Help! My Melodia stopped working!

Last thing first. Yes, C1 has an effect. I think the idea is this. There is (supposed to be) a zone extending away from the pitch antenna. At the antenna (i.e., touching it) we have C7 (3 octaves above middle C) and moving the hand out 18" the note drops to 0Hz. Any further distance results in silence. I think what C1 is supposed to do is compress and expand this zone so the 0Hz point can be closer than 18" or further away.

HOWEVER, if the (frequency) coils aren't zero-beating properly, if you move out past 18" you get the sawtooth. So by zero-beating the coils (at the appropriate place!) you should be able to set up the 18" zone with nothing beyond.

But for me, while C1 does expand and compress the zone, it does it VERY SLIGHTLY -- by maybe just an inch or two.

Clearly something is wrong. I've tried all sorts of settings with the coils (using the instructions in the construction manual, of course).

I just can't seem to get it right. Either I have some kind of zombie force field in my workshop, or a component part is going south, or I need to keep fiddling with the two coils and the variable cap (C1).

If you can think of anything else, please don't hesitate. I don't recall ever having this problem before. I've always been able to get the "silent" point (i.e., beyond 0Hz) at around 18" like Bob specifies.

Rory

Yes, C1 has an effect. Just not what I need. My experience (and understanding) is that when properly adjusted, if you touch the pitch antenna you hear C7 (3 octaves above middle-C) and moving your hand away the tone descends to 0Hz at 18". Beyond that it is silent.

What C1 does is to expand or contract the 18" point, so it is nearer or further from the pitch antenna.

But if it is improperly adjusted (i.e., the pitch circuit coils are zero-beating but they're both in too far or out too far, or the variable cap [C1] is out of whack), you don't get silence beyond 18". You start to get that sawtooth wave tone.

My problem is two-fold. The point where the tone is 0Hz is only 4" away, not 18". And beyond 5" or so, the "sawtooth" begins (and ascends as you move your hand away.) The tone is wonderful, but only within that 4" space. It is NOT possible to play that way.

Yes, I am using the technique described in the assembly manual (thankfully posted on this site, since., as I've explained, mine is still packed away in storage and I can't get to it.)

The only other clue is the way the tone sometimes "jumps" across a discontinuity. This makes me think something is really out of whack -- or else I have some kind of zombie force field in my workshop. I'm sure if I were an EE who had design experience with the circuit, the symptoms would point me to the problem, but unfortunately I've never experienced this difficulty before and I do NOT understand the circuit that well.

Before I remove all the 20 year-old components and reassembling with new parts (except my precious Bob-wound coils), I wonder what is the most likely thing to try? For example, I mentioned above I'm worried I may have over-heated one of the two glass diodes. Could a faulty diode explain my symptoms? Do you know anybody who might know? If so, can you direct them to this thread??

Rory

It sounds to me like it is working fine and you just need to do some more adjustments on the coils.  Try this:

With the top open, set the capacitor so it is half meshed.  With your hand close to the pitch rod you should hear a high frequency tone.  Move your hand away and the sound should get lower and at some point it goes silent (zero beat).  Note the distance from your hand to the pitch rod.  Then, turn coil "U" in a couple of turns and repeat the above.  When it goes silent see if the distance from your hand is closer or further from the pitch rod.  If closer then you are turning coil "U" the wrong way.  You should be able to keep making adjustments to "U" until zero beat is around 18". 

Of course, when you are making the adjustments the pitch will be all over the place because your hands are not in a normal playing position.  All this is done with the top open and without touching the trim capacitor, and don't worry about using the lock nut on the coils until you are adjusted.

It plays fine with the top open so experiment all you want. Once zero beat is at around 18" then you can fine tune with the capacitor.   When you are satisfied that it is working, then you have to close the lid and see how it behaves.  You will probably have to do some more adjustments to get it just right.  (Remove pitch rod, open top, adjust coil, close top, replace pitch rod, test) until it is properly adjusted.

Almost forgot: try pulling the transistors and reseating them in their sockets a couple of times just in case there is a dirty connection.

You are a genius! I was stubbornly stuck with making the adjustments the way I did fifty-five years ago, but when I just did like you suggested it worked great!!! Wow and double wow -- I have my theremin back again! Well, not quite perfect, yet.

First of all, it's odd. With the "senior_falcon" technique I can get the 0Hz at the 18" point, and C1 works just like it should. The only thing different is that when I touch the antenna, the note is much too high. But who cares? The whole problem was to expand the zone where you can play, and if the highest note is much too high, it doesn't matter. An inch or two away is about as high as I'd ever want to play it (above that the note is too high), and that is fine, or really, even better (!) than the original design.

OK, but now I have a little problem with the VOLUME side of things. Bob doesn't really say, but from my memory, when the volume antenna is adjusted correctly, do you get full volume with your hand at infinity, and silence when touching it? That's not really explained in the manual and I don't precisely remember.

But what I currently have (and seems like oddly like the same problem as I was having with pitch) is the "zone" of usability is too "narrow". I mean, touching the antenna is silence, and moving out to about an inch is full volume. But then if I move away further than about 2 inches, the volume drops again (and the variable cap has only a small effect.)

Sooo, with your "fiddle" approach I can probably get the volume working the right way too, but just remind me again -- how is it SUPPOSED to work?? Is it a)full volume at infinity, or b) a "zone", and if your left hand is outside of that zone (too close OR too far) you get silence?? In other words, what am I aiming for?

Thank you. I will eventually post pictures. This Melodia is entirely homemade. In fact currently there is no lid -- just an open plywood box. I think I'm going to add a lid and paint it white for my performance this Halloween at Battery Steele (where there is no electricity). I have an awesome "Freedom" amp from ElectroHarmonix that uses a sealed gelcell battery and BLASTS the sound from a 10" enclosure. I can't wait! But first, I do need help with the volume adjustment.

Rory

The normal way to set up the volume control is to be silent when your hand is close to the volume loop, or plate in the case of the Melodia.  As you move your hand away the volume should rise and reach a maximum around 10-12 inches away.  Beyond that it should stay about the same loudness. It should not get softer again.  If you go away from the theremin the volume set to full loudness, but you can drape the cable over the left side of the theremin close to the volume plate to silence it.

Quite by accident I found that you can set the coils so that the volume is reversed, that is, silent when your hand is away from the plate and rising as your hand gets closer.  Mine was set up that way for years, but when I got my etherwave I got opened it up and adjusted the coils so would behave "proper" way.  I have come to like it that way, but as you might guess there are some strongly held opinions as to which way is best.  

The screwball the Melodia throws at you is that the volume plate is vertical and ideally you would move your hand laterally and not vertically to change the volume.  So whatever you do it will not behave exactly the same as a traditional theremin.  

Both the Etherwave and Melodia jump up about an octave when you touch the pitch rod.  I have seen videos where this is used to make twittering bird-like sounds.  I also have heard that static can zap components when you touch the pitch rod and have considered putting clear tape or heat shrink tubing on the pitch rod and volume loop.  

Thanks, s-f. I haven't gone back to try to fix it, but I wonder if you might have one more piece of advice up your sleeve. You recall your "fix" for the pitch antenna was to ignore the zero beating and simply adjust ONE of the coils for hand-distance, then go back and mess with the other coil and variable cap. And that worked wonderfully.

Have you found another such heuristic approach for volume? As you know, the instructions for volume are different from pitch in that an instrument (voltmeter) is required, not just your ear. I just wondered if perhaps you had found a quick & dirty approach to the "left" side as well. It just seems odd that my volume control has a very tight spot (about an inch wide) were I can a signal, and anything closer OR further away is silent. (This is after following the manual, setting one coil to a min and the other to a max voltage.)

Thanks again.

Rory

I don't remember how I set the volume coils.  I probably used a voltmeter as described in the instructions.  In any case, the melodia will not be damaged if the coils are not set right.  Since it isn't working right, why not choose one and adjust it a bit to see what happens. If it gets better, that's good, otherwise try the other direction.  

Dumb question, but are you sure you adjusted the proper coils? Since it is home built could you have swapped their locations?

For what it's worth, I have about the same amount of threads showing on all four of my coils, but that's not very scientific.