O let me tell you a story of sorrow and woe. And ultimately, of triumph.
On Monday I had pretty much all issues resolved. Physical construction was complete, and I had it tuned to the point where it operated pretty well. An issue had arisen where it seemed that the built-in speaker was exceedingly buzzy compared to the line out, but I determined that this was actually a grounding issue, and made up a cord to ground the unit to a power outlet using a banana plug on one end connected to an RCA shell on the other so that it could be used with just the internal speaker in a satisfactory manner.
And, I thought, it would be neat to make a short recording for this thread before turning it over. So I plugged it into the computer, started looking for where the heck I could adjust the gain to a reasonable level, and whilst waving my hand past the antenna got a sharp static shock and the theremin went dead as a doornail.
Crap.
Was there some issue with the computer’s ground, that a voltage potential sufficient to create that sort of static discharge could develop? I opened the theremin case, only to see that at some point the epoxy holding the speaker to the lid had let go, and the speaker was sitting on the circuit board. My theory rapidly shifted to speaker body shorting out something.
I didn’t have the means to properly test anything beyond the caps and resistors, but a bit of swapping around rapidly suggested that a transistor amplifying the signal coming off of the pitch antenna was fried. As luck would have it, that particular transistor was in stock at a local electronics supplier. With great hope I replaced the faulty transistor, hooked everything back up…only to find that while the unit no longer had no audio output whatsoever, what output it did have sounded like someone spinning an AM radio across a bunch of radio stations just out of range.
Now what? Well, the static discharge entered the theremin via the pitch antenna. A capacitor failed short might be a critical filter at some point, maybe. I don’t really know enough about electronics to decipher what’s doing what, though. I can follow the circuit diagram well enough to get the broad picture of what’s happening, but what this particular 68 picofarad capacitor is doing? Beats me. So, I lift a bunch of components and meter away, and everything looks fine. I swap the inductive coils around, as one of those was definitely in the path of the discharge, though I’m not sure how a transient voltage spike would be likely to damage copper windings. Nonetheless, when I swap the coil at the pitch antenna with the one at the volume antenna, the theremin’s behaviour changes. Oh great. The one set of components in the kit that’s specially produced just for it, rather than being an off-the-shelf part. Something I won’t be able to get, except maybe from the Australian supplier.
Okay, well, let’s try something though. I may not be able to follow exactly what’s going on in the circuit, but I do know that the volume circuit is ultimately just supplying a variable voltage to one pin of the first integrated circuit, and I know what that voltage is supposed to be at max volume. So, if I were to use the two good coils for the pitch antenna and the reference oscillator and disconnect the volume circuit and supply the voltage it normally would with just a AA battery, I’ll have a functioning device, just that the volume will only be controlled with the pot regulating the amp instead of moving your hand by the volume antenna. Right?
Well, I make the modifications, and… it’s exactly the same, bad AM reception noise. Using my awesome powers of deduction, I conclude that the coils are fine, which leaves the integrated circuits as the only likely culprits. The two smaller chips are the amp for the little speaker, and the op amp for the line out, and if either of those had failed I wouldn’t be getting anything at all out of it. That leaves the big chip - MC1496, balanced modulator - the thing that actually generates the tone.
That same electronics supplier has this part too. Neat little shop, run by an 80-year-old guy who’ll talk your ear off if you let him, and probably even if you try not to. He volunteers the fact that he has over 40 thousand different parts in inventory, but he just can’t keep everything because his suppliers want minimum orders and the customs brokerage and young people these days just want to charge everything and he has a minimum charge for charged transactions because he loses money on them otherwise and he thinks he’d like to sell the business because he’s just getting too old and that fellow who was in yesterday…well, you get the picture. But he has the IC I need, and I cheerfully pay the $1.76 in cash and wish him a pleasant day.
And…it works! Just like it did before. The speaker I think must have shorted between the case around one of the coils (which are on the ground plane) and one of the pins on that IC, and the discharge traveled from that pin through the IC, down the line back through the transistor and up the antenna.
Did you need to know all that? Almost certainly not. But I’m feeling rather smug that I tracked down the issues without an oscilloscope or any proper electronics training, just using a bit of trial and error, common sense, and process of elimination. And before it all goes back together, that speaker is going to be fixed in place mechanically instead of just chemically.
