Vdd (Series) Regulator Troubleshooting; Voltage Levels - Fluke 76 Service

76
Service
VDD
VSS
VBT-
REFH
PS0
PS1
VOA
VOB
Make the following tests:
1. Check for +1.23V 40 mV (Vrefh) at the cathode of VR1. If Vrefh is not correct,
2. If Vrefh is correct, measure the voltage at U4 pin 6 (PS1). If Vps1 is not equal to 0V
3. Check the bias generator circuit. With the exception of resistor R35 (620 k ), the
4. Measure the ac voltage between Vdgnd and Vss. If it is greater than 10 mV ac, check
5. If the common regulator still does not work, circuitry internal to U4 is bad. Replace

Vdd (Series) Regulator Troubleshooting

If a problem still exists after the common regulator troubleshooting, continue with the
following Vdd regulator troubleshooting. Often, a short or sneak current path causes
power supply problems. Refer to the schematic for a diagram of the Vdd (Series)
Regulator. Make the following tests:
1. Measure the dc operating current from the 9V battery. If the current is greater than
2. First, visually check for both solder bridges on U4 pins and other circuit board shorts.
Isolate the current path at the negative battery terminal (Vbt-). The components
connected to Vbt- are C35, CR4, and R63. Remove these parts one at a time. Measure
I(bat) after each removal to isolate the problem.
1. If the excess battery current stops after removing R63, either R63, R57, Q9, or Q11
2. If the extra current is still present with all parts removed, remove Q10 and check for
If the power supply is not working but battery current is normal, perform the following
tests.
3-16
check VR1, R19, R20, R21 and R44 carefully. If Vrefh is still incorrect, U4 is bad.
0.15V, check R37 and R38. If Vps1 is still at an incorrect voltage, U4 is bad.
bias generator (which sets the bias level for all U4 analog circuitry) is internal to U4.
A problem with this circuit could cause the on-chip power supply op amps to fail.
Measure the dc voltage between U4 pin 8 (Vbias) and DGND. If -0.2V < Vbias <
+0.2V the bias generator is okay. If Vbias is not correct, check R35. If Vbias is still
wrong, replace U4.
C21. (An open C21 causes common regulator instability.) The dc level may also be
incorrect.
U4.
1.2 mA, a sneak current path exists. A sneak current path can be very difficult to find.
may be bad.
excess battery current. If I(bat) is now correct, Q10 is bad. If I(bat) is still excessive,
U4 is probably at fault.
Table 3-12. Voltage Levels
3.0 0.2
3.2 0.2
6.1 (battery at full 9V charge)
3.2 (battery at low charge of 6.0V)
1.23 0.04
1.23 0.15
0 0.15
2.2 to 1.7 (referenced from VSS)
1.07 to .85 (referenced from VSS)
3-27.