Supplemental Troubleshooting Procedures; Checking The Crystal Oscillator; Checking The Reference Voltage; Checking Display Drive Voltage - Fluke 77 Series III Service Manual

3-15. Supplemental Troubleshooting Procedures

3-16. Checking the Crystal Oscillator

Connect an oscilloscope or counter to pin 54 U1 or to the junction of C12 and
the Y1 crystal. Check for a 32.768-kHz sine wave approximately 600 mV p-p
in amplitude. Note that U2 and the display will not work if the clock circuit is
not working.
Probable related failures include: U1, Y1, or C12.

3-17. Checking The Reference Voltage

Check for a reference voltage of 1.00V dc (adjustable by R8) at pin 14 of U1
or at the junction of R15 and R16.
Probable related failures include: R8, R14, R15, R16, VR1, or CR2.

3-18. Checking Display Drive Voltage

Check that VM (V middle) is 1.6V dc .1V at pin 28 U2 or at the junction of
R11 and R12.
Probable related failures include: R11 or R12.

3-19. Checking Beeper Drive Signal

Put the unit in the Diode Test Mode and short the input leads. Using an
oscilloscope, measure the signal at pin 3 of U1. The signal should be a 4V p-p
symmetrical square wave at about 4.1 kHz.
A probable related failure is: U1

3-20. Tracing the VDC Signal Path

Use the following procedure to trace the VDC signal path:
1. Set the UUT to the VDC function and apply 2V dc to the input.
2. Using a DVM, measure the input at J1 for 2V dc.
3. Measure pin 1 of Z1 input divider for 2V dc. Probable related failures
include: R1, S1, RV1, or RV2.
Measurements in steps 4,5, and 6 may be affected by loading.
4. Measure Z1 pin 3 for 200 mV dc. Probable related failures are: Z1,
U1.
5. Measure for 200 mV dc at the active filter input (AFI, pin 26 of U1 or
R9). A probable failures is: U1
6. Measure for 200 mV at the active filter output (AFO, pin 27 of U1 or
R9). Probable related failures are: R9, C5, or C6.
Note
Maintenance
Troubleshooting
3
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