HP Integral Personal Computer Service Manual page 138

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CIrcuItIy
The typical symptom of bad home-switch circuitry is that the carriage will move initially to the
right, reverse direction, and crash
into
the left side frame for a few seconds, then come to rest.
Use a logic probe to troubleshoot the home-switch circuitry as desaibed below. (If you have not
already done so, prepare the computer
to
troubleshoot the logic B PCA as desaibed
in
table 8-27.)
In each of these steps, always turn off the power before
you
connect or disconnect a cable or replace a
component.
Notel The operating system ROM must be installed in the computer during the tests that
follow, and paper must be installed in the printer.
Do not troubleshoot the home-switch
circuitry
unless the carriage is capable of moving
through the home switch. If the carriage fails to move, the problem is located elsewhere.
.....
1. Verify the operation of the printer switch
panel if you have not already done
so
(refer
to table
8-14).
2. Install a known-good home switch. Probe
pin 46 of U28 on the logic B peA while you
press the top-oE-form button (the square,
white button on the switch panel).
3.
Probe pin
6
of U34 on the logic
B
PCA
while
you
press the top-of-form button.
4. Check the voltage on pin 5 of U34 on the
logic B PCA.
5. Probe pin 7 of U34 on the logic B PCA
while you press the top-of-form button.
I
e.
Press the LF and then the FF key.
7. Run the printing self test by turning off the
power, pressing the LF key and holding .t
down, and then turning the power on.
I
Indlat_ Action
If the pin does not pulse high for about two sec-
onds and then return low, replace U28.
If the pin does not pulse low for about two sec-
onds, and then return high, replace
U36
on the
logic B PCA.
If it
isn't
in the range 4.75 to 5.25 Vdc, check Ie
U34 on the logic B PCA and
its
associated resis-
tors, and replace ~ required.
If pin 7 is low, and then pulses high several
times, this indicates that the home-switch cir-
cuitry is working properly. If the home-switch is
good, IC U28 may be bad; if not, replace Ie
U34
on the logic B PCA.
If the paper does not advance normally, go to
table 8-20.
If the printer doesn't print the test pattern
shown in figure 8-9, go to table 8-21.