Zenith Z-100 PC series Service Manual page 27

Page
4.2
Troubleshooting
2. Set the polarity of the meter to positive and probe pin 1 of P110.
Meter should read
+
12 VDC (
+
5%, - 4%).
3. Probe pin 10 of P11
o.
Voltage should be
+
5 VDC (
±
3%).
4. Set the polarity of the meter to negative and probe pin 4 of P11
o.
Voltage should be -12 VDC (
+
5%, - 4%).
If any of the voltages are missing or low, the power supply is suspect.
Refer to the troubleshooting section of the power supply module.
If a voltage is present, but its LED is not lit, perform the following steps:
1. Turn off the computer, and set the meter to read ohms. Check the
value of the resistor associated with the LED that was out. R102
and R103 should read approximately 1500 ohms, and R105 should
read 560 ohms.
2. If the resistor checks good, replace the LED in question.
If the previous checks do not indicate a problem, perform the following
checks:
1. Set the voltmeter to read negative polarity, and turn the computer
back on.
2. Probe pin
0
(output) of U101. Meter should read approximately -5
VDC (± 5%). If it does not, replace U101. If replaCing U101 does
not restore - 5 VDC, check the other components associated with
U101: C117, C118, and C119.
3. If - 5 VDC is present but D101 is not lit, perform LED checks as
outlined previously.
Check the PSG signal. If the power supply is functioning properly, there
should be approximately zero volts on pin 10 of P11 0, and D104 should
be lit. If zero volts is indicated but PSG LED is not lit, turn off the computer
and check the resistance of R104 with an ohmmeter. It should read 560
ohms. If the resistor checks good, replace D1 04.
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