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Maintenance— 2440 Service
Table 6-6 (cont)
HOLDOFF
Trigger Holdoff Circuitry (schematic diagram 13):
PROBLEMS
Run Extended Diagnostic test 2600 for the SIDE-BOARD registers U761 and U762. If that fails,
troubleshoot the indicated failure.
If not, troubleshoot the Trigger Holdoff circuitry.
1.
Check the emitter voltages for logical HI/LO as follows:
SEC/DIV
Q761
Q771
Q772
Q783
500 ns
HI
LO
LO
- 1 5 V
1
ns
LO
HI
LO
- 1 5 V
10
tis
LO
LO
HI
+ 5 V
If these levels are not correct, suspect the corresponding emitter diode, or the transistor emitter-
base junctions as being defective. Observe that Q783 has no emitter diode, so suspect the
transistor itself or Q782.
Some triggering failures are an indication of possible problems with the ATHO (A trigger holdoff
signal). If ATHO is stuck HI, no triggers will be permitted by A/B Trigger Logic Array U150; if stuck
LO, the triggering will be unstable.
2. Check the signals around flip-flop U872 for proper action of that device (see the test waveforms
associated with the circuit next to schematic diagram 13).
Test scope: Select ENVELOPE 1 and AUTO TRIGGER MODE. Scope under test: Select 5 ns/div,
trigger on the CAL signal, and set HOLDOFF to minimum.
SEQUENCER
Sequencer Output circuitry (schematic diagram 20).
OUTPUT
PROBLEMS
SEQUENCE OUT doesn’t switch LO at the end of a sequence or back HI when the sequence is
exited after completion:
1. Create a sequence with one step and no PAUSE. The front-panel setup is arbitrary for the
step (see Operators Manual for operating the Sequencer).
2. RECALL the sequence. Check that Q 104's collector is HI before the sequence is recalled,
switches LO at the end of a sequence, and switches back HI when the sequence is exited
(see Operators Manual for operating the Sequencer).
If the collector switches properly, the problem is an open component in the R104-
J125/P125-Flex Cable-J1903 path.
3. If collector doesn’t switch, either the transistor, its collector supply, or its base drive is bad
(CR104 may also be shorted). Isolate base drive to Q 104 via R300 to determine whether
drive or output circuitry is bad. The driving signal comes from I/O register block of schematic
diagram 1.
SEQUENCER
OUTPUT
PROBLEMS
Sequencer Output circuitry (schematic diagram 20).
STEP OUT doesn’t switch LO at the end of a sequence step or back HI at the start of the next
sequence step:
1.
Create a sequence containing at least two steps. The front panel setup is arbitrary for both
steps, but set PAUSE on in the ACTIONS menu associated with the first step (see Operators
Manual for operating the Sequencer).
6 -4 6
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