CAUTION DEVICES  USED  IN  COMPUTERS ARE   SENSITIVE   TO   ESD   (ELEC- TROSTATIC  DISCHARGE).  ENSURE THAT YOU ARE FAMILIAR WITH THE COMPUTER’S  SAFETY  PRECAUTIONS THAT DEAL WITH ESD AND TAKE THE NECESSARY STEPS TO PROTECT THE COMPUTER.   YOU   CAN   FIND   THE REQUIREMENTS   IN   THE   COM- PUTER’S TECHNICAL MANUALS. SUBASSEMBLIES USED IN COMPUTERS Subassemblies are electronic parts of the computer that  are  a  portion  or  part  of  a  functional  area.  A subassembly can contain pcb’s or just electronic parts. Two or more components combined into a unit will form  a  subassembly.  Each  subassembly  can  contain components,  such  as  transistors,  resistors,  capacitors, and the like, and/or pcb’s to make one individual subassembly. We use a power supply module and a memory module of a large computer as our examples. A power supply module in a large computer usually has   six   or   seven   subassemblies.   Each   of   these subassemblies  contains  transformers,  transistors, diodes, resistors, capacitors, and the like. A memory module may need up to four memory stacks to make it complete. Each stack contains only the  electronic  components  necessary  to  make  it complete. Some of the items you will find in subassemblies of computers are as follows: l Memory stacks of a memory unit . Dc-to-dc converters in modules . Dc switching regulators of a power supply KEYED   SUBASSEMBLIES.—   Subassemblies are keyed to assure that only the connect subassembly is inserted  into  a  slot  and  that  each  subassembly  is inserted properly (not backwards). The manufacturer will either cut a slot into the plug-in side of the pcb or put plastic sleeving on one or more of the connector pins.  With  the  pin/plastic  sleeving  method,  the connector receptacle must match the pin(s) with the sleeving to accommodate the pcb’s connector pin(s). The arrangement of the subassembly’s connector pins (plugs) can also act as a guide when you install the subassembly. MAINTENANCE  OF  SUBASSEMBLIES.— Subassembly units can be sealed or unsealed. With the sealed units, you cannot break them down any further for repair purposes. You’ll have to discard the sealed subassembly unit and replace it or turn it in for a new subassembly.  A  subassembly  may  or  may  not  have  test points for maintenance purposes. PRINTED CIRCUIT BOARDS USED IN COMPUTERS Printed circuit boards (pcb’s) makeup the majority of the computer’s functional areas. They vary in size from small pcb’s used in modular designs to large ones used in some cage-or rack-designed computers. Let’s take  a  look  at  the  functions  and  physical  characteristics of pcb’s. Functions of Printed Circuit Boards It doesn’t matter what type of computer we are talking  about,  the  computer’s  printed  circuit  boards process all the data the computer processes. The pcb’s contain  the  circuitry  that  electronically  manipulates  the data that enters and leaves the computer. The functional areas of the computer are contained on the pcb’s. Physical Characteristics of Printed Circuit Boards The physical characteristics of a pcb depend on the type  of  computer. Let’s  examine  some  general characteristics. SIZE AND NUMBER OF PRINTED CIRCUIT BOARDS.— The size and number of pcb’s vary from the computers that require many small pcb’s for one functional area of the computer, to the computers that need only a single medium to large pcb to handle one functional area. Take a computer’s CPU as an example. Larger militarized computers may use up to 200 small pcb’s to perform the functions of the CPU. Whereas a microcomputer needs only a single “chip” on a single pcb to perform the functions of the CPU; thus requiring less circuitry to perform the CPU functions. ARRANGEMENTS OF PRINTED CIRCUIT BOARDS.— Again the type of computer will dictate the arrangement of pcb’s. The computer’s technical manual will provide the information on how the pcb’s are arranged inside the computer’s frame or cabinet. Computers that are modular in design have all the pcb’s 2-13