Etherwave Theremin Volume Problem?

Posted: 12/27/2022 8:07:00 AM

Joined: 12/27/2022

I am new to Theremins and I have one that is not working properly.
There is no audio out to the amplifier jack.  All oscillators are on frequency, and I am puzzled if I have a circuit problem or and adjustment issue.

All inductors measure within spec.
+/- 12V supplies are good.
Jumping C28 gives output, but the volume antenna does not seem to have the right effect.
Tuning L11 does not change the voltage on U3 pin 11 as it stays close to -12V (-11.5V).
The volume pot provides 0V to -12V to R18 and changes the frequency of the volume oscillator.
D1, D4 and D5 test ok with diode setting on DVM.
Q6 and Q7 emitters are at +0.2V
C12 was open and has been replaced.
U3 was replaced and there was no change.

It seems like more signal amplitude is needed from the volume oscillator to the VCA processor to drive the gain to the audio out amplifier?
If I put my thumb on L7, the signal amplitude on D1 increases, U3 pin 12 increases in voltage, and the audio output comes on.

I may be looking at amplitude issues vs frequency issues, but I can't find enough information to know for sure.

Thank you and my apologies if there is a thread on this that I may have missed.

Posted: 12/27/2022 8:26:26 AM

From: germany, kiel

Joined: 5/10/2007

The HotRodEtherwave manual says you have to read the voltage from pin 12 (not 11 which is always 12V). If turning L11 then has no effect i'd start looking around there.
L7/8/9/10 for sure are connected fine?

Posted: 12/27/2022 7:54:15 PM

Joined: 12/27/2022

Thank you, Dominik.

I made a typo, should say Tuning L11 does not change the voltage on U3 pin 12 as it stays close to -12V (-11.5V).
L11 does provide 10-20 kHz of adjustment range.

The signal on Q6-Collector = 10.1Vrms and Q7-Base = 2.1Vrms (~498 kHz).
Should the Q7 base amplitude be higher than the 2.1 Vrms measured?

Posted: 12/30/2022 2:22:25 PM

From: germany, kiel

Joined: 5/10/2007

Some quick measurements.
junction D1 / C12 / R14: -9V (hand up) to -1,5V (hand down)
U3 pin 12: -1V (hand up) to -11V (hand down)
Q6 / Q7 e: 0,14V, b: 0V, c: 11,9V
resistance: antenna > junction L7 / C12: 28 Ohm (newer board 11-211J with 3 inductors on each side)

Posted: 2/16/2023 11:26:53 PM

From: Montana, USA

Joined: 9/2/2019

OP, if you ever return, try changing D1.  It can easily be damaged by ESD.

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