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 TM 55-1905-220-14-11
5-155. 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
tantalytic 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.
5-1608
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