Loewe Xelos M37 VT Instructions De Service page 6

Allgemeiner Teil / General Section / Partie générale
Note on electrostatic shielding
1. Electrostatically shielded MOS workstations
Components sensitive to electrostatic discharge must be handled at
workstation with electrostatic shielding. An electrostatically shielded
MOS workstation is fitted with discharge resistor which earth all
conductive materials, including the technician working there.
Dielectrics are discharged by air ionisation. The use of soldering
irons and measuring equipment at shielded workstation is only
possible in conjunction with isolating transformer in each of the
devices used. Measuring equipment chassis are also earthed with
discharge resistors.
2. Shielded packaging using conductive materials
To protect against electrostatic charges, electrically conductive
plastics are used for packaging and transport purposes. Conductive
plastics are available in the form of transparent protective bags,
foam plastic, film sheeting or containers. Sensitive components
requiring the use of protective packaging must only be packed and
unpacked at shielded workstations.
Safety Precautions
General Guide Lines
1. These television sets are isolated from the electric power mains by
the power transformer. An additional isolation transformer is
necessary for servicing work on the primary side of the power
transformer.
2. When servicing, observe the original lead dress in the high voltage
circuits. If a short circuit is found, replace all parts which have been
overheated or damaged by the short circuit.
3. Since many parts in the unit have special safely related
characteristics, always use genuine producer replacement parts.
Especially critical parts in the power circuit block should not be
replaced with other makers. Critical parts marked with
circuit diagram and printed wiring board.
4. After servicing, see that all the protective devices such as insulation
barriers, insulation papers, shields and isolation R-C combinations
are correctly installed.
5. When the receiver is not being used for a long period of time, unplug
the power cord from the AC outlet.
6. Potentials as high as 29.9kV are present when this receiver is in
operation. Operation of the receiver without the rear cover involves
the danger of a shock hazard from the receiver power supply.
Servicing should not be attempted by anyone who is not familiar with
the precautions necessary when working on high voltage
equipment. Always discharge the anode of the picture tube to the
chassis before handling the tube.
7. 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
of the plug.
2. Turn on the receiver's power switch.
3. Measure the resistance value with an ohmmeter, between the
jumpered AC plug and each exposed metallic cabinet part on the
receiver, such as screw heads, aerials, connectors, control shafts
etc. When the exposed metallic part has a return path to the chassis
the reading should be between 4MΩ and 20MΩ. When the exposed
metal does not have are turn path to the chassis the reading must
be infinite.
1 - 6
Leakage current hot check
1. Plug the AC cord directly into the AC outlet. Do not use an isolation
transformer for this check.
2. Connect a 2kΩ/10W resistor in series with an exposed metallic part
on the receiver and an earth such as a water pipe (fig.1a).
3. Use an AC voltmeter with high impedance to measure the potential
across the resistor.
4. Check each exposed Metallic part and check the voltage at each
point.
5. Reverse the AC plug at the outlet and repeat each of the above
measurements.
6. The potential at any point should not exceed 1.4Vrms. In case a
measurement is outside the limits specified, there is a possibility of
a shock hazard, and the receiver should be repaired and rechecked
before it is returned to the customer.
Hot check circuit
To instrument`s
exposed
metallic parts
S
in the
Fig.1a Leakage current hot check
X-Radiation warning
1. The potential sources of X-Radiation in TV sets are the high voltage
section and the picture tube.
2. When using a picture tube test jig for service ensure that the jig is
capable of handling 31.0kV without causing X-Radiation.
Measuring high voltage
1. Set the brightness to minimum.
2. Measure the high voltage. The meter should indicate 29.0kV ± 0.7kV
if the meter indication is out of tolerance, immediate service and
correction is required to prevent the possibility of premature
component failure.
3. To prevent any X-Radiation possibility, it is essential to use the
specified tube.
NOTE: It is important to use an accurate periodically calibrated high
voltage meter
M 1000 / M 1001
A.C. Voltmeter
Water Pipe
(Earth)
2KΩ / 10W
LOEWE.-Service