RCA Theremin Service Notes

PART II-MAKING REPLACEMENTS

The various assemblies of the RCA Theremin are readily accessible and replacements are easily made. However, there are some operations that require careful procedure as noted in the following paragraphs.

(1) REPLACING PARTS IN THE THEREMIN ASSEMBLY

All parts in the Theremin assembly are readily replaceable with tools ordinarily used in servicing radio receiving sets. However the fixed condensers used across the variable and pitch oscillator coils are somewhat more critical in values than other parts and may require some experimental work before a satisfactory replacement is made. These condensers are roughly adjusted to the pitch coil and after a replacement is made, the Theremin should be adjusted for corect operation by means of the trimming condensers. If this is not possible then a substitution for the condenser just replaced should be tried and one found that will allow a correct adjustment to be made by the trimming condensers. A condenser that does not function satisfactorily in one Theremin instrument may prove O.K. in another.

Figure 9 - Internal connections of filter and by-pass condensers, and filter reactor. — Figure 9 – Internal connections of filter and by-pass condensers, and filter reactor.

When replacing a pitch control coil, sometimes a similar condition will result. If after replacing a pitch coil it is found that the maximum range is not obtained by adjustment of the trimming condensers one of the following procedures must be used.

(a) Try several pitch coils. One may be found of the correct inductance for use in place of the defective one.

(b) If the range is low, that is less than 1100 cycles, try removing a few turns from the coil and readjusting the trimming condensers.

(c) Replace both the fixed condensers across the fixed and variable pitch oscillators with ones of a greater capacity. Replacing one without the other will not remedy the situation. Both must be replaced simultaneously.

(2) REMOVAL OF PARTS AND LOCATION OF CABLE

The cable that is used to connect the Theremin chassis and the S. P. U. is clamped securely in a definite place on the inside of the cabinet. Whenever any assembly is removed, and this cable is shifted from its position, it is very important that it be returned to its original position when the Theremin is returned to normal operation. Failure to have this cable in its normal position may result in inability to adjust the Theremin for normal operation.