Thermo Scientific RO-20, Operating Manual

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3-4 RB-XXX-XXXXXX E RO-20
Löw 08.10.2008
Figure 3-4: Main circuit board layout
When operating on the two R/h ranges, K102 is pulled in to place R102, the 1.5-gigohm
resistor, in the circuit. The higher ion currents develop sufficient voltages for operation with
this smaller resistor. When using the mR/h ranges, both relays are open, leaving R101 (the
150-gigohm resistor) as the feedback element. The feedback signal to the two high- value
resistors comes from the temperature compensation circuit made up of R105, R106, R107 and
RT101. C102 is a time-constant component used to set the response time.
The relays consume about 10 times more power than the total of the rest of the circuits, with
the exception of the lamps. Because no relays are pulled in on the three mR/h ranges, the
battery life is very long if only these ranges are used.
The second stage of A101 amplifies the output of the first stage and then drives the meter
circuit. The gain of the second stage is set according to the Function Switch position. Each
range has a separate calibration control, an adjustable resistor in series with the second stage
output. This circuit then feeds the meter directly through the Function Switch. The Zero Knob
provides a slight voltage offset adjustment to the input of the first stage of A101. The
amplifiers are DC-coupled and this adjustment is reflected at the meter.
Component A203 is a 5-volt regulator whose output is connected to A202, a component
called a
rail splitter . It produces a voltage exactly one half of that which is impressed across
it. The voltage it produces is called common and this potential is used as a reference for the
rest of the electronic circuits. The negative end of the three 12V batteries is connected to
common, holding common 36 volts negative with respect to the chassis. The positive terminal
of the meter is connected directly to common so all meter upscale indications must come from
a source more negative than common. When checking the AA cells, amplifier A201-8,9,10 is
used to reflect the opposite polarity of the AA cell battery’s positive end. When checking the
three 12V batteries, amplifier A201-12,13,14 is used to reflect the opposite polarity of the
input from the more positive chassis potential.