78
Service Manual
3-20
2. If Vrefh is correct, measure the voltage at U1 pin 6 (PS1). If Vps1 is not equal to 0 V
+/-0.15 V, check R34 and R35. If Vps1 is still at an incorrect voltage, U1 is bad.
3. Check the bias generator circuit. With the exception of resistor R31 (649 kJ), the
bias generator (which sets the bias level for all U1 analog circuitry) is internal to U1.
A problem with this circuit could cause the on-chip power supply op amps to fail.
Measure the dc voltage between U1 pin 8 (Vbias) and DGND. If -0.2 V < Vbias <
+0.2 V the bias generator is okay. If Vbias is not correct, check R31. If Vbias is still
wrong, replace U1.
4. Measure the ac voltage between DGND and Vss. If it is greater than 10 mV ac,
check C17. (An open C17 causes common regulator instability.) The dc level may
also be incorrect.
5. If the common regulator still does not work, circuitry internal to U1 is bad. Replace
U1.
3-30. 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 9 V battery. If the current is greater than
1.2 mA, a sneak current path exists. Although a sneak current path can be very
difficult to find, the following troubleshooting steps may be helpful in isolating the
current path.
2. First, visually check for both solder bridges on U4 pins and other circuit board
shorts.
3. Isolate the current path at the negative battery terminal (Vbt-). The components
connected to Vbt- are LS1, Q7, CR1, and R29. Remove these parts one at a time.
Measure I(bat) after each removal to isolate the problem.
4. If the excess battery current stops after removing R29, either R27, R29, Q5, or Q6
may be bad.
5. If the extra current is still present with all parts removed, remove Q8 and check for
excess battery current. If I(bat) is now correct, Q8 is bad. If I(bat) is still excessive,
U1 is probably at fault.
If the power supply is not working but battery current is normal, perform the following
tests.
1. If Vdd - Vss is low, a problem may exist with start-up device Q8. Check Q8 by
momentarily connecting Vss to Vbt-. If both Vss and Vbt- now start up and operate
correctly, check Q8 for an open.
2. Measure the ac voltage between Vdd and Vss. An unstable Vdd regulator can be
caused by an open C16. If the voltage is greater than 10 mV ac, check C16. The dc
level may also be incorrect.
For a final check of U1, remove the battery and supply Vbt+ = +3.7 V, DGND = 0, and
Vss = -3.2 V from an external power supply. Measure the voltage at U1 pin 7 (PS0). If it
does not equal 1.23 V +/-0.15 V, check R32 and R33 carefully. If PS0 is still incorrect,
U1 is bad.
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