Model 83522A
Service
annunciators are buffered by inverters, and drive current through the LED to
ground rather than sinking current from
+5V. The outputs of these buffers can
be checked during Hex Data Rotation Write.
MKR and UNLEVELED lights are driven by pulse-stretching timers. These are
disabled by U9A during retrace. Check that U9, pin 3, is high during retrace
(approximately
+4Vdc), and low during forward sweep. The UNLEVELED
light should be lit when the available power is insufficient for leveling to the
desired reference level (typically several dB beyond specified maximum leveled
power).
If the L
MKR light is not functioning properly, set the 8350A as follows: Start
sweep
=
30 MHz, Stop sweep
=
90 MHz, Time
=
500 msec. Select 83522A
1
MHz MARKERS. Connect oscilloscope channel
B
to the 8350A Sweep Out, and
select the A vs B mode for horizontal deflection as a function of the
8350A sweep
ramp. Check the input (pin 8) and output (pin 9) of timer
U12B. The output of
U12 goes high for an initial low pulse at the Trigger input (T), and remains high
for a period of approximately 50 milliseconds. Subsequent trigger pulses,
occurring within the timing cycle, will not affect the output. However, if the
Trigger input remains low for a longer duration than the timing cycle, the output
will remain high for the duration of the trigger signal. If no trigger signal is
present, check diodes CR4 through CR7, or trace the problem back to the A7
assembly.
If the UNLEVELED light is not functioning properly, select
8350A
RF
WK
and disengage 83522A
RF
to turn the power off. In this mode,
LUNLVL, 51-12, should be low during forward sweep, and high during retrace.
Connect oscilloscope Channel
B
to 8350A Sweep Out and select the A vs. B
mode for horizontal deflection as a function of the
8350A sweep ramp. Check
the input (pin 6) and the output (pin 5) of timer
U12A. Refer to the previous
paragraph for an explanation of U12. If the circuit is functioning properly, trace
the problem back to the A4 assembly.
Keyboard
The keyboard matrix is scanned continuously by U6. This LSI device
continuously strobes the column lines, senses the row lines for depressed keys,
eliminates contact bounce, stores the key code internally, and flags the
8350A to
recover the key code. Troubleshooting is difficult because the device is so
complicated, but it is worthwhile to check all signals to and from U6, probing
directly on the pins of the chip, before replacing it.
Error codes E050 and E05 1 generally indicate U6-related problems:
E050 occurs when the microprocessor has received a flag (L PIFLG) from
the plug-in (indicating a front panel key was pressed), but cannot recover
the
keycode (indicating that the key was NOT pressed). Check the FLAG
output from
MU6 (accessible at A3P1-42). It should be TTL low,
approximately
0 volts. Pressing a front panel pushbutton should result in a
very rapid pulse. If the line appears to be locked high, replace
A2U6. If it is
good, check inverter
A3UlOF (accessible at A3J1-39) to see if it is locked
low.
E051 occurs when the key code received by the microprocessor cannot be
decoded. This indicates a failure in
A2U6 or a bad Row Sense line. If the
Row Sense lines are good, troubleshoot the keyboard matrix with a
continuity checker.
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