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3.1 SAFETY INSTRUCTIONS
Safety regulations demand that the set be restored to
its original condition and that components identical
with the original types be used.
Safety components are marked by the symbol
A.
- ESD
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All IC's and many other semi-conductors are
susceptible to electrostatic discharges (ESD).
Careless handling during repair can reduce life
drastically.
When repairing, make sure that you are connected
with the same potential as the mass of the set via a
wrist wrap with resistance.
Keep components and tools also at this potential.
For detailed information see "Handling ESD-sensitive
components".
- A set to be repaired should always be connected to
the mains via a suitable isolating transformer.
- never replace any modules or any other parts while
the set is switched on.
- Use plastic instead of metal alignment tools. This in
order to prelude short-circuit or to prevent a specific
circuit form being rendered unstable.
3.2 SERVICING OF SMDs (Surface Mounted Devices)
3.2.1 General cautions on handling and storage
a. Oxidation on the SMDs terminals results in poor
soldering. Do not handle SMDs with bare hand.
b. Avoid for storage places that are sensitive to
oxidation such as places with sulfur or chlorine gas,
direct sunlight, high temperatures or a high degree of
humidity.
As a result the capacitance or resistance value of the
SMDs may be affected.
c. Rough handling of circuit boards containing SMDs
may cause damage to the components as well as the
circuit boards. Circuit boards containing SMDs should
never be bent or flexed. Different circuit board
materials expand and contract at different rates when
heated or cooled and the components and/or solder
connections may be damaged due to the stress.
Never rub or scrape chip components as this may
cause the value of the component to change. Similary,
do not slide the circuit board across any surface.
3.2.2 Removal of SMDs
a. Heat the solder (for 2-3 seconds) at each terminal of
the chip. Small components can, by means of litz wire
and a limited horizontal force, be removed with the
soldering iron. They can also be removed with a
solder sucker (see Fig. 1 a) or
b. While holding the SMD with a pair of tweezers take it
off gently using th soldering iron's heat applied to
each terminal (see Fig. 1 b).
c. Remove the excess solder on the solder lands by
means of litz wire or a solder sucker (see Fig. 1 c).
3.2.2.1 Caution on removal:
a. When handling the soldering iron, use suitable
pressure and be careful.
b. When removing the chip, do not use undue force with
the pair of tweezers.
c. The soldering iron to be used (approx. 30 W), must
preferably be provided with a thermal control
(soldering temperature about 225 to 250°C).
d The chip, once removed, must never be used again.
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3.2.3 Attachment of SMDs
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a. Locate the SMD on the solder lands by means of
tweezers and solder the component at one side.
Ensure that the component is positioned well on the
solder lands (see Fig. 2a).
b. Next complete te soldering of the terminals of the
component (see Fig. 2b).
MOUNTING
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3.2.3.1 Caution on attachment:
a. When soldering the SMD terminals, do not touch them
directly with the soldering iron. The soldering must be
as quick as possible; care must be taken to avoid
damage to the terminals and the body itself.
b. Keep the SMD's body in contact with the printed
board when soldering.
c. The soldering iron to be used (approx. 30 W) must
preferably be provided with a thermal control
(soldering temperature about 225 to 250°C).
d. Soldering should not be done outside the solder land.
e. Soldering flux (of rosin) may be used but should not
be acidic.
f. After soldering, let the SMD cool down gradually at
room temperature.
g. The quantity of solder must be proportional with the
size of the solder land. If the quantity is too great, the
SMD might crack or the solder lands might be torn
loose from the printed board (see Fig. 3).
3.3 HANDLING ESD-SENSITIVE COMPONENTS
3.3.1 Personal safety
The testing, handling and replacing of ESD-sensitve
components requires special attention for personal
safety. A person dealing with ESD-sensitive
components should, normally speaking, be connected
via a resistance to the same potential as the chassis
of the set to protect him against direct contact with
the supply voltage.
This resistance is often applied in the connection lead
of wrist wraps. If necessary, make use of an isolating
transformer.
3.3.2 Storage and transport
Transport and store the circuits and PCBs in their
original packages.
As an alternative to the original package one may use
a conductive material or special IC package which
short-circuits all the pins of the component with one
another.
Always discharge the package before opening it
3.3.3 Testing or handling
Work on a conductive surface when testing loose
circuits and components or when transferring
components and circuits from one package to
another.
Use a conductive wrist wrap with lead to make an
electrical connection between the conductive surface
and yourself via a resistance in the connection lead of
the wrist wrap.
Connect equipment and tools also with this conductive
surface.
Do not connect any signals to inputs as long as the
power supply of the set to be tested is off.
All the inputs that are not used should be connected
either to ground or to the supply voltage. When
testing, do not use any freon sprays for under-cooling
of sensitive components.
3.3.4 Mounting ESD-sensitive components
Mount ESD-sens1tive components only after all other
components have been mounted.
Make sure that the components themselves, the metal
parts of the PCB, mounting equipment and mounting
operator are at the same potential level as the chassis
of the set.
If it is impossible to ground the PCB, the mounting
operator should pick the PCB up before bringing it
into contact with the components to be replaced.
Conductive sheet
Special attention should be paid in regions having a
dry atmosphere and when the floor is covered with a
nylon carpet or such.
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3.3.5 Soldering
Soldering iron tips, also those of low-voltage soldering
stations, should be kept at the same potential as the
components and the PCB.
It is better to use solder-removing braid than solder
suckers.
3.3.6 Electrostatic charges
One should stick to the precautionary measures also
after the ESD-sensitive components have been
mounted on the PCB. Until the sub-PCBs have been
incorporated into a complete system on which the
correct supply voltages are connected, the PCB is
nothing more than an extension of the conductors of
the components on this PCB. To prevent electrostatic
discharges from passing to the components via the
terminals, we recommend that you apply conductive
clips or conductive tape on the terminals of the PCB.
3.3.7 Transients (switch-on phenomena)
To prevent permanent damages as a result of
switch-on phenomena. no ESD-sensitive components,
or PCBs populated with these components, should be
inserted in or removed from test-sockets or systems
with the supply voltage on.
Prevent switching peeks on the mains as a
consequence of switching electric equipment, relay
and DC lines on and off.
3.3.8 Working environment
The work bench for the service technician should look
like the one shown in the figure.
3.3.9 Replacement of the Flat Pack IC's
For replacing a component see Fig. 6 Dismounting
and Mounting. Also a number of precautions and
examples is given.
When replacing a flat pack, rosin flux applied to the
device leads will ensur-e a good soldered joint.
Since rosin flux, when not properly heated by the
soldering process, is sticky, it will attract dust which
will result in component degeneration over a period of
time.
The removal of excess flux with a cleaner will not
solve this problem because the flux is then even
spread over a greater area by the cleaner. Drying of
the flux can be accomplished by blowing the area with
a common hair dryer for 1 or 2 minutes at a distance
of approx. 10 centimeters.
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Safety isolating transformer
Wrist wrap
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