Agilent Technologies 856290216 service manual CRT Supply Dropping Out

Note	Ideally, the DVM should read the voltage written on the label of the
	A6Al HV module. If necessary, perform the “High Voltage Power Supply”
	adjustment in Chapter 2, “Adjustment Procedures.”

5.If the DVM does not read approximately +llO Vdc, measure the voltage on A6U401 pin 10. This is the LHVSHUT-DOWN signal and should be near +5 Vdc.

6.If LHVSHUT-DOWN is low, suspect a bad connection along W8 between the A6 power supply and the Al7 CRT driver.

7.If LHVSHUT-DOWN is correct, connect an oscilloscope to A6TP402. Connect the scope probe negative lead to TP401. Set the oscilloscope to the following settings:

Sweep time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 psjdiv Vertical scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..lO V/div

8.A nearly-sinusoidal waveform, greater than 30 Vp-p, with an approximately i-18 Vdc offset, should be observed.

9.If the waveform is a dc voltage near 0 Vdc with narrow, positive- and negative-going pulses, the A6Al HV module is faulty. If the waveform is a dc voltage near +18 Vdc with narrow, positive- and negative-going pulses, connect the probe to TP403.

10.If the waveform at TP403 is a sawtooth waveform with a 1.8 V amplitude, the A6Al HV module is faulty.

11.If the TP403 waveform has pulses similar to those on TP402, the A6 power supply is probably faulty.

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 LHVSHUT-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 (i-5 Vdc).

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

4.Check TP405 for a voltage of approximately fll0 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.

13-16 Display/Power Supply Section