HP L1940 Service Manual page 82

TELEVISION/MONITOR SAFFTY GUIDELINES FOR THE PROFESSIONAL SERVICE TECHNICIAN
Safety Checks
After the original service problem has been corrected,a complete safety check
should be made. Be sure to check over the entire set, not just the areas
where you have worked. Some previous servicer may have left an unsafe
condition,which could be unknowingly passed on to your customer. Be sure
to check all of the following:
Fire and Shock Hazard
1. Be sure all components are positioned in such a way as to avoid the
possibility of adjacent component shorts. This is especially important
on those chassis which are transported to and from the service shop.
2. Never release a repaired unit unless all protective devices such as
insulators, barries, covers, strain reliefs, and other hardware have been
installed in accordance with the original design.
3. Soldering and wiring must be inspected to locate possible cold solder
joints,solder splashes,sharp solder points, frayed leads, pinched leads,
or damaged insulation ( including the ac cord).Be certain to remove
loose solder balls and all other loose foreign particles.
4. Check across-the-line components and other components for physical
evidence of damage or deterioration and replace if necessary. Follow
original layout,lead length, and dress.
5. No lead or component should touch a receiving tube or a resistor rated
at 1 watt or more. Lead tension around protruding metal surfaces or
edges must be avoided.
6. Critical components having special safety characteristics are identified
with ans by the Ref. No. in the parts list and enclosed within a broken line
* (where several critical components are grouped in one area) along with
the safety symbols on the schematic diagrams and/or exploded views.
7. When servicing any unit, always use a separate isolation transformer
for the chassis Failure to use a separate isolation transformer may
expose you to possible shock hazard, and may cause damage to
servicing instruments.
8. Many electronic products use a polarized ac line cord (one wide pin on
the plug.) Defeating this safety feature may create a potential hazard to
the service and the user. Extension cords which do not incorporate the
polarizing feature should never be used.
9. After reassembly of the unit, always perform an leakage test or resistance
test from the line cord to all exposed metal parts of the cabinet. Also check
all metal control shafts(with knobs removed), antenna terminals, handles,
screws, etc. to be sure the unit may be safety operated without danger of
electrical shock.
* Broken line
Implosion
1. All picture tubes used in current model receivers are equipped with an
integral implosion system. Care should always be used, and safety
glasses worn,whenever handling any picture tube. Avoid scratching or
otherwise damaging the picture tube during installation.
2. Use only replacement tubes specified by the manufacturer.
X-radiation
1. Be sure procedures and instructions to all your service personnel cover
the subject of X-radiation. Potential sources of X-rays in TV receivers are
the picture tube and the high voltage circuits. The basic precaution which
must be exercised is to keep the high voltage at the factory recommended
level.
2. To avoid possible exposure to X-radiation and electrical shock, only the
manufacturer's specified anode connectors must be used.
3. It is essential that the service technician has an accurate HV meter
available at all times. The calibration of this meter should be checked
periodically against a reference standard.
4. When the HV circuitry is operating properly there is no possibility of an x-
radiation problem. High voltage should always be kept at the
manufacturer's
rated value-no higher - for optimum performance. Every time a color set is
serviced, the brightness should be run up and down while monitoring the
HV with a meter to be certain that the HV is regulated correctly and does
not exceed the specified value. We suggest that you and your technicians
review test procedures so that HV and HV regulation are always checked as
a standard servicing procedure, and the reason for this prudent routine is
clearly understood by everyone. It is important to use an accurate and
reliable HV meter. It is recommended that the HV recorded on each
customer's invoice, which will demonstrate a proper concern for the
customer's safety.
5. When troubleshooting and making test measurements in a receiver with
a problem of excessive high voltage, reduce the line voltage by means of
a Variac to bring the HV into acceptable limits while troubleshooting. Do
WARNING Before removing the CRT anode cap, turn the unit
:
The CRT DAG is not at chassis ground.
SERVICE NOTE
:
6. New picture tubes are specifically designed to withstand higher operathng
voltages without creating undesirable X-radiation. It is strongly
recommended that any shop test fixture which is to be used with the new
higher voltage chassis be equipped with one of the new type tubes
designed for this service. Addition of a permanently connected HV meter
to the shop test fixture is advisable. The CRT types used in these new
sets should never be replaced with any other types, as this may result in
excessive X-radiation.
7. It is essential to use the specified picture tube to avoid a possible X-diation
problem.
8. Most TV receivers contain some type of emergency "Hold Down" circuit to
prevent HV from rising to excessive levels in the presence of a failure
mode. These various circuits should be understood by all technicians
servicing them, especially since many hold down circuits are inoperative
as long as the receiver performs normally.
Leakage Current Cold Check
1. Unplug the ac line cord and connect a jumper between the two prongs of
the plug.
2. Turn on the power switch.
3. Measure the resistance value between the jumpered ac plug and all
exposed cabinet parts of the receiver, such as screw heads, antennas,
and control shafts. When the exposed metallic part has a return path to
the chassis, the reading should be between 1 megohm and 5.2 megohms.
When the exposed metal does not have a return path to the chassis, the
reading must be infinity. Remove the jumper from the ac line cord.
0.15 uF
TO
INSTRUMENTS
EXPOSED
METAL PARTS
1500 ohm, 10W
Leakage Current Hot Check
1. Do not use an isolation transformer for this test. Plug the completely
reassembled receiver directly into the ac outlet.
2. Connect a 1.5k, 10w resistor paralleled by a 0.15uf. capacitor between
each exposed metallic cabinet part and a good earth ground such as a
water pipe, as shown above.
3. Use an ac voltmeter with at least 5000 ohms volt sensitivity to measure
the potential across the resistor.
4. The potential at any point should not exceed 0.75 volts. A leakage current
tester may be used to make this test; leakage current must not exceed
0.5 milliamps. If a measurement is outside of the specified limits, there is
a possibility of shock hazard. The receiver should be repaired and
rechecked before returning it to the customer.
5. Repeat the above procedure with the ac plug reversed.( Note: An ac
adapter is necessary when a polarized plug is used. Do not defeat the
polarizing feature of the plug.)
Picture Tube Replacement
The primary source of X-radiation in this television receiver is the picture
tube. The picture tube utilized in this chassis is specially constructed to limit
X-radiation emissions. For continued X-radiation protection, the replacement
tube must be the same type as the original, including suffix letter, or a Philips
approved type.
Parts Replacement
Many electrical and mechanical parts in Philips television sets have special
safety related characteristics. These characteristics are often not evident from
visual inspection nor can the protection afforded by them necessarily be
obtained by using replacement components rated for higher voltage, wattage,
etc. The use of a substitute part which does not have the same safety
characteristics as the Philips recommended replacement part shown in this
service manual may create shock, fire, or other hazards
OFF and short the HIGH VOLTAGE to the CRT DAG ground.
WATER
PIPE
EARTH
GROUND