6-43
If not there, troubleshoot A12U760 and A12U884 to determine why the select is not being gen-
erated (both shown on schematic diagram 1).
3. Test that the Band C multiplexer selection lines are HI (A12U130 pins 9 and 10) and that the
enable line is being driven from HI to LO during plotting.
2. Check that the signals at A12U120 pins 3 and 4 are the same. Replace A12U120 if they are not.
1. Check that the signals at A12U120 pins 5 and 8 are the same. Replace A12U120 if they are not.
X-Y Plotter Circuitry (schematic diagram 20):X-Y PLOTTER
OUTPUT
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.
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).
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.
If these levels are not correct, suspect the corresponding emitter diode, or the transistor emitter-
base junctions as being defective. Observe that
0783
has no emitter diode, so suspect the
transistor itself or
0782.
0783
-15
V
-15
V
+5V
0772
LO
LO
HI
0771
LO
HI
LO
0761
HI
LO
LO
SEC/DIV
500 ns
1 IlS
10
IlS
1. Check the emitter voltages for logical HI/LO as follows:
If not, troubleshoot the Trigger Holdoff circuitry.
Run Extended Diagnostic test 2600 for the SIDE-BOARD registers U761 and U762. If that fails,
troubleshoot the indicated failure.
Trigger Holdoff Circuitry (schematic diagram 13):
HOLDOFF
PROBLEMS
CDACG(DAC gain)is interactive with DACO(DAC offset), and the DACG range can be limited if DACO is not centered. Changing
either DACGor DACOwill cause the remaining DAC System outputs to be invalid until the correct settings for DAC gain and offset
are rewritten into the DAC System.
DAC DAC Ampl
COLD START Voltage Range Effect of
Output
Output DAC VALUES
Increasing Value
0
4095
HORF
U631-8 100/-3.90 V -4.11 V 4.09 V
Increases holdoff
DACO
U650-6 2048/0 V 13.92 V -13.15 V
Unbalances DAC
DACG
U660-6 3929/-0.21 V 14.02 V -13.32 V
Uncalibrates DAC
-5.18 VC
Force DAC Ranges (cont)
Table 6-6 (cont)
Maintenance-2430 Service
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