Power Supply Related Troubleshooting; Common (Shunt) Regulator Troubleshooting; Functional Description Of Power Supply Components - Fluke 76 Service

Surface-mount components are removed and replaced by reflowing all the solder
connections at the same time. Special considerations are required.
The de-solder tool should use regulated hot air to melt the solder; there should be no
direct contact between the tool and the component.
Rework surface-mount assemblies with wire solder rather than with solder paste.
Multicore X38B, no-clean wire solder is recommended.
A good connection with SMT requires only enough solder to make a positive
metallic contact. Too much solder causes bridging, while too little solder can cause
weak or open solder joints. With SMT, the anchoring effect of the through-holes is
missing; solder provides the only means of mechanical fastening. Therefore, the pca
must be especially clean to ensure a strong connection. An oxidized pca pad causes
the solder to wick up the component lead, leaving little solder on the pad itself.

Power Supply Related Troubleshooting

The two regulator circuits are interrelated; a malfunction in either the common regulator
or the Vdd regulator may cause a problem in the other. Refer to Tables 3-11 and 3-12 for
descriptions of power supply components and voltage levels. To isolate the problem
regulator circuit, disconnect the battery, and drive Vdd - Vss = 6.2V with a power supply.
This procedure tests the common regulator independently of the Vdd regulator.
Now check for Vdgnd - Vss = -3.2V
with the Vdd regulator; refer to Vdd Regulator Troubleshooting later in this section. If
this test is not successful, the problem lies with the common regulator; continue with the
Common Regulator Troubleshooting below.
Note that if the common regulator works or has been repaired, check both supplies with
the 9V battery supply.

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.
Table 3-11. Functional Description of Power Supply Components
Component
Q10 Power supply startup device. Q10 provides Q8 base startup current. Q10 is always
off during meter operations.
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
C14 Vdd regulator compensation and bypass.
C21 Common regulator compensation and Vss bypass.
C35 Battery bypass.
R24, R25 Vdd regulator voltage sensing resistors.
R37, R38 Common regulator voltage sensing resistors.
R44 Supplies bias current to VR1.
0.2V. If this test is successful, the problem lies
Function
Maintenance
3
Troubleshooting
3-25.
3-26.
3-15