Unstable Pitch?

Every idea that could be mentioned in this thread can only be a guess. I tried to avoid what electronic components could generate this effect.

You may need an expert, the power supply needs an oscilloscope peek.

In such an old house how are you achieving a good earth ground, or are you?

Since the external power supply of the Etherwave Standard/Plus is a simple AC/AC transformer and all the buffer capacitors and voltage regulators are on the main circuit board, it may not be responsible for such effects.

The "usual candidates" may rather be in this order:
a) Oxydated contacts of the 10-pin connector which leads from the main PCB to the front panel. Pull it off and put it back several times in order to clean the pins.
b) Bad solderings at the front panel potentiometers. Resolder the wires correctly.
c) Failure in the -11V helper voltage regulator stage. Take a digital Multimeter and check if you have around -11.2V between the green and the black wire which lead towards the front panel. The absolute value is not so important as long as it is between -11 and -11.5V. If it's outside these values or if it is not constant, replace transistor Q9 (2N3906) and capacitor C33 (100uF/16V or 100uF/25V)

If you don't feel comfortable with the above procedure, print this posting out and take it together with your Etherwave to the next Radio/TV repair shop. They should be able to do all this diagnostic within 15 minutes and the repair within further 30 minutes.

Thierry said:

[i] a) Oxydated contacts of the 10-pin connector which leads from the main PCB to the front panel. Pull it off and put it back several times in order to clean the pins.[/i]

This is definitely a strong contender!

I hope he finds success with his own experimenting. Finding a repair shop is going to be difficult because we live in a throw away society. To have anything fixed in today’s economy, even replacing a fuse, is probably going to cost around $400 the price of a new instrument!

Personally I hate troubleshooting and prefer the art of new & raw discovery.

If you really can't find a repair shop round the corner where an old and experimented technician will do that during his cigarette pause and only ask you to make a modest donation for his thank-you box, that means that times have really changed... :-(

I don’t want to sound discouraging for those needing repairs but today’s technicians in the states just replace boards not components. They go to these back alley advertised tech schools and have a degree in 18 months. They begin school with zero interest & knowledge and graduate with degrees in professional board substitution or board stuffing.

Thierry you know math which makes you a dinosaur not to mention having actual knowledge on how each individual component behaves. I have never replaced a board so my skills must be some where in the middle, no wait I do recycle electronics!

PS: Some of those above students do continue on developing their knowledge in electronics and come up with some outstanding results. (Got to cover my back, my full name and address was recently reveal to this board indirectly)

Edit: Sometimes noise can interfere with a theremin from external sources

External Noise Sources (http://www.oldtemecula.com/theremin/noise/index.htm)

Sorry for not posting how I resolved this. I hate when people do that.

I was in contact with moog the week leading up to the concert and the rep offered to send a replacement board. It was only $60 plus shipping... which is weird, since it's most of a $400 theremin...

Anyway, soldered the two points and plugged in the connector and everything sounded nice and new. The problem was fixed. Strangely enough, this new board interferes with the amps and instruments of the other members of my ensemble when the previous board didn't... but theremin interference is a bit of a mystery to everyone, it seems.

The rep offered to fix it and refund me the cost of the replacement board, but my group has a busy performance schedule this month. I don't think I'll have time to send it in. Besides, I'd have to remove the line level/instrument level switch I put in...