MAINTENANCE
(ELECTRONIC BOARD ADJUSTMENTS)
PROCESSOR BOARD ADJUSTMENTS (Refer to Fig. 3 & 11)
The Model 924 has a test mode (Mode 0) of operation. This mode is entered by closing switch S4-1 located on
the processor board near the bottom of the front panel. With S4-1 closed, all sampling of inputs from the input
conditioner board is suspended. (It should be noted that the inputs are not physically disconnected, so that it is
possible that some failure modes of the input conditioner could cause operational failure of Mode 0.) In Mode
0, the voltage at the arm of R35, located at the lower right corner of the processor/display, is used for the input
parameter selected for display on the LCD readout. All other parameters retain the same input value as when
the particular parameter was last displayed. For example, if R35 is set at the middle of its range and the front
panel display selector is moved through all positions, the median value of all input parameters will be used to
calculate the output parameters. An exception to this is the analog temperature output which is not
calculated. This output is always determined by the value of resistance between the thermistor input terminals
(3 and 5). Fig. 3. In Mode 0 the display will flash continuously.
There are three other potentiometers on the processor/display board, which should not require adjustment
unless a D/A converter or voltage reference is changed. R5 is a single turn potentiometer used to trim the 2.5
volt reference to 2.500 volts. The total range of adjustment is approximately 200 millivolts. R24 and R25 are
used to trim analog outputs 1 and 2 respectively. R24 is adjusted to give 10 volts between output terminals 1
and 7 (or between TP4 and analog return on the processor board) when the corresponding parameter display is
at full scale. R 25 is adjusted to give 10 volts between output terminals 2 and 7 (or TP5 and analog return)
when the corresponding parameter display is at full scale.
The 924 has three functional modes depending on the setting of the front panel FUNCTION switch. The FAC
mode requires a chlorine sensor to be connected to input terminals 6 (red) and 7 (black) and a 3.00 kilohm
thermistor to input terminals 5 and 3. A pH sensor may be connected to the BNC connector, but if not, it is
good practice to short the BNC input to its shield connection. The TFC/AUTO mode requires both chlorine
and pH sensors and a 3.00 kilohm thermistor. (The thermistor in the standard pH sensors is 1.00 kilohm.) In
this mode, the chlorine sensor output is corrected by the processor using the thermistor and pH sensor outputs.
In the TFC/MAN mode a fixed value is used for the pH correction. (Again it is good practice to short the BNC
input.) This fixed value is entered by setting the display switch to pH and setting the screw-driver adjustment to
the left of the FUNCTION selector to the appropriate value on the display. In this mode when pH is displayed,
all calculations are suspended, the TFC output is held at its last value, and the pH output tracks the display.
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