ISCO 4230 - 5.9 Precautions for Servicing CMOS Circuitry

4230 Flow Meter
Section 5 Maintenance and Service
5-15
If the unit is powered from the AC supply, make sure the
branch circuit is delivering at least 110 VAC to the power
supply cord.
4. Check the wiring harnesses, connectors, and solder joints.
Under normal conditions, these should stay in good condi-
tion. However, abuse or operation of the unit with the door
open or the desiccator saturated could cause corrosion of
the connectors in certain atmospheres.
5. Look for physical damage.
Burnt or broken components, overly hot components, stuck
or inoperative switches, or the evidence of water damage,
again brought about by neglect or abuse, may be apparent
if you look closely.
6. Look for shorted or open diodes and transistors, especially
driver transistors.
7. Check the voltage regulators.
The output voltage from the regulators should be within
5% of their rated value. Check to see that rated voltages
are available at various places on the boards.
8. Look for excessive current draw from some or all the cir-
cuitry.
This will usually be accompanied by an unusual amount of
heat coming from some component or group of components,
and the voltage on the power rails may be depressed.
9. Make sure the air pump and associated circuitry are func-
tioning correctly.
Check both the electronic and pneumatic circuits. A leaky
piece of tubing will seriously affect accuracy.
10. Check to see that the crystal oscillator is operating and at
the proper frequency.
11. Check the reset circuitry to see that it is working properly.
5.9 Precautions for
Servicing CMOS
Circuitry
Most of the circuitry in the 4230 Flow Meter is made up of CMOS
components. Because of the oxide gate structure of these devices,
they are extremely susceptible to destruction caused by the dis-
charge of static electricity through their inputs.
Many of the driver transistors in the 4230 are power MOS
devices; they are just as susceptible to static damage as CMOS
ICs are. Because of this risk, certain precautions must be taken
when working on these circuits.
5.9.1 Hazard of Static
Electricity
The voltage levels present from static buildup due to walking
over carpeted floors, movement of woolen or synthetic clothes
over chair seats, workbenches, etc., are high enough to destroy
CMOS circuitry when performing repair work.
Ideally, all tools, soldering irons, etc., should be grounded, and
work should be conducted on a grounded metal workbench, with
grounding straps worn on the wrists of personnel. It is recog-
nized that in most field repair situations, such precautions are
impractical. However, certain extreme hazards must be avoided.