434 Fault Finding of Control Head with 1- 2- or 3-Digit Display TM8100/TM8200 Service Manual
© Tait Electronics Limited June 2006
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 149. If it has not, replace
the control-head board and return to “Initial Tasks” on page 149.
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 149. If it has not, replace the
control-head board and return to “Initial Tasks” on page 149.
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