Th-1 Maestro volume antenna weirdness (video)

Generic advice because I've never messed with one of these:

Is it properly grounded?  This can be via the mains or via your amplifier connection if it is grounded via the mains.

Once tuned, analog Theremins are quite picky about antenna capacitance.  It is likely you simply aren't using a large enough volume antenna, and oddly there doesn't appear to be a front panel tuning control for the volume side to compensate for this.

There is a diode on the volume side (CR1) that I would check to see if it is blown or not.  There's the chance that someone may have been monkeying around on the inside and the volume inductors need tweaking.  There are two such inductors on the schematic, don't use anything metallic to tune them, use a plastic screwdriver / hex tool or else you might destroy them, and mark their current locations with a sharpie or something before you turn anything.  Small turns here to see if anything improves / degrades.

Do you have access to an oscilloscope?  How about a DMM?  Electrons are invisible.

Thanks for the advice! Knew I had the name wrong, (I was referring to the inductors as "trim pots" haha). I'll try a longer volume antenna, or maybe even try to recreate the original plate style ones. I'll check the diode too and get back once I find something. I don't have an oscilloscope unfortunately, but I do have a digital multimeter.

Ok diode seems fine, and the larger antenna theory worked! I'll probably either go back to the goodwill and see if they separated the antennas, but if that isn't the case do you think it would be a good idea to just make new ones? The measurements exist and they're pretty simple. I was thinking of making the volume one go off to the side because them being so close together is really not good for playing since they interfere with each other, but people have mentioned that will decrease the value. Realistically though, with how damaged this thing is cosmetically I'm not confident in it fetching a huge amount so I might just go for it. Open to suggestions of course!

"Ok diode seems fine, and the larger antenna theory worked!"  - Cortinoias

Yay!

"I'll probably either go back to the goodwill and see if they separated the antennas, but if that isn't the case do you think it would be a good idea to just make new ones?"

Is there any other option if they're not at the goodwill?

"The measurements exist and they're pretty simple. I was thinking of making the volume one go off to the side because them being so close together is really not good for playing since they interfere with each other, but people have mentioned that will decrease the value. Realistically though, with how damaged this thing is cosmetically I'm not confident in it fetching a huge amount so I might just go for it. Open to suggestions of course!"

I guess it all depends on whether you want to keep it to play or sell it.  Even if the former, I would adhere to the originals at least as functional mock-ups of the real thing just to get a feel of how it played out-of-the-box.  Never hurts to experience whatever the real thing was, even if you end up not liking it and changing things afterward.  The TH-1 fetches some do-re-mi on-line, you really scored finding it in a thrift shop!  You might look into professional restoration of the cabinet.

Ended up not finding the antennas at goodwill, and due to the rarity of this thing there's pretty much no way to buy replacements, so I just made some. I decided I wanted the pitch antenna to look as goofy as possible (gotta outdo the og goofiness), and for the volume i did a pretty conventional coil shape and moved it to the side (an extra hole drilled is pretty much nothing on top of the existing wear to the cabinet).

It seems to work well for the most part, however there are a couple more noise related things going on:

Thanks for the help so far! Not sure if I should be making a new post since it's technically a different problem now but I'll just continue this one for now

I'ld say best at this point to make really sure you have it grounded properly.

I concur with DreadVox.  I looked at the audio spectra of your video and there is a big peak around 120Hz, which to me means mains hum / a grounding problem.  The low end sounding bad is intermodulation with the mains hum.  Not sure what is causing the noise though, that's pretty strange.  The schematic doesn't show series inductors going to the antennas, so the fields probably aren't that strong in the first place, meaning environmental noise can likely get in easier.  Try to keep it away from lamps and such when playing.

I have such dirt at low frequencies in circuits with field-effect transistors, where the shutter is on ground (on a common wire). And I think that the reason is that such circuits require an increased need for grounding. What is the fatal flaw of such schemes. Their unpredictability depends on conditions, locations, time of day, network 220 conditions, and many other factors. We have to abandon such circuits in favor of independent of grounding.

Sorry kinda late here with this update, I switched out the mains cable for a grounded one and connected the ground to the chassis. If there's a difference, I didn't notice a huge one, although I did take it and an amp outside to test how it wound sound away from indoor interference and the noise basically vanished. Only issue now is, when I touch the volume antenna the pitch raises about half an octave, probably a calibration thing (need to fiddle with the inductors a bit more). Thanks for all the help once again, will update once I make some progress!