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RCA Theremin Service Notes

by RCA Service Department January 01, 2002

Page 14

(12) METHODS FOR CONTINUITY TESTS

In making a continuity test whether it be for the Theremin assembly or the S. P. U., the following procedure is recommended:
Disconnect the cable connecting the socket power unit to the Theremin assembly, the loudspeaker, and the A.C. supply cord at its outlet.
A pair of headphones, with at least 41/2 volts in series, or preferably a voltmeter with sufficient voltage to give a full scale deflection when connected across the battery terminals, should be used in making these tests, for example, a 0-50 volt meter with a 4,5volt "B" battery. The Radiotron socket contacts, numbers and lugs used in these tests are shown in Figure 6. The Theremin continuity wiring diagram is illustrated in Figure 8, the S. P. U. terminal numbers are shown in Figure 7.
Test leads should be of the flexible insulated type with partially insulated testing tips, so that false readings will not be obtained through contact with the hands. Similarly the hand should not touch the chassis or component parts.
The contacts of the test equipment should be placed across the terminals or leads indicated in the following test table under the column marked "Terminals." If the results are negative the cause of such negative effect will be found in the last column under the heading, "Incorrect Effect Caused By." The second column indicates the approximate correct resistance in ohms of most of the circuits tested.

(13) S.P.U. CONTINUITY TESTS

Remove UX-280 Radiotron and Disconnect Cable -Refer to Figure 8

Test Terminals	Correct Effect	Incorrect Effect
Test Terminals	Correct Effect	Indication	Caused by
Terminal No. 1 to filament of rectifier socket	1000 ohms	Open	Open reactor
No. 1 to No. 7	22,000 ohms	Shorted	Shorted filter condenser
No. 2 to one loudspeaker pin jack	Closed	Open	Open lead
No. 2 to other pin jack	Open	Closed	Shorted 0,02 condenser
No. 3 to No. 4	Shorted	Open	Open connection
No. 3 to No. 1	2500 ohms	14,000 ohms 2250 ohms	Open 2250-ohms resistor Open 14,300-ohm resistor
No. 3 to No. 6	20,000 ohms	Open Shorted	Open 20,000-ohm resistor Shorted 0.5 mfd. condenser
No. 5 to No. 8 or 9	13 ohms	25 to 30 ohms	Open power transformer winding
No. 6 to No. 10 or 11	13 ohms	25 to 30 ohms	Open power transformer winding
No. 7 to either plate contact of rectifier socket	190 ohms	Open	Open connection or open winding in power transformer
No. 8 to No. 9	Shorted	60 ohms	Open 5-volt filament winding
No. 10 to No. 11	Shorted	60 ohms	Open 2.5-volt filament winding
Across contacts of auxiliary power receptacle	3 to 6 ohms	Open	Open primary of power transformer or open 110-220-volt switch
Across blades of AC input plug with auxiliary receptacle shorted	Closed	Open	Open operating switch, open lead or connection
From either contact of auxiliary receptacle to frame	Open	Closed	Grounded primary in power transformer

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