SAFElY PRECAUTIONS
GENERAL GUIDEUNES
1.
When
servicing, observe the original
lead
dress. If a
short circuit
Is
found, replace
all
parts which
have
been
overheated or damaged by the short circuit.
2.
After servicing,
see
to
it
that
all
the protective devices
such
as
Insulation barriers, insulation papers shields
are
properly Installed.
3.
After servicing,
make
the following leakage current
checks to prevent the customer from being exposed to
shock hazards.
LEAKAGE CURRENT COLD CHECK
1.
Unplug the
AC
cord
and
connect a jumper
between
the
two prongs
on
the plug.
2.
Measure
the resistance value, with
an
ohm
meter,
be-
tween
the jumpered
AC
plug
and
each exposed metallic
cabinet part
on
the equipment such
as
screwheads con-
nectors, control shafts,etc.
When
the exposed metallic
part has a return path to the chassis, the reading should
be
between
1M
ohm and
5.2
M ohm.
When
the exposed metal does not
have
a
return
path
to
the chassis, the reading must
be
oo.
LEAKAGE CURRENT
HOT
CHECK
(See Figure 1)
1.
Plug
the
AC
cord directly Into the
AC
outlet.
Do
not
use
an
Isolation transformer for this check.
2.
Connect a
1.5k
ohm,
10
watts resistor,
In
parallel with a
0.15uF
capacitor, between each exposed metallic part
on
the set
and
a good
earth
ground such
as
a water
pipe,
as
shown
In
Figure
1.
3.
Use
an
AC
voltmeter, with
1000
ohms/Volt or more sen-
sitivity, to measure the potential across the resistor.
4.
Check each exposed metallic
part,
and
measure the
voltage
at
each point.
5.
Reverse
the
AC
plug
in
the
AC
outlet
repeat
each of the
above measurements.
6.
The
potential
at
any
point should not exceed
0.75
volts
RMS.
A leakage current tester (Simpson Model
229
equivalent) may
be
used to
make
the hot checks,
leak-
age
current must not exceed 1/2 milliamp.
In
case a
measurement
Is
outside of the limits specified,
there
is
a
possibility of a shock
hazard,
and
the equipment should
be
repaired
and
rechecked before
it
Is
returned to the
customer.
AC
VOLTMETER
Hot-Check Circuit
Figure 1
- 2 -
ELECTROSTATICALLY
SENSITIVE (ES) DEVICES
Some semiconductor (solid
state)
devices
can
be
dam-
aged
easily
by
static electricity. Such components com-
monly
are
called Electrostatically Sensitive
(ES)
Devices.
Examples of typical
ES
devices
are
integrated circuits
and
some field-effect transistors
and
semiconductor chip
components.
The
following techniques should
be
used to
help reduce the incidence of component damage caused
by
static electricity.
1 . Immediately before handling
any
semiconductor com-
ponent or semiconductor-equipped assembly, drain off
any
electrostatic charge
on
your body by touching a
known
earth
ground.
Alternatively, obtain
and
wear a commercially available
discharging wrist strap device, which should
be
re-
moved for potential shock reasons prior to applying
power to the unit under test.
2.
After removing
an
electrical assembly equipped with
ES
devices, place the assembly
on
a conductive surface
such
as
aluminum foil, to prevent electrostatic charge
buildup or exposure of the assembly.
3.
Use
only a grounded-tip soldering Iron to solder or
un-
solder
ES
devices.
4.
Use
only
an
anti-static solder removal device classified
as
anti-static
can
generate electrical charges sufficient
to damage
ES
devices.
5.
Do
not use freon-propelled chemicals.
These
can
gener-
ate
electrical charges sufficient to damage
ES
devices.
6.
Do
not remove a replacement
ES
device from its protec-
tive package untilimmedlately before you
are
ready
to
Install
it.
(Most replacementES devices
are
packaged
with leads eletcrically shorted together by conductive
foam,
aluminum foil or comparable conductive
mate-
rial).
7.
Immediately before removing the protective
materia!
from the leads of replacement
ES
device, touch the
protective material to the chassis or circuit assembly
Into
which the device will
be
installed.
CAUTION
:
Be
sure no power
Is
applied to the chassis or
circuit,
and
observe
all
otner asfety precautions.
8.
Minimize bodily motions when handling unpackaged
replacement
ES
devices. (Otherwise harmless motion
such
as
the brushing together of your clothes fabric or
the lifting of your foot from a carpeted floor
can
generate
static electricity sufficient to damage
an
ES
device).
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