DEVICES USED IN COMPUTERS
ARE SENSITIVE TO ESD (ELEC-
TROSTATIC DISCHARGE). ENSURE
THAT YOU ARE FAMILIAR WITH THE
COMPUTERS SAFETY PRECAUTIONS
THAT DEAL WITH ESD AND TAKE THE
NECESSARY STEPS TO PROTECT THE
COMPUTER. YOU CAN FIND THE
REQUIREMENTS IN THE COM-
PUTERS TECHNICAL MANUALS.
SUBASSEMBLIES USED IN
Subassemblies are electronic parts of the computer
that are a portion or part of a functional area. A
subassembly can contain pcbs 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 pcbs to make one individual
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
Some of the items you will find in subassemblies
of computers are as follows:
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 correct 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 pcbs connector pin(s).
The arrangement of the subassemblys connector pins
(plugs) can also act as a guide when you install the
MAINTENANCE OF SUBASSEMBLIES.
Subassembly units can be sealed or unsealed. With the
sealed units, you cannot break them down any further
for repair purposes. Youll have to discard the sailed
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
Printed circuit boards (pcbs) makeup the majority
of the computers functional areas. They vary in size
from small pcbs used in modular designs to large ones
used in some cage-or rack-designed computers. Lets
take a look at the functions and physical characteristics
Functions of Printed Circuit Boards
It doesnt matter what type of computer we are
talking about, the computers printed circuit boards
process all the data the computer processes. The pcbs
contain the circuitry that electronically manipulates the
data that enters and leaves the computer. The functional
areas of the computer are contained on the pcbs.
Physical Characteristics of Printed Circuit
The physical characteristics of a pcb depend on the
type of computer. Let's examine some general
SIZE AND NUMBER OF PRINTED CIRCUIT
BOARDS. The size and number of pcbs vary from
the computers that require many small pcbs 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 computers CPU as an example.
Larger militarized computers may use up to 200 small
pcbs 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 pcbs. The computers technical
manual will provide the information on how the pcbs
are arranged inside the computers frame or cabinet.
Computers that are modular in design have all the pcbs