Agilent Technologies 856290216 service manual Focus Problems

12. If signals are correct and cables to CRT are good, suspect the CRT.

Focus Problems

Focus problems may be due to a defective A18Vl CRT, the Al7 CRT driver (especially the grid level shifter section), or the A2 controller focus-control circuitry. Focus problems may also be due to improper adjustments, improper connections, or absence of high voltage.

The Al7 focus grid level shifter (function block D) is the leading cause of Al7 focus problems. Function blocks C, E, F, and H generally have less effect on focus, but may cause poor focus that is a function of screen position, length of line, or intensity.

1.Connect the positive lead of a DVM to A2J202, pin 2. Connect the negative lead to A2J202 pin 6.

2.Use the knob to change the focus DAC value from 0 to 255.

3.If the DVM reads near 0 Vdc with the focus set to 0, and near -10 Vdc with the focus set to 255, A2 focus control circuitry is working properly.

4.Perform the “Display Adjustment” in Chapter 2, “Adjustment Procedures.” Note that A17R34 COARSE FOCUS has the greatest effect on focus. Adjustment A17R93 ASTIG and A17R92 DDD have a lesser effect, and A17R21 2 FOCUS, A17R26 X FOCUS and front-panel adjustment (press (DISPLAY), MORE i OF 2, FOCUS, and turn knob) have very little effect on focus.

5.If the focus of some areas of the screen are worse than normal, continue with step 11. If no part of the screen can be brought to sharp focus, continue with step 6. (CRTs have some normal focus variation across their face.)

6.Turn off the analyzer and place Al7 in the service position. Connect the ground lead of a high-voltage probe (HP 34111A) to the chassis, and use it with a DVM to measure A17J7( 10).

7.The nominal A17J7(10) voltage is -1600 Vdc, but the CRT will function if this voltage is within 200 V of -1600 Vdc.

8.Adjusting A17R34 COARSE FOCUS should vary the A17J7(10) voltage by 150 V. If these voltages are correct, suspect the CRT.

9.Check the A6Al high voltage module cathode supply output at A17TP16 using a high voltage probe. If the cathode voltage is -2450 V f250 V, check the focus grid level shifter.

10.If the cathode voltage is not correct, check the A6Al high voltage module and its connections.

11.Connect an oscilloscope probe to A17TP9. This signal corrects the focus for the X position of the CRT beam, and for intensity level. It also provides the front-panel focus adjusting voltage.

12.Press (DISPLAY), MflRF, 1 OF 2 , and FOCUS. While turning the front-panel knob, verify the dc level of the signal at TP9 adjusts about 30 Vp-p.

Display/Power Supply Section 13-9