Theremax - L1 and L4 circuits not oscillating

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Theremax - L1 and L4 circuits not oscillating

Post by Geebeex »


I have trouble making my Theremax oscillating.

I have the clip on D1.
The Theremax is powered by 12V DC.
I have ~1,7v DC on all Q1,Q2,Q3,Q4 emitters.
With the oscilloscope I can see the oscillations on Q2 and Q3 emitters around 900Khz, but I'm not able to make L1 and L4 circuits oscillating, whatever the position of the coils.

Could a polarity inversion with the power supply have burned something ?
Can be please give me some advice on how to troubleshoot this issue ?

Thank you very much.
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Re: Theremax - L1 and L4 circuits not oscillating

Post by PAiA-Scott »

The first and fourth oscillator circuit sections include wiring and parts on the front panel and to the antennae. If the SG wire from the board to the panel output connector sleeve terminal (S) is not completing the circuit out to the panel Trim controls R79/R75 and R80/R76 (via the S-S bare wire jumpers), it will prevent oscillation. R79 and R80 must be the specified 1K (the value is marked on the side of the control facing the panel, but the codes on the back of these two should match each other and differ from the other controls.

The shielded cable extends the oscillator signals on the Q1 and Q4 transistor collectors out to the antennae mounts. This is an easy spot to get a short between these oscillator nodes and the grounded shield wire due to the shield wire either touching the antenna mount or it melting through or being too near the internal wire and a stray strand causing a short or the heat from soldering and tension resulting in melting through the insulation of the internal wire.

Look at the trouble-shooting tips and suggestions over in the Technical Bulletins and Assembly Notes section for Theremax for other possibilities. Images are included showing the way the wires must be routed away from parts (and the antenna cable ends) in the oscillator circuit sections.
Scott Lee
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