TM 9-6625-1753-14
C3
sight, smell, hearing and touch method. These tables list the malfunctions, probable causes, and corrective actions for the
Table 5-2. Troubleshooting the Main Frame - Continued
main frame, dual trace plug-in, and delaying sweep plug-in. Obviously, such tables can't include every trouble symptom that
may occur. However, using personnel should use the tables, in conjunction with the functional and schematic diagrams, as a
Malfunction
Probable case
Correct action
guide in analyzing faults that may not be listed in the tables.
No high voltage (765MH and 765
Check high-voltage rectifiers (fig. 5-2).
Table 5-2. Troubleshooting the Main Frame
MH/F).
Check high-voltage oscillator (oscillator
Malfunction
Probable case
Correct action
grid voltage is -25 volts when oscillator is
working) (fig. 5-3).
Power-on
indicator
does
not
Lamp open.
Replace lamp DS1001.
Check transformer T2001 for open or
illuminate.
short.
No input power.
Check power source.
No high voltage (765MA).
Check for the presence of +23 2 volts at
Check power input cable.
the emitter of Q2004 using a DC
Check that both plug-in assemblies are
securely locked to the main-frame
Check for the presence of a 40 kHz sine
connectors.
wave at the collector of Q2005 using the
Thermal relay cut-out (relay S103)
Check thermal relay S103.
test oscilloscope. This signal should be
has opened because of excessive
Check operation of fan (thermal relay S1 is
approximately 30 volts p-p at a
heat.
closed when fan is on).
frequency of 35-45 kHz. If no oscillation
Line fuse F101 is open.
Replace fuse (fig. 5-1).
is observed, turn off the power and
Primary of transformer T1001 is not
Check that 230v/115v switch S101 is set to
check Q2005, CR3001, CR3002 and the
connected to proper voltage source.
the proper input voltage (fig. 5-2).
continuity of windings on T2001.
CRT defective.
Replace CRT.
Line fuse opens after power is turned
Shorted rectifier and/or shorted filter
Starting with the -50 volts power supply,
on.
capacitor in low-voltage power
check power supply rectifiers and filter
Poor regulation of high voltage.
High-voltage regulator tube defective.
Replace V201 (fig. 5-3).
Shorted
negative
high-voltage
Check CR203-CR205 (fig. 5-2).
supply or fan circuit.
capacitors for shorts.
Short in transformer T1001.
Check for short in transformer primary and
rectifiers.
secondary windings (fig. 5-3).
Incorrect
bias
on
high-voltage
Using HV ADJ resistor, adjust bias to -51
Fan fails to operate when thermal
Burned-out fan motor.
Replace fan assembly (fig. 5-3)
regulator.
volts.
relay S1 closes.
Heater fails to operate when
Thermal relay S1201 remains opens.
Check relay. Replace if defective.
temperature drops below 41F.
Complete failure in one or more of the
Fuse is open.
Check and replace any opened fuses in
power supply voltage regulators,
the regulated power supplies (fig. 5-1).
and line voltage correct.
Incorrect output voltages from one or
-50 volts regulators misadjusted.
Set the -50 volts regulator for an output
more regulators.
voltage of -50 volts and then, in turn,
read just the 50 volts, 100 volts, and 200
volts regulators to the correct voltage
Defective component within the power
Starting with the -50 volts regulator,
supply voltage regulators.
systematically check each stage for
proper
voltages
until
defective
component is isolated.
Replace
component.
Power-on indicator illuminates but no
Broken lead from CRT filament
Replace lead.
trace on CRT.
winding.
INTENSITY control too low.
Adjust INTENSITY control.
Power-on indicator illuminates but no
Delaying trigger LEVEL control,
Set controls to proper level.
trace on CRT.
normal trigger LEVEL control, or the
delaying sweep plug-in POSITION
control is improperly set.
Figure 5-1. Main frame, oscilloscope 765MH, right side view showing fuses.
5-2
