Maintenance— 2440 Service
Table 6-7 (cont)
INCORRECT
See INITIAL SETUP in VIDEO Trigger Problem for control settings and signal application. Set the test
LINE
scope Sec/Div setting to 5
tis
and the Volts/Div to 2 V.
COUNTING
1.
Check that the correct protocol and Counter Restart choices are selected for the applied Video
2.
signal. (TV OPT under the EXTENDED FUNCTIONS— SYSTEM choices.)
Check that the HORIZCLK signal at U220B pin 12 is stable. If not, troubleshoot that problem.
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?)
4.
If the FIELD and HORIZCLK signals are ok, suspect a problem with Line Counter U530, NAND-
gate U 541, or U424.
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.)
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.)
7.
If not correct, replace U530.
8.
Check that the clock at pin 8 of U424D is stable and has a LO-to-HI transition. If no transition,
replace U424.
TV CLAMP
PROBLEM
N O T E
T h e V id eo O p tio n m u s t h a v e a c o m p o s ite -s y n c o r c o m p o s ite -v id e o s ig n a l s o u rc e a p p lie d
fo r th e C h a n n e l 2 D is p la y C la m p to fu n c tio n p ro p e rly . C la m p in g a c tio n is u n p re d ic ta b le if a n
in c o rre c t s ig n a l is a p p lie d . T h e C L A M P c irc u it re m a in s o n, e v e n i f V ID E O C O U P L IN G is n o t
s e le c te d , a n d m a y b e u s e d to d a m p a C h a n n e l 2 d is p la y if th e s e le c te d V ID E O s o u rc e sig
n a l is a c o m p o s ite -s y n c o r c o m p o s ite -v id e o sig n al.
1.
Set the Trigger CPLG to TV, Trigger SOURCE to CH 2, Trigger SLOPE to - (neg-sync), A COU
PLING to FIELD1— Line count to 50, CH 2 input coupling to DC, SEC/DIV to 5
n s,
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.
2.
Is the display triggered and stable? If not, the CLAMP circuit will not be properly enabled in any
case, and some other problem may exist. Check that the collector of Q330 is LO. If not LO, either
the PLL (U314) is not locked or Q 330 or its associated circuitry is defective; go to PHASE-
LOCKED LOOP PROBLEM troubleshooting.
3.
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).
4.
Set the offset voltage for — 1.5 V offset, and verify that the back-porch level is offset from ground
- 1 . 5 V.
5.
Set CLAMP ON. Is the back-porch level clamped to ground level? If so, the VIDEO clamp
is functioning.
6-107
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