TROUBLESHOOTING AND REPAIR
DIGITAL CONTROLLER
Edit Knob Interface 6B-33.
The edit knob interface circuitry receives two input signals (WINDOWL and
TRIGGERL) from the opto-interrupters on the A19 switch PCA.
If the trigger signal makes a high to low transition while the window signal is low, an
edit up request is generated. This information is transmitted to the Controller PCA by
setting the knob interrupt signal KNOBINTL low and the knob direction signal
KNOBUP high.
If the trigger signal makes a low to high transition while the window signal is low, an
edit down request is generated. This information is transmitted to the Controller PCA
by setting the knob interrupt signal KNOBINTL low and the knob direction signal
KNOBUP low.
The trigger signal is ignored when the window signal is high.
After servicing the interrupt and reading the directional information, the controller
resets the knob circuitry by toggling the reset signal KNOBRSTL.
Display Blanking 6B-34.
Monostable (U11) and NOR gate (U12) clear the display if new field or segment
strobes are not received. This protects the display if the microprocessor stops
refreshing.
Operate/Standby Selection 6B-35.
The front panel POWER switch selects the operate or standby modes. When in the
standby position, the switch is closed, the STANDBY signal is set high, and LED CR1
is lit. When in the operate position, the switch is open, the STANDBY signal is pulled
low,
and LED CR1 is
off.
FRONT PANEL TROUBLESHOOTING 6B-36.
Display and Controls 6B-37.
If the display shows signs of activity but has missing or bright digits or segments, the
problem is most likely one of the data latches or drivers on the A1 Display PCA. If the
display is blank and the controller is operational, check the power supplies.
Use the I/O diagnostic tests described in the A13 Controller troubleshooting section to
continuously write data to the display latches. Verify that the correct data is written to
each latch and is present at the outputs of the display drivers. The display blanking
output of U11 (pin 13) should remain high while the data is written.
As the edit knob is rotated, the window and trigger signals should generate transitions
on the input signals to U18 resulting in an interrupt at U12 pin 12. If the signals do not
change as the knob is turned, suspect the opto-interrupters on the A19 switch PCA or
the interconnection with the switch PCA.
Two special-function service tests are available to test the front panel indicators and
keys. Special function 901 checks the front panel displays by lighting all segments. The
test is exited by pressing any key.
Special function 902 initiates the key check. As each key is pressed, its identifier code is
displayed in the center of the FREQUENCY display field. The key identifier codes are
assigned in order from top to bottom and from left to right. This test is exited by a clear
entry.
6B-11/6B-12
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