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 TM 55-1905-219-14-11
5-152. LOUDHAILER - MAINTENANCE INSTRUCTIONS (Continued).
LOCATION
ITEM
ACTION
REMARKS
TESTS (Cont)
Remove transistors from circuit before making resistance measurements; use a
heat sink in soldering and unsoldering transistor leads.
(2) Resistance measurements.
(a) Most causes of malfunctioning are due to a faulty component, open or shorted wiring, or a loose or
cold soldered joint which may have developed. These defects can usually be localized by conventional resistance
measurements when these vary markedly from the normal values that should be obtained. Table 1 lists the normal
resistance readings to be obtained across all points in the circuit necessary to localize defects, and in addition lists the
possible J faults indicated by abnormal resistance reading. Test point A is the positive bus of the circuit, and test point
B is the negative bus. All readings are in ohms and are measured with any standard 20,000 ohm per volt ohmmeter.
Transistors Q2, Q3, and Q4 are removed for this test and at least two leads of Q1 are unsoldered from their lugs. Use a
hot iron and hold the leads with a long nose plier to conduct heat away from Q1 and unsolder quickly.
(b) Note that the nominal resistance of the particular component is not measured in all cases, in as
much as there are other components forming a network shunting the component. The median readings are given in
each case for the combination of shunting effects - for instance in measuring R2 a reading of 1300 ohms nominal is
obtained, instead of 27K. Here R2 is shunted by R3 in series with RT1 and the combination of T2 terminals 4-5, R9, L1
terminals 2-3 in parallel with T2 terminals 3-4, R11, L1 terminals 1-2. Confirmation of a defective transformer winding
can be checked against the values given in the transformer table 2 after unsoldering the connections to this transformer.
(c) Attention is called to the tantalytic capacitors which y have a different resistance in either direction of
polarity. For this reason, ohmmeter test prods must be used as table 1 indicates - use the plus test prod on test point A
and the negative prod on test point B as called for in column 1. Be sure the ohmmeter used actually shows the polarity
indicated by the test prod jacks or terminals as there are some meters which have reverse polarity on the ohmmeter
range from that on the d.c. volts range.
5-1509
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