Service manual

PT300, Service Manual
Rev B, October, 2000
23
If the display backplane drive is missing, TURN THE SCALE OFF IMMEDIATELY.
Operation without a backplane signal will damage the display quickly. Remove the
display while determining the cause of the missing backplane signal. Display damage
will be evident as display segments that are on permanently, even with the scale off.
Amplifier:
No load on scale, or a load cell simulator set to 0.05 mV/V.
U18, Pin 11, zero input = 0.05 volts.
Apply 20000Lb, or a simulator set to 2.0 mV/V.
U18, Pin 11, full scale = 2.0V.
A/D converter:
U13, pin 14, zero input = About 1 millisecond width pulse.
U13, pin 14, full scale input = About 30 milliseconds width pulse.
Lamp:
Press the lamp key. All eight lamps light up.
Keys:
For each key, observe about five volts on the indicated pin. Press the key and
see zero volts. The display should show two minus signs. Release the key and
see five volts again.
U4, pin 6, press the Print key.
U4, pin 8, press the Lb/Kg key.
U4, pin 11, press the Local/Total key.
U4, pin 13, press the Hold/Release key.
U4, pin 15, press the Test/Zero key.
Low batt indicator:
Connect a variable supply to the input connector and remove the batteries.
Set the supply to 12V and turn the scale on.
Reduce the input voltage until the low battery just turns on.
Q6, collector, 6.5V to 6.8v.
Power down shutoff:
Continue the above test, reducing the power supply until the scale turns off.
Q6, collector, 5.25V to 6.0V.