TROUBLESHOOTING AND REPAIR
DIGITAL CONTROLLER
If the signals pass the above tests, check the data and address signals at any suspicious
latch or DAC on the suspect RF circuit board. If a DAC problem is suspected, use
special functions 941, 942, and 943, which set all DACs to zero, half scale, and full scale
respectively.
Microprocessor Kernel 6B-13.
A blank front panel is a symptom of many controller-related problems. Micropro-
cessor-related problems are difficult to troubleshoot because of the volume of activity
at any given time. However, one can systematically verify the independent circuit
functions and quickly spot some of the most obvious problems. Read the following
paragraphs and verify the related circuitry.
Clock 6B-14.
Connect an oscilloscope probe to the clock oscillator output (U18 pin 4). There should
be a symmetrical 8-MHz square wave with adequate logic levels. If the signal appears
abnormal, determine if the problem is with the oscillator circuit or the ICs connected to
the clock output by checking the input signal at pin 3 of U18. It should be an inverted
version of the same 8-MHz square wave. (It may be slightly distorted due to its
loading.)
Power-On Reset 6B-15.
Connect an oscilloscope probe to the RESET input (pin 18) of U1. The signal should
generate a low to high transition on power-up and remain high during normal
operation. Turning the power off and on generates an active low reset pulse
approximately 200 ms wide.
If the reset pulse to U1 appears abnormal, compare it to the reset output (pin 5) of the
power supply monitor IC U13. Suspect problems with U13, and all ICs connected to
the RESETL and RESETH signals.
Also check the HALT input (pin 17) of U1; it should look like the RESET (pin 18)
input.
Unused Microprocessor Inputs 6B-16.
Input signals to U1, BR (pin 13) and BGACK (pin 12) should both be high. If either of
these signals is not high, correct the fault before continuing.
Bus Error 6B-17.
The bus error input BERR (pin 22) notifies the microprocessor when a memory cycle
cannot be completed as a result of a hardware fault. Normally, the BERR signal should
always remain high.
If the BERR signal goes low, verify that pin 1 of U14 is clocked by an 800-kHz signal
with a 60/40 duty cycle. Also verify that pin 2 of U14 is receiving continuous activity
from the address strobe signal (AS).
Interrupts 6B-18.
The front panel edit knob interrupt is generated on the A1 Display PCA when the knob
is turned. The interrupt signal from the display PCA connects to U12 pin 13. If it is low
when the knob is in the rest position, refer to "Edit Knob Interface" later in Section 6B.
6B-5
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