Common (Shunt) Regulator Troubleshooting - Test Equipment Depot 88 Service Manual

3-28. Common (Shunt) Regulator Troubleshooting

Refer to Figure 3-7 for a simplified diagram of the common regulator. 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. Make the following tests:
1. Check for +1.235V ±20 mV (VREFH) at the cathode of VR1. If VREFH is not
correct, carefully check VR1, R19, R20, and R44 and the traces to these parts. If
VREFH is still incorrect, U4 is bad.
2. If VREFH is correct, measure the voltage at U4 pin 6 (PS1). If VPS1 is not equal to
0V ±0.15V, check R37 and R38 and their associated traces. If VPS1 is still at an
incorrect voltage, U4 is bad.
3. A DGND and VSS current much larger than 100 mA may indicate a problem with
Q12 (VDGND-to-VSS quick turn-off clamp) or its base drive signal from U6. The
Q12 clamp is off when the Meter is on and on when the Meter is off.
4. Check the bias generator circuit. With the exception of resistor R35 (620 kΩ), the
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 not to
work. 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.
5. Measure the ac voltage between VDGND and VSS. If the voltage is greater than 10
mV ac, check C21. (An open C21 causes common regulator instability.) The dc level
may also be incorrect.
6. If the common regulator still does not work, circuitry internal to U4 is bad. Replace
U4.
Maintenance
Troubleshooting the Power Supply
3
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