RCA Theremin Service Notes

(10) S.P.U. TERMINAL STRIP VOLTAGES

Line 120 volts-Adjustment switch in 120-volt position

Use D.C. voltmeter with a 0-300 volt scale and at least 1,000 ohm, per scale volt such as incorporated in Weston Model 537, Type 2, test set, for the D.C. voltage. The AC. voltage may be measured with an A.C. voltmeter of suitable range. Refer to Figure 6.

Terminal. Nos.	Cable on Tubes Lighted Volts	Cable Off Volts
1 to 6 (D.C.)	190	260
2 to 6*	190	260
3 to 6	140	230
5 to 6	29.0	0
8 to 9 (A.C.) rms	2.5	2.8
10 to 11 (AC) rms	4.7	5.0

* With Model 106 Loudspeaker connected to audio outputput jacks.
(These jacks may be shorted together for the purpose of taking a reading with negligible error in the result).

(11) THEREMIN RADIOTRON SOCKET VOLTAGES

Line 120 volts – Adjustment switch in 120-volt position

The following voltages taken at each Radiotron socket with the Theremin in operating condition should prove of value when checking with test sets such as the Weston Model 537, Type 2, or others giving similar readings. The plate currents shown are not necessarily accurate for each tube, as the cable in the test set will cause some circuits to oscillate, due to its added capacity. Small variations of voltages will be caused by different tubes and line voltages. Therefore the following values must be taken as approximately those that will be found under varying conditions. Refer to Figure 6.

Tube No.	Cathode to Grid Volts	Cathode to Screen Grid Volts	Cathode to Plate Volts	Plate Milamps	Filament or Heater Volts
1 UY-227	11.5	–	60.	7.1	1.95
2 UY-224	11.5	10.2	135.	0	1.95
3 UY-227	11.0	–	60.	1.95	5.5
4* UY-227	0	–	-2 to 40	Zero to 3.5	1.95
5* UX-120	0	–	110 to 64	0.5 to 4.1	unreadable
6* UX-I7IA	26 to 31	–	95.	12 to 17	4.6
7* UXI71A	25 to 30	–	140 to 145	26 to 30	4.6

* The range of variation of the readings taken on Radiotrons Nos 4, 5, 6 and 7 is caused by a change in the resonance point of the volume control loop circuit. Any object (such as the measuring instrument cable, body of the operator etc.) coming in proximity to this circuit will give the variations noted above.