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 TM 55-1905-219-14-11
5-152. LOUDHAILER - MAINTENANCE INSTRUCTIONS (Continued).
LOCATION
ITEM
ACTION
REMARKS
TESTS (Cont)
(e) If normal indications are obtained in the above test, energize the oscillator with the volume control
turned down. Bridge a standard vacuum tube voltmeter (VTVM) across the output load terminals. Select a scale to read
12.65 volts, and advance the oscillator volume control until this output voltage is obtained. The d.c. ammeter will read
between 1.2 and 1.4 amperes with a reading of 11.5 to 12 volts on the DC voltmeter. Proceed with the Full Signal series
of measurements shown in table 3.
(f) Any appreciable departure in either test from the median values shown in table 3 indicates defects in
the components or wiring, not evidenced in the Resistance Measurements made per table 1. Conventional methods of
isolating, removing, and checking or replacing suspected defective parts are followed from this point on.
(g) Gain, frequency response, and distortion are measured in accordance with standard test practices.
The same test set-up as shown is suitable for this purpose with a variable audio frequency oscillator as a source, and the
VTVM used to set the input signal voltage. An oscilloscope may be used across the output to check distortion content.
Frequency response and gain of an average amplifier are shown on the schematic.
(h) Other than conventional tests on electronic circuits and those described above, testing a transistor
amplifier requires making sure that undamaged transistors are in circuit and that base bias voltages are correct. Note
that the bias networks of both the driver and output stages includes the DC resistance of their respective transformer
secondaries, and that the feedback resistors R7, R9, and R11 are also part of these bias networks. It is also good
practice to check catalytic capacitors to make sure that their resistive value has not fallen to a low value, that is that their
leakage is not excessive. If it becomes necessary to replace a driver transformer, be sure the phasing of the circuit has
not been reversed, as the negative feedback link =, R8 and C3 is connected in a degenerative sense. Transformer
winding reversal will cause R8 and C3 to give positive feedback and the amplifier will oscillate strongly.
5-1514
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