Tait TM8100 mobiles, Service Manual

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452 Fault Finding of Control Head with 1-, 2- or 3-Digit Display TM8100/TM8200 Service Manual
© Tait Electronics Limited November 2007
15.3 All LEDs Faulty
If the display functions correctly but all the LEDs are faulty:
1. Disconnect the control-head loom.
2. Replace IC3 which drives the switching transistors for the LEDs.
See
Figure 15.2 .
3. Reconnect the loom and test the user interface. If the fault has been
removed, return to
“Initial Tasks” on page 155 . If it has not, replace
the control-head board and return to “Initial Tasks” on page 155 .
15.4 Display and All LEDs Faulty
If the display and all LEDs are faulty:
1. Use a multimeter to measure the 3.3 V DC supply voltage across C1
(see
Figure 15.2 ). If it is correct, go to Step 3 . If it is not, go to Step 2 .
2. Check for shorts to ground of the 3.3 V supply. Repair any fault and
go to
Step 8 .
3. Use the multimeter to check that the
RST
line at pin 6 of IC7 is high.
The level should be 3.3 V. If it is, go to Step 5 . If it is not, go to
Step 4 .
4. Check for continuity in the LCD driver circuitry D22 (not fitted for
3-digit control head), C12 and R27 (see
Figure 15.2 ). Repair any
fault and go to Step 8 . If there is no continuity fault, replace IC7 and
go to Step 8 .
5. Use the multimeter to check that the
OE
line at pin 8 of IC5 is low.
The level should be less than 0.6 V. If it is, replace IC7 and go to
Step 8 . If it is not, go to Step 6 .
6. Check that the voltage at pins 4, 10 and 14 of IC5 is 3.3 V DC.
Also check that pin 7 of IC5 is at ground. If the voltages are correct,
replace IC7 and go to
Step 8 . If they are not, go to Step 7 .
7. Check for continuity between IC5 and the control-head connector
SK1. Also check for shorts to ground between IC5 and SK1. Repair
any fault and go to
Step 8 .
8. Confirm the removal of the fault. If the fault has been removed,
return to
“Initial Tasks” on page 155 . If it has not, replace the
control-head board and return to “Initial Tasks” on page 155 .