Common (Shunt) Regulator Troubleshooting - Fluke 78 Service Manual

Table 3-11. Functional Description of Power Supply Components
Component
Q8 Power supply startup device. Q8 provides Q7 base startup current. Q8 is always off
during meter operation.
VR1 VR1 provides the system reference voltage. It is used for the a/d converter reference
and as a reference for both power supply regulators.
C16 Vdd regulator compensation and bypass.
C17 Common regulator compensation and Vss bypass.
R32, R33 Vdd regulator voltage sensing resistors.
R34, R35 Common regulator voltage sensing resistors.
R23 Supplies bias current to VR1.
Signals
VBT+ 3.7 ±0.2
VDD 3.1 ±0.2
VSS -3.2 ±0.2
VBT- -5.3 (battery at full 9 V charge)
-3.2 (battery at low charge of 6.9 V)
REFH 1.23 ±0.04
PS0 1.23 ±0.15
PS1 0 ±0.15
VOA 2.2 to 1.7 (referenced from VSS)
VOB 1.07 to 0.85 (referenced from VSS)

3-29. Common (Shunt) Regulator Troubleshooting

To troubleshoot the common regulator, connect the power supply so that Vdd, Vss, and
DGND (digital ground) are supplied from an external power supply. This procedure
overdrives the large on-chip shunt transistors; the bias current from the power supply
ranges from 10 mA to 100 mA. Refer to the schematic for a diagram of the common
regulator.
Make the following tests:
1. Check for +1.23 V +/-40 mV (Vrefh) at the cathode of VR1. If Vrefh is not correct,
check VR1, R23, R24, R25 and R26 carefully. If Vrefh is still incorrect, U1 is bad.
Function
Table 3-12. Voltage Levels
Levels
Maintenance
3
Troubleshooting
3-19