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*VM-8020
F-501
SAFETY
CHECK-OUT
After
correcting
the original
service
problem,
perform the following safety checks before releasing
the set to the customer:
1.
Check the area of your repair for unsoldered or
poorly-soldered
connections.
Check the entire
board surface for solder splashes and bridges.
2.
Check
the interboard
wiring to ensure that no
wires
are
'"'pinched"
or contact
high-wattage
resistors.
3.
Check
that
all control
knobs, shields, covers,
ground
straps,
and
mounting
hardware
have
been replaced.
Be absolutely certain that you
have replaced all the insulators.
4.
Look for unauthorized replacement parts, par-
ticularly transistors, that were installed during a
previous repair. Point them out to the customer
and recommend
their replacement.
5.
Look for parts which, though functioning, show
obvious signs of deterioration.
Point them out
to the customer and recommend
their replace-
ment.
6.
Check
the line cord
for cracks and abrasion.
Recommend
the replacement
of any such line
cord to the customer.
7.
Check the condition of the monopole antenna
(if any).
Make sure the end is not broken off, and has
the plastic cap on it. Point out the danger of
impalement
on
a
broken
antenna
to
the
customer,
and
recommend
the
antenna's
replacement.
8.
Check
the B+ and HV to see they are at the
values
specified.
Make sure your instruments
are accurate; be suspicious of your
HV meter
if sets always have low HV.
9.
Check
the
antenna
terminals,
metal
trim,
"metallized"
knobs,
screws,
and
all
other
exposed
metal
parts for AC
leakage.
Check
leakage as described below.
To Exposed Metal
Parts on Set
AC
voltmeter
|
|
{0.75 V)
0.15 uF
=
Earth Ground
Using an AC voltmeter to check AC leakage.
—4—
LEAKAGE
TEST
The AC leakage from any exposed metal part to
earth ground and from all exposed metal parts to any
exposed
metal part having a return to chassis, must
not
exceed
0.5mA
(500
microampers).
Leakage
current
can
be
measured
by
any
one
of three
methods.
1.
A
commercial
leakage
tester,
such
as
the
Simpson
229
or RCA
WT-540A.
Follow the
manufacturers' instructions to use these instru+
ments.
2.
<A battery-operated AC milliammeter.
The Data
Precision
245 digital multimeter is suitable for
this job.
3.
Measuring the voltage drop across a resistor by
means
of a VOM
or battery-operated AC volt-
meter.
The
"limit"
indication
is 0.75 V, so
analog
meters
must
have
an
accurate
low-
voltage
scale.
The
Simpson
250 and Sanwa
SH-63Trd
are examples of a passive VOM that
is suitable.
Nearly all battery operated
digital
multimeters
that
have
a 2V
AC
range
are
suitable.
(See Fig. A)
HOW TO FIND A GOOD
EARTH
GROUND
A cold-water pipe is guaranteed earth ground; the
cover-plate retaining screw on most AC outlet boxes is
also at earth ground.
If the retaining screw is to be
used as your earth-ground, verify that it is at ground
by measuring
the resistance between
it and a cold-
water pipe with an ohmmeter.
The reading should be
zero
ohms.
If a cold-water
pipe is not accessible,
connect
a 60—100
watts trouble light (not a neon
lamp) between the hot side of the receptacle and the
retaining screw.
Try both slots, if necessary, to locate
the hot side of the line, the lamp should
light at
normal brilliance if the screw is at ground potential.
(See Fig. B)
Trouble Light
-
Ohmmeter
= AC Outlet Box
N
Cold-water Pipe
Fig. B.
Checking for earth ground.
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