TM8100/TM8200 Service Manual Fault Finding of Control Head with 1-, 2- or 3-Digit Display 451
© Tait Electronics Limited November 2007
Control-Head Board
Faulty
If neither the elastomeric strips nor the LCD are faulty, check the relevant
components on the control-head board:
1. Use an oscilloscope to display the signal at pin 5 of IC5
(see Figure 15.2). The signal should be a square wave with a fre-
quency of about 60Hz and an amplitude that alternates between 0.0
and 3.3V. If the signal is correct, go to Step 3. If it is not, go to Step 2.
2. Replace IC2 (see Figure 15.2). Test the user interface. If the fault has
been removed, return to “Initial Tasks” on page 155. If it has not, go
to Step 3.
3. Use the oscilloscope to display the signal at pin 12 of IC7 (see
Figure 15.2). The signal should be a square wave with a frequency of
about 120Hz and an amplitude that alternates between 0.0 and 3.3V.
If the signal is correct, replace IC5 and go to Step 4. If it is not,
replace IC7 and go to Step 4.
4. Test the user interface. If the fault has been removed, return to “Initial
Tasks” on page 155. If it has not, the repair failed; replace the
control-head board.
15.2 Some LEDs Faulty
If the display functions correctly but one or more (but not all) of the LEDs
D1 to D18 are faulty:
1. Disconnect the control-head loom. Remove the control-head board.
Reconnect the loom to the board.
2. Use a multimeter to measure the forward voltage across each faulty
LED. See Figure 15.1. The voltage should be 2.0±0.4V DC. If it is,
go to Step 3. If it is not, replace the LED and go to Step 4.
3. If the forward voltage is correct, the LED is functional but the
associated switching transistor is suspect. Replace the transistor
corresponding to the LED in question. The switching transistors
associated with the LEDs D1 to D18 are Q1 to Q4. See Figure 15.2.
Continue with Step 4.
4. Test the user interface. If the fault has been removed, re-assemble the
control-head board, and return to “Initial Tasks” on page 155. If it
has not, replace the control-head board and return to “Initial Tasks”
on page 155.
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