6-102
Set TV CLAMP ON. Is the back-porch level clamped to ground level? If so, the TV clamp is
functioning.
5.
Set the offset voltage for -1.5 V offset, and verify that the back-porch level is offset from ground
-1.5 V.
4.
If the display is triggered correctly, check that the back-porch level of the displayed video signal is
at approximately ground level. If not, run SELF CAL and check again. If there is a large offset
present, troubleshoot CH 2 Preamp U320 and U230 (schematic diagram 9).
3.
Is the display triggered and stable? If not, the TV CLAMP circuit will not
be
properly enabled in
any case, and some other problem may exist. Check that the collector of 0330 is LO. If not La,
either the PLL (U314) is not locked or 0330 or its associated circuitry is defective; go to PHASE-
LOCKED LOOP PROBLEM troubleshooting.
2.
Set the Trigger CPLG to TV, Trigger SOURCE to CH 2, Trigger SLOPE to - (neg-sync), A TV
COUPLING to FIELD1-Line count to 50, CH 2 input coupling to DC, SEC/DIV to 5
j.LS,
TV
CLAMP OFF, and VOLTS/DIV to 1 V (for a two-division signal amplitude). Connect the negative
sync composite video signal to the CH 2 input in series with a dc offset voltage source. Set the
offset level for 0 V offset.
1.
NOTE
The Video Option must have a composite-sync or composite-video signal source applied
for the Channel
2
Display Clamp to function properly. Clamping action is unpredictable if an
incorrect signal is applied. The TV CLAMP circuit remains on, even if TV COUPLING is not
selected and may be used to clamp a Channel
2
display if the selected trigger source signal
is a composite-sync or composite-video Signal.
TV CLAMP
PROBLEM
8.
Check that the clock at pin
8
of U424D is stable and has a LO-to-HI transition. If no transition,
replace U424.
7. If not correct, replace U530.
6. Check Line Counter outputs at pin 27 and pin 3 for a LO-to-HI transition during the vertical sync
pulse time. (View the composite video on channel 2 of the test scope and use the channel 1
probe to check the signal at pin 27 and pin 3. Trigger the test scope on the channel 1 signal. Use
delayed sweep to view the signals if using an analog test scope.)
5. Check that the FLD1 signal at U541 is HI when FLD1 is selected and alternates HI-to-LO when
ALT is selected. If not, troubleshoot A12U750 (schematic diagram 20). (This assumes that the
FLD2 diagnostic test passed the power-on diagnostics.)
4. If the FIELD and HORIZCLK signals are ok, suspect a problem with Line Counter U530, NAND-
gate U541, or U424.
3. Check that the FIELD signal at U424E pin 13 is stable and correct (waveform 164). If not, trouble-
shoot that problem. (Is the trigger signal amplitude excessive, causing erratic triggering?)
2. Check that the HORIZCLK signal at U220B pin 12 is stable. If not, troubleshoot that problem.
1. Check that the correct protocol and Counter Restart choices are selected for the applied Video
signal. (TV OPT under the EXTENDED FUNCTIONS-SYSTEM choices.)
See INITIAL SETUP in TV Trigger Problem for control settings and signal application. Set the test
scope Sec/Div setting to 5 ,.,.sand the Volts/Div to 2 V.
INCORRECT
LINE
COUNTING
Table
6-7
(cont)
Maintenance-2430 Service
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