Service manual

PT300, Service Manual
Rev B, October, 2000
16
Troubleshooting
Caution
Changing some parts may cause a large change in calibration whereas others may or
may not change the calibration, depending on the nature of the problem. The most
sensitive parts are in the analog and power supply sections; this includes Q3, U17,
U18, U19, and their related resistors and capacitors. The areas that normally would
have no effect are: CPU, display and driver, memory chips, and serial interface
sections. Normally, the calibration should be checked after any repairs.
Symptoms
No power up, nothing on display.
If any power reaches the scale the display driver turns on some random
segments. Since we are not seeing this we will assume that no power is
reaching the scale circuitry. Some possible causes: blown fuse, shorted battery
pack, bad switch circuit, bad keypad, bad regulator. The power supply may be
delivering power, but it might be eaten up with a shorted circuit board or filter
capacitors.
Power up to random display.
We know that some power is reaching the display driver circuit, but the
microprocessor is not able run the display. Some possibilities: bad
microprocessor (U2), bad memory (U1, U5), bad chip select logic to memory or
display driver. Finally, the microprocessor may be working but the display driver
(U10) circuitry may not be working.
Power up to test pattern.
The microprocessor is running well enough to write to the display, but gets hung
up at some point after that. The usual symptom is the display stays on EEE0.
During the time between the display test and normal operation the scale is trying
to read a stable A/D value between certain limits. The A/D converter (U12, U13,
U17) may not be running at all. The converter may be returning values that are
outside of allowed ranges. Two reasons for this could be bad load cell outputs
or a bad amplifier chip (U18). The microprocessor may be defective, not reading
end of conversion interrupts, getting hung up in a loop, shorted address lines,
etc.