I need theremin construction help. :(

Posted: 10/20/2007 2:59:55 PM

From: Massachusetts

Joined: 8/18/2007

I took a quick look at the pictures and can't see
any obvious problems.

Is C32 optional? It appears to be missing.

You should mount L12 flush to the board. Unsolder it, push it flush to the board, solder it and trim the leads.

If C32 is optional and moving L12 doesn't help
(which it may not) you will need to get
a meter and an oscilloscope to troubleshoot

(* jcl *)

Posted: 10/20/2007 4:11:28 PM

Joined: 10/13/2007

I don't know what flush means. So... yeah.

Also, some extra stuff:

The wire between the board and the volume antenna lowers volume when touched. The wire on the pitch side does not.

L1 does nothing when touched, but all of the other ones like that do.

Also, L14 looks the same as L12, should that be "flush" as well?
Posted: 10/20/2007 5:06:52 PM

From: Massachusetts

Joined: 8/18/2007

Does "all of the other ones" mean L2 & L3 ?

If L1 does nothing when touched but L2 & L3 do then the problem is in L1 or the connection of L1 to the PCB. Check for a bad connection to L1 or a broken wire on L1.

I do not see L14. If it is part of the volume circuit I would leave it alone since the volume circuit is working.

(* jcl *)

Posted: 10/20/2007 5:45:39 PM
RS Theremin

From: 60 mi. N of San Diego CA

Joined: 2/15/2005

I agree with “* jcl *”

[i]“L1 does nothing when touched, but all of the other ones like that do.”[/i]

I can’t take the suspense. . . great troubleshooting clue though, you must be pointing to the problem area. There is a break in the continuity (electrical path) between the antenna solder tab wire with focus on the coil L1 and L2 wires. Temporarily jump or short an alligator test lead across both L1 and L2 or maybe even L1 to L4. The problem is most likely how the last coil wrap is soldered to its connecting lead or the solder joint on circuit board.

A theremin at idle tuned to a very high pitch will seem to present a very low response sensitivity, this is normal. Your theremin construction sounds like it is working fine except the antenna is disconnected somewhere between the antenna solder tab and the (collector) C spot on the Q1 transistor.

Also check the foil side of the board for a defect or cut in the copper trace along this same area.

Posted: 10/21/2007 10:49:24 AM

Joined: 10/13/2007

I think I found the problem!

http://img510.imageshack.us/img510/7849/theremin017kl2.jpg (This one I circled it in photoshop)

I don't think you can see it too well in the picture, but I'm wondering what I can do about it?

See, my mom was poking around, trying to help me, but she was being annoying about it, so I was about to tell her, and she touched J1 and the pitch went up, and I just froze.

It appears that the wire that leads into J1 from J2 works. The problem is apprently an infinitesimally small break in the wire between the gray wire and the red wire. the two aren't touching each other at all.

Can I fix that?

EDIT: Another thing! It appears that the two wires simply aren't long enough to reach each other. How do I remedy this? Would a tiny drop of solder work?
Posted: 10/21/2007 12:21:40 PM

From: Massachusetts

Joined: 8/18/2007

You should be able to connect the broken wire
with a piece of uninsulated wire and solder.

With your soldering iron you may be able to melt the insulation off the two ends of the broken wire
on L1. If it does not melt back you could scrape
it off with an Xacto knife.

(* jcl *)
Posted: 10/21/2007 12:27:48 PM

Joined: 10/13/2007

There's no insulation- it's the really really thin wire that connects the metal legs to the copper wire that's wrapped around things like L1.
Posted: 10/22/2007 9:01:10 PM

From: Toledo, Ohio United States of America

Joined: 2/22/2006

flush (LEVEL)
at the same level as another surface,such as:
'I want the light fittings to be flush with the ceiling.
Good Luck!
Posted: 10/24/2007 5:15:06 PM
Ian Nexus

From: Lower Stondon, Bedfordshire UK & Morton MS USA

Joined: 10/21/2007

To anyone interested that part looks like Hammond RF Choke 10uH part no. 1535G (digikey.com), my personal experience with varnished open inductors like these that they are VERY delicate and are often damaged in transit after PCBs are populated ( probably happened when kit was packed/transit)
Its also interesting that Moog are using 3 inductors from pitch oscillator to antenna rather than 4 on earlier versions of this circuit.

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