7
KV-27HS420/29DRC420/30HS420/32HS420/34HS420/36HS420
KV-27HS420/29DRC420/30HS420/32HS420/34HS420/36HS420
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
touching 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, particularly 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
replacement.
6. Check the line cords for cracks and abrasion. Recommend the
replacement of any such line cord to the customer.
7. Check the B+ and HV to see if they are specifi ed values. Make sure
your instruments are accurate; be suspicious of your HV meter if
sets always have low HV.
8. Check the antenna terminals, metal trim, “metallized” knobs, screws,
and all other exposed metal parts for AC leakage. Check leakage as
described below.
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 0.5 mA (500 microamperes). 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 manµFacturers’ instructions to use these
instructions.
2. A battery-operated AC milliampmeter. 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 voltmeter. The “limit” indication is 0.75 V,
so analog meters must have an accurate low voltage scale. The
Simpson’s 250 and Sanwa SH-63TRD are examples of passive
VOMs that are suitable. Nearly all battery-operated digital multimeters
that have a 2 VAC range are suitable (see Figure A).
How to Find a Good Earth Ground
A cold-water pipe is a 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- to 100-watt 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 on the
line; the lamp should light at normal brilliance if the screw is at ground
potential (see Figure B).
To Exposed Metal
Parts on Set
AC Voltmeter
(0.75 V)
Earth Ground
0.15
µ
F
1.5 k
Ω
g
AC Outlet Box
Ohmmeter
Cold-water Pipe
Figure A. Using an AC voltmeter to check AC leakage.
Figure B. Checking for earth ground.
