TM8400B-L-00
1 November 2004
the capacitor/resistor filter formed by C230, C231, C233, C234, R253 and R254 detect any DC
bias on the chopped signal, and output a signal proportional to the bias. This signal is fed to
U208C, which buffers it and sums it into the cell voltage comparator/balancing circuit, to
remove any bias or polarization. The chopped conductivity signal is also sampled by U212 and
filtered by the R-C output filter C229, C232, C235 and R256 to convert the conductivity signal
into a DC level.
U206 and the surrounding passive components provide a final gain and offset adjustment stage
for the signal. The final DC conductivity signal, along with the reference signal from the
reference circuit now go to the meter circuit.
9.5. METER ASSEMBLY
The meter assembly converts the conductivity signal into a digital number, and drives the front
panel display and the BCD* circuit if equipped. The meter assembly is made up of two circuit
boards, the meter circuit board and the display circuit board.
9.5.1. METER CIRCUIT
The meter circuit consists of a 4½ digit digital voltmeter chip set, clocks and drive and
decoding. The following references are to the Meter PCB schematic 16504.01.04 and
assembly 16504.01.02. Z316 is the clock generator for the meter. R306 is the frequency
adjustment. R309 is the meter gain adjustment. Z317 and Z314 are the digital voltmeter
set. They are a 4½ digit dual slope integration auto-zero dvm. Z301 and Z302 are
driver/buffers. Z304 to Z308 are latches for the BCD
output data. Z303 is a BCD* to
seven segment decoder. Q301-Q304 are the common anode drivers for the display light
emitting diodes (Q305 is not used). Z315 decodes the sign bit. S301 is the meter test
switch which disconnects the conductivity signal from the meter circuit and substitutes a
fixed voltage.
9.5.2. DISPLAY CIRCUIT
The display circuit contains logic to decode the settings of the front panel switches for
display, and light emitting diode displays. The following references are to the Display
PCB schematic 16503.01.04 and assembly 16503.01.02. Z401-Z407 encode the front
panel switch settings into BCD. Z408 and Z409 drive the BCD outputs. Z410 and Z411
are BCD to seven segment decoders, Z412, Z413, Z414 are resistor packages, and DS1 to
DS7 are the seven segment L.E.D. displays.
Binary Coded Decimal
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