Initial Troubleshooting Operations; Power Supply Check; Internal Clock Check - Garmin GNC 250 Maintenance And Repair Manual

2.3

INITIAL TROUBLESHOOTING OPERATIONS

The first step in troubleshooting the GNC 250/300 is to remove the top and bottom covers (see
Section 3.1) and the CPU bottom shield, and swing the comm assembly open. Verify that the unit is
being properly powered and the internal clock is functional.
NOTE: The NavData® card should be removed before disassembling the unit.

2.3.1 Power Supply Check

1. Check the DC voltage between D511 and ground as shown in Figure 6. The voltage should be between -9.60
and -14.40 volts. If the voltage is out of the limits specified, replace the CPU board.
2. Check the DC voltage between L502 and ground as shown in Figure 6. The voltage should be between 4.8 and
5.2 volts. If the voltage is out of the limits specified, replace the CPU board.
3. Check the DC voltage between Q515 and ground as shown in Figure 7. The voltage should be between 17.00
and 67.00 volts. If the voltage is out of the limits specified, replace the CPU board.
4. Check the DC voltage between L504 and ground as shown in Figure 6. The voltage should be between 9.6 and
14.4 volts. If the voltage is out of the limits specified, replace the CPU board.
5. Check the DC voltage between Q511 and ground as shown in Figure 7. The voltage should be between 25 and
45 volts. If the voltage is out of the limits specified, replace the CPU board.
6. Check the DC voltage between D508 and ground as shown in Figure 6. The voltage should read between 12 and
18 volts. If the voltage is out of the limits specified, replace the CPU board.

2.3.2 Internal Clock Check

Check the clock speed by placing an oscilloscope probe at I101 pin 31 as shown in Figure 7. The
frequency should be approximately 16.4 MHz. If the clock is not operational, replace the GPS
receiver. If the clock still fails to function, replace the CPU board.
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