CET1171 Lecture #13 - Disassemble/Reassemble a PC

Materials:
Lecture Only
Objectives:
The student will be able to properly handle:
All major MPC'97 components including,
Case and power supply,
Drives and cables,
Motherboards and attachments including,
CPU, RAM, expansion cards,
Standard ports and the video controller,
External peripherals.
Competency:
The student will perform the procedure of a complete disassembly and reassembly of the PC learning and practicing proper anti-static procedures as well as physical handling of the individual components. The student will learn how to identify all major subsystems and components on sight and be able to inventory and diagram the component layout in an organized fashion to facilitate accurate and timely reassembly this time and in the future.

Procedures

  1. Boot the PC to the DOS prompt to be sure that the PC is fully functional prior to disassembly. Once it is seen that the PC works; turn the unit off including the monitor. On the worksheet at the end of this lesson make a note of the type of case: desktop or tower. Detach all external peripherals and identify the type of keyboard and type of mouse. In the case of a serial mouse note exactly which serial port it is attached to. Note the location of the power supply. In desktops it is located in the rear right hand corner facing the PC from the front. In towers, it is located in the rear top facing the PC from the front.

  2. Remove the case cover and lay it down in a location out of the way. Do not stand it up, it will only get knocked over at some point anyway. If you have a tower lay it on its side so that the interior is facing up and the front is facing toward you to start.

  3. It is critical to be aware that the ATX form factor encompasses the layout of the case down to the precise position of the mounting screw holes as well as the precise locations of the motherboard rear panel and expansion slot positions. The ATX power supply form factor is an engineering level specification which covers the AC input tolerances the DC output tolerances as well as the dimensions and mounting screw hole positions. The ATX motherboard form factor is also a well defined specification. Note the position of the CPU next to the power supply and directly behind the rear connector panel. Note the location of the RAM slots in front of the CPU. Note the positions of the drive data cable connectors on the very front edge of the motherboard which prevents data ribbon cables from having to cross the motherboard and even have to wedge down in between expansion cards as they did in the AT motherboard.

  4. With AT or Baby-AT based systems the CPU is located toward the front of the motherboard in front of the expansion card slots. A very inconvenient location in which the large heatsink/cooling fan assemblies can interfere with full sized expansion cards. Data ribbon cable connectors for the IDE/ATA controllers and floppy disk drive controllers are not well defined and often end up located in between expansion card slots and the cards can interfere with the attachment of the cables and the cables can interfere with the attachment of the cards.

  5. The PROPER ESD procedures have already been fully discussed in a previous lecture. The ESD protocol differs between AT-type systems and ATX-type systems. Be sure to properly identify the system type based on the power supply that is attached to the motherboard. Then review the ESD procedures to ensure that you will initiate proper ESD procedures at this time.


    AT power supply-to-motherboard connectors


    ATX 1.0 power supply-to-motherboard connector

  6. Once ESD procedures are in effect, the system may be safely disassembled. The general disassembly procedure in full is:

    1. Remove case cover
    2. Establish proper ESD protocol appropriate to the type of system (AT or ATX)
    3. Remove data ribbon cables and record connector positions and Pin #1 orientations
    4. Remove all drives and record locations and orientations
    5. Remove all expansion cards and slot mounted connectors and record slot locations and motherboard connections
    6. Remove RAM
    7. Remove CPU
    8. Remove Motherboard
    9. Remove Power Supply (and Switch if it is an AT Power Supply)

  7. At this point remove all data ribbon cables. Start with the Floppy Disk Drive data ribbon cable. This cable is 34-wires wide and attaches the standard floppy disk controller or FDC to the Floppy Disk Drive (FDD). Note that the wire down one edge is marked. All literature calls this a "red" stripe and if you strain your eyes you might be able to glimpse a very faint pink thread like stripe painted onto this outermost wire down one side. This indicates Pin #1 and guides the orientation of the cable to the controller connector and the FDD connector. As you remove the cable from the motherboard connector diagram the orientation of Pin #1 so that you will not accidentally reverse the cable when you reassemble the system later.

  8. Following the cable from the motherboard connector outward you will encounter two physically distinct connectors. The larger one is a female card-edge connector and provides attachment for 5 ¼" FDD's. The smaller connector is a standard 34-pin female block pin jumper connector (or IDC – Insulation Displacement Connector) and provides attachment for 3 ½" FDD's. You may attach an FDD to either of these connectors but never both at the same time. Both connectors are at the same position on the cable and are interpreted by the system as the connection point for the B: drive. Continuing away from the motherboard beyond these connectors there is a split in the cable and a section of it twists and then continues out to two more connectors. Another female card-edge connector for attaching a 5 ¼" FDD and another 34-pin female IDC connector for attaching a 3 ½" FDD. Again either can be used but never both at the same time. A drive attached to either of these connectors that lie beyond the twist in the cable will be interpreted by the system as the A: drive. Note the orientation of Pin #1 on the cable as you remove the cable from the FDD. Diagram the direction it is facing in the connector of the drive so that you will not reattach the cable backwards when you reassemble the system later. Place the floppy cable aside.

  9. Remove the standard IDE ribbon cables. These examples in this lecture module use the older IDE technology and these cables are standard 40-pin/40-wire ribbon cables. Note that these cables possess three 40-pin female block pin jumper (IDC) connectors and attach the hard disk drives or HDD's and ATAPI devices to the ATA controller. In almost all Pentium systems and beyond, the ATA controller is embedded in the motherboard. Note that the cable also has a Pin #1 indicated on it. Note the direction that Pin #1 is facing in the motherboard connector as you remove the cable and diagram this so that reversing the cable when reattaching it during the reassembly process can be avoided. Accidentally reversing the IDE/ATA ribbon cable can and does DESTROY hard drives and ATA controllers. Since the controller is embedded in the motherboard this can cost the system a new motherboard, an expensive and very troublesome component to replace. Take extra effort to be absolutely sure that accidentally reversing these cables never occurs. Double check them multiple times before deciding to power on the system.

  10. Follow the cable up from the motherboard and diagram the specific HDD's and or ATAPI devices attached to the middle connector and to the end connector. Diagram the cable attachments. Note that the motherboard should have some small notation next to the connector on the motherboard along the lines of "IDE1" and "IDE2" Be sure that the diagrams indicate which devices are attached to which controller.

  11. The motherboard chipset contains a single ATA controller component which is usually embedded within a chip that contains many standard devices like the serial and parallel ports as well. Low level software calls never have to distinguish which controller when attempting to access a hard drive, but the two connectors are called the Primary Controller and the Secondary Controller. Technically speaking they are IDE channels of a single controller chip, much more than they are two independent controllers, but this is the modern situation. In older ATA specifications there were motherboards in which the Primary and Secondary controllers were physically independent chips usually the Primary was the latest and fastest and more expensive technology while the secondary was an older surplus chip. Since ATA-4 (UDMA-33 and integrated ATAPI support for bootable CD-ROMs) these systems have virtually disappeared and all systems carry a single controller chip that manages both controllers at the same speed and feature set.

  12. Note the orientation of Pin #1 of the cable in the connector on the device as you remove it. As already stated, every effort must be taken to avoid ever reversing this cable as it can destroy either the device or the controller in the motherboard or both. As a technician your objective is to fix the computer not destroy it. Set the ribbon cables aside. Note that the standard 40-wire cable is NOT position selective like the floppy data ribbon cable is. However, recent hard drives attach to the system with position selective 80-wire UDMA cables that appear to "misbehave" based on their position even on the 40-wire cables. On these older systems using the 40-wire cable the devices identity is established by setting jumpers on each individual device. If there is more than one device, different jumper settings may be necessary from when there is only one device present on the cable (i.e. alone on the controller versus master in the presence of a slave) therefore it is critical to keep the positions of every device attachment diagrammed so that the system can be reassembled correctly.

  13. Once the IDE/ATA ribbon cables have been removed a general inspection can be made of the interior of the system. Correlate the interior with the diagrams made during lecture module "Component Identification #2."

  14. Now proceed to remove all drives. In the case of the system used in this example, this will consist of a single floppy disk drive, a single hard disk drive and a single optical (CD-ROM) read-only drive. Note the orientation of each drive especially vertically mounted floppy disk drives so that they can be mounted again in the correct orientation when the system is reassembled later. Note the number and locations of screws and if any are missing. In general, floppy disk drives are mounted to the case with 4 small finer threaded screws, two on each side. These same screws are used to mount optical drives which have eight screw holes. A minimum of four of these should be used, two on each side. Hard drives are held by larger screws with a larger thread. These are the standard screws that are found throughout the rest of the system from anchoring expansion cards to attaching the case cover. Hard drives should be attached to the case or a removable bay housing by four screws, two on each side. Some proprietary case systems attach the hard drive (and others as well) to racks that then slide in and latch in place. These are fine, although sometimes tricky to figure out. The drives should use all possible screws to attach to these racks.

  15. Now remove all expansion cards. Note what type of card each is, and diagram the exact locations in the slots of each card. You should know what the card does, what type of bus it attaches to and the actual slot it occupies. In the example systems each has a video controller card and a network interface card. Once you have identified each one, the bus and slots they are attached to, remove the retaining screw at the top rear of the system that holds the card in place and then carefully grasp the card between your index finger and thumb of one hand near the back of the system and the other hand at the front edge of the card and pull it upward slightly from the back and then pull it upward slightly from the front of the card rocking it until it is loose enough to pull straight up and out of the slot. Remove the rear slot external port connectors if the system has these and note the slots they occupy and which motherboard block pin connector each attaches to by the small ribbon cables. Note the orientation of pin #1 on each of the cables as well. You must attach these to the correct connectors on the motherboard and you must never reverse the connectors as this can prevent the system from booting up and can also damage the motherboard; destroying it. Once diagrammed remove the retaining screws at the top of these expansion slot external port connectors and lift them out of the slot and drape them over the back of the system, diagram carefully their connectors and then pull straight up on the ribbon cable connectors. Place all of these items aside and out of the way.

  16. At this point the student should practice on a dead demo motherboard. These can be obtained from any computer store. Practice the removal and installation of RAM modules. You should practice with 30-pin SIMM's, 72-pin SIMM's and 168-pin DIMM's. Be sure to learn not only the proper procedure for removal and installation but also how to determine the orientation of the module in the slot so that you will not attempt to insert a module backwards which will do more damage to the slot than to the module possibly destroying the motherboard. While you are practicing with the RAM modules practice the removal and insertion of the CPU as well. You should practice with the processor type that is installed on the motherboard you are handling as well as others. Be very sure that you are aware of the orientation of the CPU fan power connector, modern CPUs can reach over 600º F in seconds which can weld the pins into the socket, burn circuitry below the socket, and could even potentially catch fire!

  17. Once you feel satisfied with your proficiency in the handling of RAM and the CPU you may return to your system and remove the RAM and note the orientation of the notch in the case of 72-pin SIMM modules (DIMM’s have a notch on the edge connectors itself) as well as which slots they are mounted in. Diagram these keys so that you will not insert them into the wrong slots nor attempt to insert them backwards when you perform the reassembly later.

  18. Now remove the CPU. First disconnect the power connector to the CPU fan and make a careful note of exactly how it is oriented in the connector. The CPU usually has a corner indicating pin #1 as does the socket. These MUST match up when it is inserted or the motherboard and CPU will be destroyed. Then remove the CPU and place it to the side resting upside down (in the case of Socket CPUs). Do not place the CPU on its pins as they can easily be bent and this will destroy the chip. The CPU will be far less likely to be damaged by ESD from the surface where it is placed also. The main worry is the bending of the pins, so avoid placing the CPU on the pins even on an anti-static mat.

  19. Now the motherboard is completely bare. If the system is an AT then disconnect the "P8" and "P9" AT power supply to motherboard connectors by firmly grasping each one and pulling it straight up. Make an extremely careful inspection of their orientation as you remove them. Reattaching these skewed or reversed can instantly and completely destroy the motherboard. Diagram their orientation carefully so that such a mistake will not be made during reassembly later. Drape these connectors out of the way. Locate the front panel connectors to the motherboard. Note that these are block female jumper connectors at the ends of fine twisted pairs of wires and they carry signals to and from the motherboard to the front panel switches and LEDs; LED - Light Emitting Diode.


    Once unlatched a ZIF socket processor
    lifts straight out with no effort

  20. Carefully diagram how these are attached to the pins on the front panel connector area of the motherboard as you remove them so that you will be able to attach them correctly when you reassemble the system. Note that LED’s are direct current devices and can be attached backwards. This can damage the LED and of course it will not light when it should if it is attached backwards. Attaching them backwards could also damage the circuits to which they are attached in the motherboard.

  21. Note the orientation to the motherboard pins of the white and the colored wire of the block pin jumper female connector at the end of the front panel cable in order to be able to reattach the connector with the correct polarity later.

  22. In AT systems these connectors are unnecessary and the system will boot up and function without any of them attached. When you are in doubt as to how to connect them on an AT simply leave them off until you can find the motherboard manual. However, in the case of the ATX which you will work with in the PC Repair Class #2, they must be attached and they must be attached correctly. Attaching these connectors backwards can damage the circuit to which they belong which can destroy the motherboard. Be very sure of their orientation before you proceed.

  23. Find and remove all motherboard retaining screws. On quality systems the motherboard is attached by screws that screw into brass "stand-offs." The brass stand-offs in turn screw into the case bottom and the motherboard rests on top of these, with the screw holes lined up to the screw holes in them. This arrangement holds the motherboard very firmly in place and is preferred to the inferior mounting systems often found in OEM name brand computers. These motherboards have plastic stand-offs that push into the screw holes and snap into place. These all push into large open holes in the case and then the entire motherboard is pushed sideways and the plastic standoffs "grab" into the narrowing channels where they were inserted into. This mounting arrangement can allow the motherboard to wobble loosely if the expansion cards are all removed. Whenever you are working on a machine with plastic standoffs try to replace them with the brass screw mounts which are far superior. Note that the stand-offs not only elevate the motherboard so that expansion cards will line up to their slot openings in the case, they also separate the underside circuitry from the case and are electrically neutral (insulators) to prevent short circuiting the motherboard to the grounded case.

  24. Once all of the screws have been removed lift the motherboard out carefully. Make every effort not to stress the motherboard. So lift it from opposite corners with your hands underneath the motherboard and allow it to rest on your hands as you lift it out. The slightest bending or momentary warping of the motherboard can cause irreparable damage to the fine copper circuitry tracings that cross it. Place the motherboard gently on the desktop out of the way. Never rest anything on the motherboard as it could potentially scratch the fine circuitry tracings. Once this is done, ESD protocols are no longer necessary in working with the computer because all electronic circuitry components have been removed. The unopened power supply is in general not susceptible to ESD damage. All circuitry components that have already been removed are highly susceptible to ESD damage and should not be handled unless proper ESD protocols are observed.

  25. Locate and remove the retaining screw that holds the front panel power switch in place This only applies to AT based systems, ATX systems have a small plastic momentary contact switch that attaches to the motherboard along with the rest of the front panel connectors shown above. Note how the power switch hangs into place as you remove it. Drape the power switch, at the end of its well insulated cable to the power supply, out of the way. Now circle around to behind the system and remove the retaining screws that hold the power supply in place. Remove the power supply from the case and place it to the side.

  26. At this point the disassembly of the system is complete. Call the instructor over to verify that this phase of the module is complete. Then proceed to reassemble the system in exactly the reverse steps that were taken disassembling it. The reassembly will proceed as follows:

    1. Install the power supply and then the front panel power switch
    2. Install the motherboard
    3. Attach the front panel connectors, and verify them
    4. Attach the power supply to motherboard connectors, and verify them
    5. Install the CPU, carefully watch for the orientation of Pin #1
    6. Connect the CPU fan power connector, verify it
    7. Install the RAM modules, verify their slots and orientation to prevent damaging the modules or the slots
    8. Connect the external port cables to the motherboard and mount the ports into the rear panel slots (AT only)
    9. Install expansion cards. Press them firmly straight down until they snap into place, and secure with the single screw to the rear panel of the case
    10. Attach the drive data cables to the motherboard and the drives, connect power to the drives
    11. Mount the drives into their bays

  27. Do not mount the drives to the case yet. This is the last step once you have tested the system and found that it is working correctly. A significant number of the problems that arise in PC assembly are caused by a drive that is attached or configured improperly that is preventing the system from booting up so it makes no sense to waste the time mounting them until you are sure that everything is correct.

  28. DO NOT ATTEMPT TO BOOT THE SYSTEM. Call over the instructor and once the instructor has thoroughly inspected and verified your work, then you may attempt to start the PC. Even if this is not your first complete disassemble and reassemble of a PC, it is the first time that you have done so with THIS system. Quite often the over-familiarity of experience can lead to carelessness which is just as destructive as the oversight of a novice. Disassembly and assembly of a PC is NEVER a test of speed, it IS a test of carefulness and correctness.

  29. If the system boots to the C: prompt as it did before you began this exercise then turn the system off and mount the drives into their bays without bending or damaging anything with their cables and attach them to the case with their mounting screws. Inspect the interior of the system especially the connectors to the drives themselves to be certain that nothing has been knocked loose. Test the system for a good boot up once more. If it boots to the prompt again, then turn it off, remove your wrist strap, and reattach the case cover.

Copyright©2000-2007 Brian Robinson ALL RIGHTS RESERVED