RCA Theremin Service Notes

(4) PITCH ADJUSTMENT CONDENSER NOT AT CENTER

If the conditions indicated in Part I, Section 2 are fulfilled but with the pitch adjustment condenser off center, it may be corrected as follows:
(a) If the pitch adjustment is too far to the right, indicating too much capacity, the remedy is to increase the capacity of the fixed pitch oscillator trimming condenser located on the rear of the chassis. This is the center of the trimming condensers.
(b) If the pitch adjustment is too far to the left, indicating too little capacity, reduce the capacity of the fixed pitch oscillator trimming condenser.
Both of these adjustments must be made by changing the trimming condenser a little at a time, closing the rear doors and checking from the front for normal operation.

(5) RANGE BELOW 1100 CYCLES

This indicates the fixed pitch oscillator and variable pitch oscillator are at too great a frequency difference from the pitch control circuit The remedy is to increase both the fixed pitch oscillator and the variable pitch oscillator trimming condensers until the necessary high note is obtained. These are the left and center trimming condensers respectively at the rear of the Theremin. Care should he taken not to adjust the oscillators so that the note will be higher than 1400 cycles, as unstab1e operation may resu1t. 1100 cycles is about C sharp, two octaves above middle C, and 1400 cycles about F sharp, two octaves above middle C on a piano. A tuning fork or piano – if available – can be used for a pitch standard. A reversal of connections to the concentrated coil located inside of the large tone coil will also reduce the range of the Theremin.

(6) REVERSED ACTION OF PITCH CONTROL

This condition is caused by the fixed pitch oscillator being at a lesser frequency than the variable pitch oscillator. Such action is caused by the fixed pitch oscillator trimming condense, having too much capacity, the variable pitch oscillator having too little capacity or both. The remedy is to decrease and increase the capacity of the respective trimming condensers until normal operation is secured.

(7) HIGH PITCHED NOTE

Should the Theremin give a high pitched squeal when no one is close to it the fixed pitch oscillator trimming condenser is adjusted to too great a capacity. Also too small an amount of capacity at the variable pitch oscillator, or both maladjustment, will cause the conditions represented by the effects noted in Section 6 – only to a lesser degree. Also this condition may be caused by the two pitch oscillators being too close to the frequency of the tone coil. This is caused by too much capacity at each of the tvo pitch oscillator trimming condensers. An open tone coil will cause a constant high frequency note with little variation caused by various positions of the hand in relation to the pitch control rod.

(8) SKIP EFFECT IN PITCH CONTROL

A pitch control abruptly changing from one frequency to another, instead of giving a smooth variation of tone indicates a poor Radiotron UY-227 in one of the pitch control oscillators. Usually in the three Radiotrons UY-227 used with each Theremin, there will be found two that will function sarisfactorily in the pitch oscillator sockets. An erratic UY-227 in the oscillator socket will probably function normally as an audio amplifier. Too much capacity at each of the trimming condensers on the pitch oscillators may cause a skip effect in the range in addition to a greater than normal range. The effect noted in (7) may also be noted.