RCA Theremin Service Notes

(3) VOLUME CONTROL ADJUSTMENTS

Should normal operation of the volume control not be obtained proceed as follows:
(a) Volume control cuts off when the hand is at a greater distance than three inches from the loop, the volume adjustment condenser being to the extreme right or maximum position. Also full volume is not obtained when the hand is completely removed. The remedy in this case is to increase the capacity of the volume control oscillator trimming condenser. This condenser is the extreme right one when looking at the Theremin from the rear with the doors open.
(b) Volume control cuts off when the hand is less than three inches from the loop, the volume adjustment condenser being at maximum. The remedy in this case is to reduce the capacity at the volume control trimming condenser on the rear of the chassis until a normal condition is obtained.
(c) Volume control has reversed action or goes from low volume to high and then low while the hand approaches the loop. This is due to being on the wrong side of the resonance curve or in thr latter condition going from one side of resonance through resonance and then to the other side. The remedy in both cases is to reduce the capacity at the volume control oscillator trimming condenser until normal operation is secured.
(d) A more accurate method of adjusting the volume control is by the use of a milliammeter. An adapter is necessary, which may be either a UY or UX type and having means for breaking the plate circuit. This adapter may be placed either in the UX-120 socket in the case of the four-prong UX type or in the first audio UY22, socket. A 0-5 milliammeter ” placed in series with the plate of either tubs through the connections provided in the adapter. The volume adjustment on the front of the Theremin is then set at maximum. The right rear door is now opened, the safety switch held down with one hand and the volume control oscillator trimming condenser adjusted until the milliammeter indicates 2 M.A. is flowing with door closed. The pitch should be around zero beat when making this adjustment. This is the cosrect adjustment provided the effect noted in (c) is not obtained, in which case the capacity should be reduced until another reading of 2 M.A. is obtained. Thus the correct adjustment is fonud when the milliammeter reads 2 with the least capacity at which such a reading may be obtained.