Sony FD-42A - Safety Check-Out; Leakage Test; How to Find a Good Earth Ground

FD-42A
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
Check the entire
board surface for soider spiashes and bridges.
poorly-soldered connections.
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.
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
|
se eed
AC
0.75 pF 15K vo/tmeter
{0.75 V)
= Earth Ground
[Fig A. Using an AC voltmeter to check AC leakage.
—3—
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 0.5SmA (S00 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.
Measuring the voltage drop across a resistor by
means of a VOM or battery-operated AC volt-
meter. The “limit” indication is 0.75V, 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 reon
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
a Ohmmeter
- AC Outlet Box
Cold-water Five
Fig. B. Checking for earth ground.