LG M1721A Service Manual Download Page 5

Page: 5 / 38

- 5 -

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 or 500 F to 600 F.

2. Use an appropriate gauge of RMA 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

mall wire-bristle (0.5 inch, or 1.25cm) brush with a
metal handle.
Do not use freon-propelled spray-on cleaners.

5. Use the following unsoldering 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.
c. Quickly draw the melted solder with an anti-static,

suction-type solder removal device or with solder
braid.

CAUTION:

Work quickly to avoid overheating the

circuitboard printed foil.

6. Use the following soldering technique.

a. Allow the soldering iron tip to reach a normal

temperature (500 F to 600 F)

b. First, hold the soldering iron tip and solder the 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.

d. Closely inspect the solder area and remove any

excess or splashed solder with a small wire-bristle
brush.

IC Remove/Replacement

Some 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
the 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 the 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 Device
Removal/Replacement

1. Heat and remove all solder from around the transistor

leads.

2. Remove the heat sink mounting screw (if so equipped).
3. Carefully remove the transistor from the heat sink of the

circuit board.

4. Insert new transistor in the circuit board.
5. Solder each transistor lead, and clip off excess lead.
6. Replace heat sink.

Diode Removal/Replacement

1. Remove defective diode by clipping its leads as close

as possible to diode body.

2. Bend the two remaining leads perpendicular y to the

circuit board.

3. Observing diode polarity, wrap each lead of the new