RCA Theremin Service Notes

(9) SERVICE DATA CHART

Indication	Cause	Remedy
Radiotrons Fail to Light	No supply voltage	Test with A.C. voltmeter at loudspeaker auxiliary power receptacle (operating and safety switches should be closed)
	Defective safety switch	Test as above. Repair or replace
	Defective operating switch	Test as above. Replace
	Defective Power Transformer	Check by continuity tests (Section 13). Replace
	Defective Radiotrons	Replace
	UX-120 (only) fails to light	Low emission UX-280 or 171-A (no. 6) tube not oscillating. (A Neon glow lamp – such as the Cooper Hewitt type G10, 115-V – is a convenient test for oscillation. The brass base of the lamp should be held in the hand and the glass bulb near the pitch control rod or volume control loop). Circuits out of adjustment
	Defective wiring	Check by means of continuity tests, Sections 12-13
Tubes light – Theremin fails to play	Defective “Play-Off” switch	Replace
	Loudspeaker defective or disconnected	Connect O. K. speaker
	Tube Nos. 1, 3, or 6 not oscillating	Test tubes and continuity of circuits, Section 12
	Defective audio input transformer	Check with continuity test
	Grounds or opens	Check continuity Sections 12-13
	Control grid connection open at UY-224	Attach contact cap
Distorted Reproduction	Low emission or defective Radiotrons	Replace
	Poor wave form of A. C. supply current	Try resistances from 500 ohms to 2000 ohms in series with control grid of UY-224
	Open 550-ohm resistor	Replace
	Defective audio transformer	Replace
	Open or imperfect chassis ground	Repair
	Improper audio voltages at audio sockets	Check with tables Sections 10-11, Isolate trouble by continuity test Section 12
	Defective Loudspeaker	Replace
Hum	Defective Radiotrons	Replace
	Open or shorted center tapped fixed condensers	Repair or replace
	Open by-pass or filter condensers	Test and replace
	Shorted filter reactor	Check by resistance measurement. Replace
	Open chassis grounds	Check from diagrams Figures 7-8
	Loose laminations in power transformer	Repair or replace