Table 5-2. Troubleshooting the Main Frame

TM 9-6625-1753-14

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

voltmeter.

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

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 41°F.

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

(fig. 5-5).

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