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 TM 55-1905-220-14-11
5-155. LOUDHAILER - MAINTENANCE INSTRUCTIONS (Continued).
LOCATION
ITEM
ACTION
REMARKS
TESTS (Cont)
(3)
Voltage measurements.
(a) Table 3 shows the nominal DC voltage measurements, which are obtained with a normally functioning
amplifier. These are made with a 20,000 ohm per volt ohmmeter from either test point A or B as indicated, polarity of the
test prod being observed accordingly. The values given are those obtained with a battery supply of 12 volts, so it is best
to use a storage battery for these tests. The battery circuit should be fused, to avoid damage due to accidental shorts.
(b) The values in the NO SIGNAL column are those obtained with transistors in circuit, the amplifier
terminated normally and the battery circuit closed'. The FULL SIGNAL column shows the normal d.c. voltages obtained
with the amplifier is driven by a test signal to produce 10 watts output. Although it is possible to make amplifier tests with
the amplifier module removed from the rear housing and opened up, and a 1000 c.p.s. test signal fed into the "HI" and
"LO" terminals it is more convient and conductive to careful testing to set it up for a bench test.
(c)
To set up for a bench test, disconnect all six wires with snap-on connectors from chassis and remove the
amplifier module entirely from the rear housing. Provide a 1000 c.p.s. adjustable signal source of any available
impedance between 100 and 1000 ohms, to connect to terminals "HI" and "LO". Use a 16 ohm power resistor (25 to 50
watt rating) to terminate the output across terminals "LINE" and "COM". For test power use a 12 volt storage battery
connected to the plus and minus terminals of the amplifier, through a 2 ampere fuse and a 2 ampere d.c. meter. The
figure shows the amplifier connected in this manner ready for testing. Spring test clips are convenient means of making
connection to the terminal tabs. The module is of course opened up as described above to permit access to all terminals.
(d) For the NO SIGNAL tests, shutoff the oscillator, but operate. the switch S1. Observe the DC ammeter
reading. This will be on the order of 100 m.a. if there are no defective components or wiring in the amplifier. Proceed with
the NO SIGNAL measurements in table 3.
5-1607
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