Service manual

PT300, Service Manual
Rev B, October, 2000
42
Selecting the Initial Span Range.
It may be necessary to change the span resistor if you run out of travel when setting
scale span. Likewise the span resistor may have to be changed when new circuit
boards or load cells are installed.
There are two revision boards; 300B and 300C. With the 300B board the span resistor
(R24) must be unsoldered to change values. The 300C board has a shorting plug
selecting a span resistor (R24, R52 or R53).
If you need more span (run out of travel going clockwise), select the next lower resistor
in this series: 274Ω, 301Ω, 330Ω, 365Ω, 392Ω. If you need less span (run out of
travel going counter-clockwise), select the next higher resistor in the series. These
resistors must be 1% tolerance parts.
With the revision B board, you must unsolder the old resistor and solder in the new
part.
With the revision C board the two most common values, 330Ω and 365Ω are selectable
with a shorting plug. JP1-1 goes to R24/330Ω and JP1-2 goes to R53/365Ω. Any
values outside this range will need to be soldered in place. Put the new resistor in
place of R52, and move the jumper to JP1-3.
With either board, clean off any soldering flux after changing a span resistor. Failure to
do this may result in inaccurate weight measurement.
Now try the span adjustment, and insure that you have enough adjustment range.