Thermo Scientific RO-20, Operating Manual

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Ro-20 RB-XXX-XXXXXX E 4-7
Löw 08.10.2008
4.1.7 Troubleshooting
The system block diagram shown in Figure 3-2 , the general schematic diagram shown in
Figure 4-2 , and Chapter 3: “Theory of Operation” are the primary troubleshooting aids.
Check the batteries. The RO-20 has only four active circuit components, amplifiers A101 and
A201, the voltage regulator A203 and the reference divider A202. A quick check with a
voltmeter will determine whether the outputs of A203 and A202 are proper. The two
amplifiers are in sockets for easy substitution. (Pin 2 of A101 is bent outward to avoid
touching the socket. Pin 2 is solder-connected; Pin 2 or a new component must be reinstalled
the same way for air insulation.)
If substituting new amplifiers does not solve the problem, then track the circuit functions to
find the fault. Several components connect to the point in the circuit where the extremely
small ion chamber current flows. It is of the utmost importance that they be clean, free of
fingerprints, and carefully installed. They include Pin 2 of A101, the two feedback resistors
R101 and R102, relays K101 and K102, and C102. The critical leads of these components are
not supported on insulators. They are air-insulated at all points.
If leakage currents appear to be a problem, make sure the desiccant is dark blue. Temperature
cycling may correct the problem. It may help to use a high-purity, mild Freon solvent spray to
clean the high-impedance components. Do not use alcohol spray on the center electrode disk
because it will dissolve the conductive Aquadag
®
coating.