Dc Voltage Signal Tracing - Fluke 27 Service Manual

27
Service Manual
isolating a malfunction to the component area, and Table 3-4 provides further fault
isolation within the component area. Refer to the disassembly and reassembly procedures
as necessary (paragraphs 3-9 and 3-10).
1. Set up the instrument as follows:
2. Select the dc voltage function (F).
3. Use a Digital Voltmeter (DVM) to check the battery voltage at the battery connector
4. Connect either an oscilloscope or frequency counter, using a low-capacitance probe,
5. Use a DVM to check for a reference voltage of 1.00V dc (adjustable through R l 9)
6. Use a DVM to check for Vm (voltage middle) at pin 55 of U2, pin 16 of J8, or at the

3-25. DC Voltage Signal Tracing

The following procedure is a step-by-step method of tracing a dc voltage input through the
instrument's circuits to the output of the active filter. Faulty components in the input signal
path can be identified using this procedure. All measurements are taken with respect to
common.
1. Select the dc voltage function ( F ), then apply a 2 V dc input through the
2. Using a DVM, measure the 2 V input at the input terminal (Jl).
3. Check pin l of the input divider (Z l ) for 2 V dc. If 2 V dc is not present, R2, Sl, RV1,
4. Check pin 3 of Z1 for approximately 200 mV dc. If 200 mV is not present, either Z1
5. Check the active filter input (pin 28 of U1) for approximately 200 mV dc. If 200 mV
3-18
a. Perform steps 1 through 10 of the disassembly procedure to remove the battery
cover, bottom case, and main pca/shield assembly from the instrument.
b. Perform steps 21 through 23 of the disassembly procedure to remove the top and
bottom shields from the main pca.
c. Perform step 14 of the reassembly procedure to install the main pca in the top
case.
d. Perform step 15 of the reassembly procedure to secure the main pca to the input
terminals.
(battery voltage must be above 6.3 V). If battery voltage is less than 7.5 V, battery
replacement is advised. Normal current drain is less than 600 µA.
to pin 54 of U1 or to the junction of C17 and Y1 (the crystal). A 32.768 kHz sine
wave with an amplitude of approximately 600 mV peak-to-peak should be present at
the junction of C17 and Y1 (3 V peak-to-peak at U1 pin 54). Note that U2 and the
display will not operate if the clock signal is not present. If the clock signal is not
present, the most likely causes are U1, Y1 or C17.
at pin 13 of U1 or at the junction of R15 and R18. If the reference voltage is not
present, the most likely causes are VR1, R12, R15, R18, R19, R37, or U1.
junction of R3 and R4. Vm should be 1.6 V dc ±0.1 V. If Vm is not present, the most
likely causes are R3, R4, or C13.
volts/ohms/diode-test input terminal.
RV2, RV3, RV4, or RV5 may be defective.
RU 8 PD\ EH GHIHFWLYH 0HDVXUHPHQW PD\ EH DSSUR[LPDWHO\ 
input impedance DVM is being used for signal tracing.)
is not present, U1 may be defective. (Measurement may be affected by loading.)
ORZ LI D 0