As a twelve year-old I purchased the six Melodia coils from Bob out of the Jan. 1961 Electronic World magazine. I ordered the rest of the parts from Allied, successfully put it together following Bob's instructions, and entered my Theremin in the science fair. I won and Bob included a picture of me in his Moog Music catalog that year. Eventually I took the instrument apart, saved the coils., knobs, and switches and 30 years later my daughter and I rebuilt it. After showing it off and playing it for about a year, it stopped working (no output). I put it aside, moved several times, and now, at sixty-nine, I've gotten it out again and am trying to troubleshoot it.
In the original magazine article, and the instructions that came with the coils (and thanks, by the way, for posting the pages from that manual) Bob says to do two tests after assembly prior to turning it on. One is to verify 500 to 2500 ohms resistance between ground and the red wires (the transistor bases). The other, after inserting the battery and turning it ON, is to measure D.C. for 0.3 v. and 6 v at two points listed on the schematic.
So the 6v measurement is fine. The 0.3v point is giving me about 5v! The resistance from the V1, V2, and V3 bases is checking out at 2500 ohms. The resistance from V4's base to ground is closer to 8K ohms.
So why is the one voltage measurement too high? (And why is the resistance on the red wire to V4 too high?)
I don't know where to begin to troubleshoot. I tried re-soldering all the joints (except on the xistor sockets and coil terminals) but it didn't help.
Is it possible the glass diodes have gotten damaged? I may not have always been careful with heat-sinking when I soldered their lugs. When turned OFF, the diode over by L3 measures about 5K ohms in one direction and 54K ohms in the other. Obviously the diode is working. But the one by T1 only measures 2.4K ohms in one direction and 2.2K ohms in the other. So I guess it's working?
Do you have any suggestions? My alternative is to carefully take it apart again, and start over with new resistors, capacitors, diodes, and transistors. I hate to do this if the bug is some silly minor problem. But I have no clue.
Thank you for any help.
Rory Sellers