CRT Supply Dropping Out

See function block K of A6 Power Supply Schematic Diagram in the Component-Level Information binder.

The CRT Supply is a separate switching supply which provides the +llO Vdc for the Al7 CRT Driver from a winding on the A6Al HV Module. The CRT Supply operates at approximately 30 kHz. The exact frequency is determined by the inductance of the primary winding of A6AlTl and A6C407. The supply will only operate if the HV-SHUT-DOWN line is high.

If the power supply keeps dropping out, there is probably a short on the Al7 CRT Driver assembly.

1.Disconnect W8 from A6J4.

2.Connect an IC clip to U401 and connect a jumper between U401 pin 10 and TP308 (+5 Vdc).

3.Connect a voltmeter to TP405 and set the (LINE) switch on.

4.Check TP405 for a voltage of approximately +llO Vdc. It will probably measure higher since there is no load on the supply.

5.If the voltage at TP405 is correct, suspect a short on A17. If the voltage at TP405 is not correct, check pin U401 pin 8 for a sawtooth signal. The sawtooth should be flat-topped and about 5 Vp-p at a frequency of about 30 kHz.

6.If the sawtooth is not flat-topped, suspect U402A and its associated circuitry.

7.If the sawtooth is correct, check the base of Q401 for 30 kHz pulses.

8.If the duty cycle is high, but there is no +llO Vdc, suspect the bridge rectifier, CR401 through CR404.

Blanking Signal

1.Connect an oscilloscope probe to A25202 pin 3. Connect the oscilloscope ground lead to TP3. Set the oscilloscope to the following settings:

Sweep Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2ms/div

Vertical Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 V/div

2.If a 4 Vp-p signal is not observed, the A2 Controller Assembly is faulty.

3.Repeat steps 1 and 2 with the oscilloscope probe on A25202 pin 14.

4.Set the oscilloscope to the following settings:

Sweep Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . lps/div Vertical Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 V/div

5.Connect the positive probe lead to A2J202 pin 15. This is the blanking output.

6.TTL-level pulses should be observed. If the signal is either always high or always low, the display will be blanked; suspect the A2 Controller Assembly.

7.If the signals on A2J202 pins 3, 14, and 15 are correct, troubleshoot the Al7 CRT Driver.

Display/Power Supply Section 12-15