MC1496PG no output?

Hello all.
I have just constructed my first Theremin!
I have been tuning T2 pitch oscellator and have no sound.
I think that C100N (.1Uf) was shorting my carrier + to ground. With it out of the ckt I can scope a carrier signal at pin8 of IC1.
However, I still have no output to IC2A from pin 12...
I feel that the balanced modulator chip ( MC1498PG ) has failed. Is this reasonable, or could something else cause no output to my preamp Ic?
I await the twangy magic to begin.
Thank you

Woops!
I forgot to mention I'm building the most recent Jaycar (Silicon Chip) model.

The MC 1496 does normally not fail. Did you build the theremin from scratch or did you buy a kit, i.e. from Jaycar? In the latter case, which is the kit number (there are several revision)?

Although the MC 1496 is a balanced mixer, it is used in an unbalanced mode in the Silicon Chip theremin. So pins 4 (Signal-) and 8 (Carrier+) are virtually grounded through both 0.1uF capacitors. The signals to be mixed are applied to pin 1 (Signal+) from the fixed pitch oscillator and pin 10 from the variable pitch oscillator.
If both input signals are present but you see no output signal, it's most probably an issue with the gain and bias setting of the MC 1496. Please check the resistor between pins 2 and 3 (gain) to have really 1k. Then check for correct bias current, which is applied to pin 5 through a 6.8k resistor. At its other end (not at pin 5 of the MC 1496) you should have 4.3 V for a maximum output amplitude provided from pin 1 of IC2. If not, adjust VR2 until you get this voltage.

If checking the RF signals of the oscillators and around the mixer circuit, please use a well compensated 1:10 probe, standard 1:1 probes have too much capacitance and may cause the oscillators to stop.

Thank you for the detailed reply.

Yes, it was a Jaycar kit. I bought it last week and it arrived on Friday. I can check the model number if you still need it.

I'll definitely take a look at what you suggested and report back.

We adjusted T2 to 4.3v and still no sound.

The model number (I'm assuming this is on the little orange paper inside the package?) is either KC5295 or 9P4070.

Maybe we're missing something big. So far we've
- Soldered the parts into position on the board
- Soldered and wired the speaker, volume knob, volume plate and power supply
- Adjusted the piece that the instructions tell you to adjust "until you hear a tone". We're not hearing a tone.

What other information could you use to help diagnose our problem?

Are you sure that tuning T2 didn't bring a result? It is a subtle question of 1/100 of a turn... Out of that, you should take care that the voltage at pin 1 of IC2 keeps at about 4.3V, because it tends to decrease when your hand (or any metallic tool/cable approaches the volume plate...

Perhaps there is still a problem in the later audio stages, so if you have an oscilloscope...

...you should make sure first that both pitch oscillators are working, so that you find sine waves with about 400kHz and 600mV p-p at pins 1 and 10 of IC1 and finally also at the mixer output (pin12), but only if the frequency difference between both oscillators is < 1.5kHz. At a bigger difference, the low pass filter will make decreasing the signal.

If one or both oscillators don't work, check them and make them work.

If both oscillators work, try still once more tuning T2 carefully and slowly until you get an output signal at pin 12. If that doesn't help, please get a frequency counter and come back here.

Aha!
I didn't think of moving everything away from the volume plate. We noticed that the 4.3v was changing eradically - probably because we were adjusting the thing on a metal workbench surrounded by old TVs!

We'll go ahead with your suggestions. I'll get back to you soon.

Aaaargh!

When adjusting or playing a theremin, you should keep all metal parts at least 1 meter or 3 feet away from the instrument, since tens of a picofarad have already an important impact on it!

Well, we've got it working!

We went through everything you suggested, then finally gave up and went through the circuit with an oscilloscope piece by piece. The _absolute last_ solder, the one leading to the speaker, had a bridge...a pretty big one, too!

Thank you for your help in any case.

It's always a pleasure for me to help

Thierry
Theremingénieur franco-allemand