HP 8560E Service Manual page 635

Display/Power Supply Section
Focus Problems
Focus Problems
Focus problems may be due to a defective A18V1 CRT, the A17 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.
Although A17 grid level shifter (function block D) is the leading cause of
A17 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, the A2 focus control circuitry is
working properly.
4. Perform the "Display Adjustment" in Chapter 2. Note that A17R34
COARSE FOCUS has the greatest effect on focus. Adjustment
A17R93 ASTIG and A17R92 DDD have a lesser effect, and A17R21 Z
FOCUS, A17R26 X FOCUS and front panel adjustment (press
,
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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 A17 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 A6A1 high voltage module cathode supply output at
A17TP16 using a high voltage probe. If the cathode voltage is −2450
V ±250 V, check the focus grid level shifter.
10.If the cathode voltage is not correct, check the A6A1 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.
634
, , and turn knob) have less effect on
FOCUS
Chapter 12