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Maintenance
1. Remove the AC power cord from the analyzer.
2. Measure resistance across the primary and secondary
transformer windings. The nominal resistance readings of a
good high-voltage transformer are 60 ohms across the primary
winding and 47.2 K ohms across the secondary winding
(output).
5.4.9 Amplifier Board
The amplifier board output is set at 0 to 100 mVDC at the time of
installation. Check it by measuring the voltage across the terminal strip
test point connections. See Figure 1-2. This reading varies with sample
impurity level.
5.4.10 Hamamatsu PMT High Voltage Supply
The high voltage DC power for the PMT is supplied by a power
supply that is integral with the PMT socket. The socket contains the high
voltage supply, the resistor ladder for the intermediate dynodes in the
PMT, and a 1.22 VDC reference voltage generator.
The socket works in conjunction with the 10-turn 10K variable
potentiometer plate voltage adjustment on the back panel of the analyzer
(see Figure 1-2) to provide the proper plate voltage to the PMT. The
blue wire supplies a 1.22 VDC reference voltage to the plate adjustment
potentiometer. This voltage is divided down by the potentiometer and a
fraction of this voltage is sent back as a control voltage to the socket on
the white wire. Both of these voltages can be measured on the back of
the plate voltage adjustment potentiometer. The high voltage supply
amplifies this control voltage 1000x. The raised voltage is then used by
the PMT.
To check the system, proceed as follows:
1. Measure the +15 VDC supply to the socket at the test jack
panel. See Figure 1-4.
2. If the +15 VDC measures correctly, measure the reference
voltage on terminal 3 of the 1P28 potentiometer. It should
measure 1 223 ± 5 mV. Ground may also be found on the black
wire on the potentiometer.
3. If the reference voltage measures correctly, measure the control
voltage terminal 2 of the potentiometer. You should read a
fraction of the reference voltage, depending on the plate voltage
Teledyne Analytical Instruments
48
Model 2120
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