A & A Scales PT300, Руководство по обслуживанию

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PT300, Service Manual
Rev B, October, 2000
17
Scale shuts off.
Normally the scale shuts itself off after a long period of inactivity. The length of
time is defined at the time the scale is configured, but is normally one hour. If
the microprocessor is acting up, one of the symptoms may be that the scale
shuts off after the A/D is initialized and starts to run. All this tells up is that the
microprocessor is not resetting the counter U12, so its reaching MIN/MAX count.
We do know that the microprocessor is getting far enough through the startup
routine to set up U13, as U13 does not emit any pulses until it's told to. If the
scale turns off IMMEDIATELY after you take your finger off the off button, you
may have defective power switch circuitry.
"Locks up."
Bad microprocessor (U2) or support circuitry.
No backlight.
Look for a bad keypad, connector. If only one string of lamps refuses to shine,
short out the unlit LEDs until you bridge the bad one.
No response to one or more keys.
The keypad or connector may be bad. U4 may be defective.
"Bad" weights.
First, check weighing technique. Are the scales being used by a new operator?
Are the scales being used on level ground? Are the operators using dummy
blocks on the unweighed wheels? If on a calibration press, are you using weight
distribution blocks and rubber pads? Is the reference scale correct? Assuming
all this, is the scale spanned correctly? Are the load cells bottoming out? If the
weight is exactly half of what you would expect, one of the two signal leads may
not be providing signal. This would probably be in the load cell or amplifier
(U18). Non-linearity would be in the load cell or the analog section. The
interconnecting wiring inside the scale may be pinched under the printed circuit
board.
Jumpy/drifty weights.
This can usually be traced to contamination on circuit boards, a bad load cell, or
a defective U18.
Slow operation.
If the clock is running at some sub-multiple of the clock frequency, the scale runs
very slowly, but correctly. It is also possible that the A/D converter circuitry is
not working well. The scale uses the A/D cycles to pace all its internal
operations. This problem is usually caused by a bad crystal.