4220 Flow Meter Section 6 Maintenance and Service
6.7Precautions for Servicing CMOS Circuitry
6.7.1Hazard of Static Electricity
9.Check the input signals to the unit and see that they are correct.
This may require the use of an oscilloscope. You may need to consult Customer Service for the proper appearance of wave forms.
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.
Most of the circuitry in the 4220 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.
Note that many of the driver transistors in the 4220 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.
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.
•Never perform any work in a room with a carpeted floor.
•Always roll up sleeves so that your arms are in contact with the working surface.
•Avoid using a work surface made of an extremely good insulator.
Plastic and glass are good insulators and should be avoided. A metal surface is best; a wood surface is acceptable. Conductive grounding mats are available for work stations and are worthwhile if much repair is to be done.
•The degree of hazard depends on the level of humidity. Be particularly careful if the work area is extremely dry, or if the work is being done in cold seasons, when indoor forced heating and outdoor low temperatures cause the relative humidity level to be very low.
•Keep yourself grounded when handling disassembled equipment.
After a unit has been opened for repair, always touch the metal chassis before touching any of the circuit compo- nents.
•Be especially careful handling the CMOS integrated circuits when they are removed from the rest of the
