Zenith H-100 - Page 86

5-170
LOGIC LEVEL CHECKS
On the following pages is a logic probe analysis of the
H-207 board. When performing the tests, you need only to
test the ICs in the left column indicated by an asterisk
(*). If you don't get the suggested logic state, then check
each IC listed immediately to the right. The logic states
for these ICs are listed in the left column below the ICs
with the asterisks.
Continue tracing backwards using this procedure until you
test an IC that matches the suggested logic state. The
previous IC that you tested is likely the bad IC.
Before you replace the suspected IC, check the other lines
lead ing up to it. You must do this because this checkout
procedure gives only the most likely causes to the problem.
It doesn't cover such things as open ground foils, shorted
foil runs, or open resistors.
As you make the following checks, press the (B)oot key and
press RETURN. Logic states located inside parenthesis
indicates that the probe pulses one or more times while
"Read Completed" is printed on the screen. In the case
of a (P) indication, the pulse rate (as indicated by the
logic probe)' will momentarily change during the "Read
Completed" interval.
The schematic shows the logic states after a CTRL/RESET
has been performed. Refer to these logic states for
troubleshooting areas not covered in the following tests.
To setup the H-207 for the following test. connect at least
one 48 TPI, 5-114" disk drive to P1 and turn on the
computer.