the unregulated B+ voltage falls below 400
ground. Use the 250-volt scale of a voltmeter
when the line voltage is 100, the tube emission
having a resistance of 1,000 ohms per volt or
is too low for satisfactory operation. Tube V22
higher. The voltage across the regulator lamp
should be replaced after each 500 transmissions.
should be between 82 and 92 volts. If the voltage
is found to be correct, test V9 or try a new tube.
Weak tubes in stages V7 and V8 maybe responsible
for a low RB+ voltage. There is a possibility
a. Signal Tracing. For a quick check of satis-
that an excessive load is responsible for the low
factory operation, measure the voltage across the
output voltage. If there is reason to suspect this,
exciter lamp. It should be between 6.1 and 6.3
disconnect the load from the regulator panel and
volts at any line voltage between 100 and 130.
measure the open-circuit voltage. If it is neces-
The measurement is made at the terminal strip
sary to trace further, test the components for
in the power supply. The voltage drop to the
the correct value. Capacitors, especially C57,
lamp socket is approximately .2 volt. If the
must be tested for leakage.
voltage is not correct, measure the voltage on grid
4 of stage V17 (7S7). This should be about 4
volts ac, as measured with a vacuum-tube volt-
a. General. The term rectifier power unit is
meter. If further signal tracing is necessary,
used to describe the complete unit, which includes
follow the steps shown in the signal-tracing chart.
the conventional rectifier power supply and the
The exciter lamp should be replaced after each 500
exciter lamp power supply system. In this para-
transmissions.
graph, the term rectifier power supply does not
include the exciter lamp supply system.
lamp burns out, the load on transformer T15
becomes practically zero, resulting in a very high
may require replacement after a few hundred
voltage across secondary No. 1. There have been
hours of operation, especially if the equipment is
transformer failures as a result of the high voltage.
to operate on low line voltages; that is, 100 to 110
In tracing trouble, consideration must be given to
volts. A tube selected for hugh emission permits
the probability of failure of this transformer.
operation on voltages between 90 and 100. If
Section Ill. REPAIRS AND ADJUSTMENTS
the bearing has been thoroughly cleaned, dry out
103. General
all traces of the solvent before re-oiling.
Except in cases of simple trouble, do not attempt
to repair mechanical parts such as the motor, the
serial numbers below 1157, do not attempt to
lead screw, the drum, and the tuning fork unless
replace the side-wall bearing in the field. Clean
new parts are not available. Put replacements
and re-oil. In motors with serial numbers above
and lead wires in the same physical location as the
1157, side-wall bearings (C, fig. 105) are of the
original. When replacing components in the
ball-bearing type. These bearings may be re-
amplifier system, take into account protection
placed when defective. If tests indicate that
necessary because of high voltages.
synthetic rubber pad P is too soft, replace it.
If the pad is too hard, a portion of the center can
be cut away. If bearing ball R is replaced,
select one that will not bind on the side wall of
Caution: Work on the synchronous motor
should be done in a clean, dry place. Disassemble
the bearing.
c. Synchronous Rotor Bearing (D, fig. 105).
and assemble motor parts with care, avoiding
This bearing cannot be replaced in the field.
risks of scratching and damaging any of the
If the rotor turns too freely on the shaft and does
machined surfaces. Be careful not to lose the
not fall into synchronism as it should, the free
lower thrust bearing ball (R, fig. 105) when dis-
oscillating action must be reduced by adding some
assembling the motor.
end thrust. Insert a thin washer (U) between
the felt washer (V) and collar (S) Remove the
to replace this bearing in the field. Use solvent
collar after driving out the taper pin (T) with a
(SD) to remove gummed-up oil and grease. After
