Hitachi 50gx49b Service Manual page 7

General Soldering Guidelines
1. Use a grounded-tip, low-wattage soldering iron and
appropriate tip size and shape that will maintain tip
temperature within the range 500°F to 600°F.
2. Use an appropriate gauge of resin-core solder composed
of 60 parts tin/40 parts lead.
3. Keep the soldering iron tip clean and well-tinned.
4. Thoroughly clean the surfaces to be soldered. Use a
small wire-bristle (0.5 inch or 1.25 cm) brush with a metal
handle. Do not use freon-propelled spray-on cleaners.
5. Use the following desoldering technique.
a. Allow the soldering iron tip to reach normal
temperature (500°F to 600°F).
b. Heat the component lead until the solder melts. Quickly
draw away the melted solder with an anti-static,
suction-type solder removal device or with solder
braid.
CAUTION: Work quickly to avoid overheating the
circuit board printed foil.
6. Use the following soldering technique.
a. Allow the soldering iron tip to reach normal
temperature (500°F to 600°F).
b. First, hold the soldering iron tip and solder strand
against the component lead until the solder melts.
c. Quickly move the soldering iron tip to the junction of
the component lead and the printed circuit foil, and
hold it there only until the solder flows onto and
around both the component lead and the foil.
CAUTION: Work quickly to avoid overheating the
circuit board printed foil or components.
d. Closely inspect the solder area and remove any excess
or splashed solder with a small wire-bristle brush.
Use Solding Iron to Pry Leads
IC Removal/Replacement
Some Hitachi unitized chassis circuit boards have slotted
holes (oblong) through which the IC leads are inserted and
then bent flat against the circuit foil. When holes are the
slotted type, the following technique should be used to
remove and replace the IC. When working with boards using
the familiar round hole, use the standard technique as
outlined in paragraphs 5 and 6 above.
Removal
1. Desolder and straighten each IC lead in one operation by
gently prying up on the lead with the soldering iron tip as
the solder melts.2. Draw away the melted solder
with an anti-static suction-type solder removal device (or
with solder braid) before removing the IC.
Replacement
1. Carefully insert the replacement IC in the circuit board.
2. Carefully bend each IC lead against the circuit foil pad
and solder it.
3. Clean the soldered areas with a small wire-bristle brush.
(It is not necessary to reapply acrylic coating to areas.)
"Small-signal" Discrete Transistor Removal/Replacement
1. Remove the defective transistor by clipping its leads as
close as possible to the component body.
2. Bend into a "U" shape the end of each of three leads
remaining on the circuit board.
3. Bend into a "U" shape the replacement transistor leads.
4. Connect to replacement transistor leads to the
corresponding leads extending from the circuit board and
crimp the "U" with long nose pliers to insure metal to
metal contact, then solder each connection.
Power Output Transistor Devices Removal/Replacements
1. Heat and remove all solder from around the transistor
leads.
2. Remove the heatsink mounting screw (if so equipped).
3. Carefully remove the transistor from the circuit board.
4. Insert new transistor in circuit board.
5. Solder each transistor lead, and clip off excess lead.
6. Replace heatsink.
Diode Removal/Replacement
1.
2.
3.
4.
5.
Remove defective diode by clipping its leads as close as
possible to diode body.
Bend the two remaining leads perpendicularly to the
circuit board.
Observing diode polarity, wrap each lead of the new
diode around the corresponding lead on the circuit board.
Securely crimp each connection and solder it.
Inspect (on the circuit board copper side) the solder joints
of the two "original leads". If they are not shiny, reheat
them and, if necessary, apply additional solder.