzel.[/i]

If it's a kosher one, why not?


:-)))

Thanks for the offer Thierry, I'll keep it in mind, but for now I'll be holding off.

I tinkered around with my theremin, and I now get a charge running from the IC points as the instructions say, good sign, but I'm still not getting anything from the transformer coils. Any ideas on that?

I'm beginning to worry two things. That I may have fried the circuit board by accident, or that when I wiggled the 7805 in the corner before I damaged that. Anyway to test if I did either of those?