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Maintenance— 2440 Service
Table 6-6 (cont)
TIMING ERROR
AT 50 MS/div
AND FASTER
Phase-Locked Loop Circuit (schematic diagram 11).
1.
Check the 10-MHz input to U381 pin 6. If there is no 10-M Hz clock at TP174 then go to the
Timebase troubleshooting chart (located in the “Diagrams” section) and troubleshoot the System
Clocks.
NOTE
Use 2 44 0 C U RSO R function o f 1/T IM E to m easure the frequency. The cursor posi
tion difference will read out directly in frequency.
2.
Check U381 pin 9 for 10 MHz.
Frequency too low at pin 9:
a. Check that U381 pin 3 has negative pulses and that the voltage at U381 pin 12 is positive with
respect to U381 pin 3. The VCO CTL voltage at TP581 can be as high as + 1 2 V.
Frequency too high at pin 9:
b. Check that U381 pin 12 is ramping negative with respect to U381 pin 3 (average not absolute)
and TP581 can be as negative as —0.6 V.
c. If these conditions are not true, the problem is probably Phase/Frequency Detector U381 or
amplifier U580.
MISSING DATA
POINTS
IN REPET
Jitter Correction Troubleshooting (schematic diagrams 12 and 13):
On the scope under test, select REPET acquisition mode, AUTO LEVEL, VERT trigger, DC
Trigger COUPLING, and set the SEC/DIV setting to 5 ns. Then select ACQUIRE and connect a
probe from the CH 1 input to TP291 (TTLB1) (found above A10U450, the CH 1 CCD, on the
main board).
If there are bands of missing data points every two divisions, or only a few data points are placed
every two divisions or the waveforms are distorted, the problem may be in the Jitter Correction cir
cuitry.
The Jitter Correction circuit has both analog and digital circuits. First check the digital portion to insure
that it is working. If that is ok, then assume that the problem is in the analog portion of the Jitter
Correction circuit.
DIGITAL SECTION TROUBLESHOOTING
1.
Check that START1 and START2 are present at U841 pin 2 and U842 pin 2 (located on the Side
board and shown on diagram 13) respectively and that they are coincident.
a. Test the collector of Q492 and Q391 (located on the Main board and shown on diagram 12)
for the START pulses.
If missing:
b. Check for SRAMP1 and SRAMP1 at the bases of Q492 and Q 491.
c. Check for SRAM P2 and SRA M P2 at the bases of Q391 and U390.
d. Check for RAMP and RAMP at the bases of Q392 and Q490.
If any gating signals are absent, backtrack to U470 and/or U370 (on diagram 11) and locate the
defective component.
6-3 8
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