Tait TP9100 - Checking Linear Regulators

156 Power Supply Fault Finding TP9100 Service Manual
© Tait Electronics Limited May 2005
Task 4 —
Check Linear
Regulators
This section gives the general procedure for checking any linear regulator.
There are two possible faults: either the 1.8V regulator has failed and
prevents the radio from powering up, or, with the other linear regulators,
the radio will power up but will not function correctly. (The regulator IC
might or might not have been removed during earlier checks.)
Table 8.1 gives an overview of the ICs and the relevant voltages.
Note If the +3
V0 RX voltage is missing, check the 3.0V linear regulator,
the
DIG RX EN signal, and the 3V RX switch (Q604).
1. Measure the input voltage to the relevant regulator is correct.
Repair any fault.
2. Check the control/enable signal to the relevant regulator is active.
Repair any fault.
3. Check the output for shorts to ground. Repair any fault.
4. Reconnect the 7.5V external supply and turn the power/volume
control on. If the radio powers up or the correct regulator voltage is
restored, go to “Final Tasks” on page 134. If the repair failed, go to
Step 5.
5. Disconnect the 7.5V external supply. Replace the regulator IC with
a spare. Reconnect the 7.5V external supply and turn the
power/volume control on. If the radio powers up or the correct
regulator voltage is restored, go to “Final Tasks” on page 134. If the
repair failed, replace the main board and go to “Final Tasks” on
page 134.
Table 8.1 Input, output and control/enable voltages of the linear regulators
Regulator Input Control/Enable Output
Voltage IC Pin Voltage Pin Voltage Pin Voltage
5V (TX) IC600 pin 6 7.5V pin 1 >2V pin 4 5V
5V (AN) IC601 pin 6 7.5V pin 1 >2V pin 4 5V
3.0V IC604 pin 1 3.3V pin 3 >2V pin 5 3.0V
2.5V IC605 pin 1 3.3V pin 3 >2V pin 5 2.5V
1.8V IC608 pin 1 3.3V pin 3 >2V pin 5 1.8V