Teledyne 402REU - Hydrocarbon analyzer - Manual (Page 49)

Total Hydrocarbon Analyzer Maintenance and Troubleshooting
5-15 Teledyne Analytical Instruments
5.5.4 Electrometer-Amplifier PC Board
Schematic No. B-34436
Assembly Dwg. No. C-34434
The ions formed in the process of burning hydrogen in the presence of
hydrocarbon components of the sample gas cause an electrical conduction
between two electrodes in the combustion chamber (or detector cell) that is
amplified by a high sensitivity and high input impedance electrometer-
amplifier circuit. The electrical output of the electrometer-amplifier is
directly proportional to the quantity of flame ionizable hydrocarbons
present.
The electrometer amplifier PC board is not located on the
motherboard, as are the other plug-in printed circuit boards; rather, it is
mounted to the backplate assembly and interconnected to the electronics
circuitry by means of a single card edge connector, so that the ease of
replacement of a plug-in board is maintained. The high input impedance
requires a shield, or cover, which is removable for access, as well as a
shielded input conductor. Interconnection with the collector is made by a
coaxial cable. Although the cable and fittings are intended for coaxial
service, the cable is actually being used as a shielded single-conductor
connection. The collector cable plugs into a coaxial connector on the
electrometer amplifier PC board, which is located at the rear cover.
The circuit consists of an electrometer amplifier and an operational
amplifier. It is a very high-gain, current-to-voltage converter circuit,
having an input impedance measuring in the billions of ohms. It is static
sensitive and highly susceptible to contamination, and special handling
precautions must be taken.
Because of its high impedance, the input circuitry to the electrometer
has had careful design consideration. The glass resistors (R2 and R3) in the
input gain circuit (see schematic) are installed on Teflon-insulated stand-
offs, instead of directly to the printed circuit, to eliminate the possibility of
leakage currents.
To eliminate any possibility of contamination of the insulating materi-
als employed, the completed PC board is ultrasonically cleaned in labora-
tory grade alcohol. Under no circumstances should the parts described
be handled with bare fingers. A freshly-scrubbed finger, stroked along