Nash - Scientific Computing with PCs (523165), страница 34
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Dobb’s Journalthe equipment is subjected, on the relative availability or suitability of different types of servicing, andon the urgency with which the PC must be restored to operation.Many users buy service contracts with repair companies or PC makers.
The contracts vary, but generallythe user pays a certain sum per year to a service company that agrees to repair the PC when it goeswrong. The contract details cover points such as speed of response to a request for service, where servicewill be carried out (on site, at a service depot or in the factory) and who pays for transport, what serviceand parts are covered, what the user must do to keep the service contract in force and how any disputeover any of the above points may be resolved.Typical equipment service contracts cost about 1% of the original purchase price for each month of service.Rates may be reduced if the user agrees to "carry in" the equipment to a service depot.
Costs are higherfor contracts involving round-the-clock response to service calls or for installations in hard-to-reachlocations.We prefer to repair our equipment as failures occur. We see PCs under service contracts receiving heavyhanded use, kept in hostile conditions, or attached to unsuitable power outlets. A service contract isdesigned to make money under average conditions of treatment of the equipment. Our experience hasbeen that the money costs for repairs are much less than half the costs of service contracts, especially forwell-treated PCs.A "cost" that is often forgotten is that of arranging for the service call. Waiting for a technician to arriveis costly, either in lost productivity or salaries.
Many PCs are installed in places where the servicetechnician cannot enter unless someone is specifically waiting. Moreover, a minimum time charge mayapply. This is especially frustrating for intermittent faults. We once paid a 2-hour minimum charge fora technician to conclude "The output shows a definite printer fault, but it’s working fine now." Weeventually fixed it ourselves.If you are "handy", some do-it-yourself servicing is possible by:•Careful recording of symptoms;•Verification, if possible, of hardware operation by self-tests of system or components, or by use ofdiagnostic software;•Location and repair of fault by cleaning, adjustment, or component exchange, where possible.
We donot recommend repairs to parts that are not easily exchangeable unless you are adept at soldering.If one has several similar machines, it is possible to maintain a modest selection of components. TheFaculty of Administration computer labs do not carry a service contract on PCs. Instead a stock of a fewkeyboards, screens, disk drives, and circuit boards is kept to swap into machines as needed. This hasturned out to be several times cheaper than a service contract, even given the heavy usage of these PCs.The exchange of circuit boards is straightforward. First, turn off the power! Label any cables andconnectors before disassembly so they can be reassembled correctly. We use sticky labels, marking pensor typewriter correction fluid to do this. We mark connectors for orientation that may be critical, e.g., fordisk control cables.
Take care in removing or replacing retaining screws so that such small parts and theirwashers do not drop into awkward places or get lost. Removal or insertion of boards should be donegently but firmly, pulling or pushing the board so connector pins are straight. It is easy to put sidewaysstress on pins or connectors that can break soldered joints or mechanical supports.Note the position of any switches, jumpers or headers on our board, as these are critical for theconfiguration of such boards.
Ignore such settings at your peril. We usually check that socketed integratedcircuits are well-seated by pressing each gently but firmly. Inspection with a magnifying glass under astrong light may reveal that the pins of such circuits are bent double under the "chip".
Contact may stillbe made with the socket most of the time, giving unfortunate intermittent faults. We have twice seen such11: HARDWARE AND OPERATING PRACTICE99problems on our own machines. They are extremely difficult to diagnose. We suspect such "bent pin"faults may be a major source of motherboard (the main PC circuit board) replacements.To fix bent pins, we proceed as if we were exchanging the integrated circuit.
That is, gently remove thecircuit by prying it up a little at a time, first from one end, then the other, until it is free of the socket.Special tools are available to "pull" circuits, but it can be done easily by loosening the chip with ajeweller’s screwdriver. Once the chip is out, it should be placed on a grounded conductive surface toavoid static damage. Then use fine point pliers to gently straighten the pin.
Breaking the pin meansgetting a new chip. Carefully reposition the chip, ensuring correct orientation and that each pin is directedto the proper socket position. A gentle but firm push will seat the chip.Cables are a frequent source of problems.
It is easy to accidentally pull them in a way that puts a greatdeal of mechanical stress on plugs and connectors. Where provided, screws or clips that give mechanicalsupport to a connection should be used. However, we feel manufacturers do not always provide enoughbracing for these. To see just how little support there is, try giving a well-secured cable a small sidewayspull. Watch the flexing of the circuit board to which it is attached! On most PCs, the keyboard connectionis a circular DIN plug with no extra mechanical support, even though keyboards get moved a lot in use.We use foldback clips or similar clamps as a cable strain relief. This serves to attach the cable firmly toeither the PC case or to furniture so that there is no stress on the connector to the PC itself.
After all, akeyboard is a lot cheaper than a motherboard, and we can replace it without technical assistance.Apart from computer viruses, strange or intermittent effects are often caused by:•Dust, dirt, or loose parts causing short circuits;•Overheating of some components due to poor ventilation;•Memory failure.Dust and dirt can be dealt with by cleaning.
Overheating requires that we ensure adequate ventilation ofall components. Memory failure, which is usually in a single chip or bank of chips, can be fixed byreplacing the offending parts. Finding which chips to replace may be possible using special diagnosticsoftware. Sometimes this is supplied with a computer.
If not, we recommend contacting a local users’group and asking the software librarian or similar member about such diagnostic programs.Diagnostic software can also help in improving the configuration of our PC. That is, we can adjust varioussettings of both hardware and software to improve performance or make our PC simpler to use. This isa continually changing field, and we will not mention any products by name here.A servicing task that users may need to carry out from time to time is the reformatting of fixed disks. Thetracks that carry the magnetization recording our data are laid down either in the disk factory or thecomputer supplier’s store by special programs. Over time, temperature and mechanical stress may causethe read-write mechanism to drift off-specification or the tracks to lose some magnetization. In either case,we may be unable to persuade the PC to read our data. Assuming we have a backup, the main issue isto restore the disk to working condition.
This requires a low-level reformatting, a task requiring specialsoftware. Users’ groups or a friendly computer store probably can supply this. (Many vendors seem tobelieve users should not be trusted with such programs.) Software also exists that will "renew" the lowlevel format and test a disk simultaneously, e.g., SpinRite.Clocks commonly need battery replacement every couple of years.
Some PCs use a rechargeable batterythat can lose charge if the PC is switched off for extended periods of time. Usually the configurationmemory is also lost and must be restored. See Section 11.8.11.8 Configuration MaintenanceMany problems arise due to the different ways in which personal computers may be configured.100Copyright © 1984, 1994 J C & M M NashNash Information Services Inc., 1975 Bel Air Drive, Ottawa, ON K2C 0X1 CanadaSCIENTIFIC COMPUTING WITH PCsCopy for:Dr. Dobb’s JournalRecommendations made by one user may be totally inappropriate for another.
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