DOWNLOAD LG 42LH20FD (CHASSIS:LT91A) Service Manual ↓ Size: 4.29 MB | Pages: 75 in PDF or view online for FREE

Model
42LH20FD (CHASSIS:LT91A)
Pages
75
Size
4.29 MB
Type
PDF
Document
Service Manual
Brand
Device
TV / LCD
File
42lh20fd-chassis-lt91a.pdf
Date

LG 42LH20FD (CHASSIS:LT91A) Service Manual ▷ View online

LCD TV
SERVICE MANUAL
CAUTION
BEFORE SERVICING THE CHASSIS,
READ THE SAFETY PRECAUTIONS IN THIS MANUAL.
CHASSIS : LT91A
MODEL : 42LH20
F
D
42LH20FD-DA
- 2 -
CONTENTS
CONTENTS ............................................................................................. 2
PRODUCT SAFETY ................................................................................. 3
SPECIFICATION ....................................................................................... 6
ADJUSTMENT INSTRUCTION .............................................................. 10
BLOCK DIAGRAM.................................................................................. 14
EXPLODED VIEW .................................................................................. 15
SVC. SHEET  ...............................................................................................
- 3 -
SAFETY PRECAUTIONS
Many electrical and mechanical parts in this chassis have special safety-related characteristics. These parts are identified by      in the
Schematic Diagram and Exploded View. 
It is essential that these special safety parts should be replaced with the same components as recommended in this manual to prevent
Shock, Fire, or other Hazards. 
Do not modify the original design without permission of manufacturer.
General Guidance
An  isolation  Transformer  should  always  be  used during  the
servicing  of  a  receiver  whose  chassis  is  not  isolated  from  the  AC
power  line.  Use  a  transformer  of  adequate  power  rating  as  this
protects  the  technician  from  accidents  resulting  in  personal  injury
from electrical shocks.
It  will  also  protect  the  receiver  and  it's  components  from  being
damaged  by  accidental  shorts  of  the  circuitry  that  may  be
inadvertently introduced during the service operation.
If  any  fuse  (or  Fusible  Resistor)  in  this  TV  receiver  is  blown,
replace it with the specified.
When replacing a high wattage resistor (Oxide Metal Film Resistor,
over 1W), keep the resistor 10mm away from PCB.
Keep wires away from high voltage or high temperature parts.
Before returning the receiver to the customer,
always  perform  an  AC  leakage  current  check on  the  exposed
metallic  parts  of  the  cabinet,  such  as  antennas,  terminals,  etc.,  to
be  sure  the  set  is  safe  to  operate  without  damage  of  electrical
shock. 
Leakage Current Cold Check(Antenna Cold Check)
With the instrument AC plug removed from AC source, connect an
electrical  jumper  across  the  two  AC  plug  prongs.  Place  the  AC
switch in the on position, connect one lead of ohm-meter to the AC
plug prongs tied together and touch other ohm-meter lead in turn to
each  exposed  metallic  parts  such  as  antenna  terminals,  phone
jacks, etc. 
If  the  exposed  metallic  part  has  a  return  path  to  the  chassis,  the
measured resistance should be between 1MΩ and 5.2MΩ. 
When  the  exposed  metal  has  no  return  path  to  the  chassis  the
reading must be infinite.
An  other  abnormality  exists  that  must  be  corrected  before  the
receiver is returned to the customer.
Leakage Current Hot Check
(See below Figure) 
Plug the AC cord directly into the AC outlet.
Do not use a line Isolation Transformer during this check.
Connect  1.5K/10watt  resistor  in  parallel  with  a  0.15uF  capacitor
between  a  known  good  earth  ground  (Water  Pipe,  Conduit,  etc.)
and the exposed metallic parts.
Measure  the  AC  voltage  across  the  resistor  using  AC  voltmeter
with 1000 ohms/volt or more sensitivity.
Reverse plug the AC cord into the AC outlet and repeat AC voltage
measurements  for  each  exposed  metallic  part.  Any  voltage
measured must not exceed 0.75 volt RMS which is corresponds to
0.5mA.
In  case  any  measurement  is  out  of  the  limits  specified,  there  is
possibility  of  shock  hazard  and  the  set  must  be  checked  and
repaired before it is returned to the customer.
Leakage Current Hot Check circuit
1.5 Kohm/10W
To Instrument's
exposed 
METALLIC PARTS
Good Earth Ground
such as WATER PIPE,
CONDUIT etc.
AC Volt-meter
IMPORTANT SAFETY NOTICE
0.15uF
- 4 -
CAUTION:  Before  servicing  receivers  covered  by  this  service
manual  and  its  supplements  and  addenda,  read  and  follow  the
SAFETY PRECAUTIONS on page 3 of this publication.
NOTE:  If  unforeseen  circumstances  create  conflict  between  the
following servicing precautions and any of the safety precautions on
page  3  of  this  publication,  always  follow  the  safety  precautions.
Remember: Safety First.
General Servicing Precautions
1.  Always  unplug  the  receiver  AC  power  cord  from  the  AC  power
source before;
a.  Removing  or  reinstalling  any  component,  circuit  board
module or any other receiver assembly.
b.  Disconnecting or reconnecting any receiver electrical plug or
other electrical connection.
c. Connecting  a  test  substitute  in  parallel  with  an  electrolytic
capacitor in the receiver.
CAUTION: A  wrong  part  substitution  or  incorrect  polarity
installation  of  electrolytic  capacitors  may  result  in  an
explosion hazard.
2.  Test high voltage only by measuring it with an appropriate high
voltage  meter  or  other  voltage  measuring  device  (DVM,
FETVOM, etc) equipped with a suitable high voltage probe.
Do not test high voltage by "drawing an arc".
3.  Do  not  spray  chemicals  on  or  near  this  receiver  or  any  of  its
assemblies.
4. Unless  specified  otherwise  in  this  service  manual,  clean
electrical  contacts  only  by  applying  the  following  mixture  to  the
contacts  with  a  pipe  cleaner,  cotton-tipped  stick  or  comparable
non-abrasive applicator; 10% (by volume) Acetone and 90% (by
volume) isopropyl alcohol (90%-99% strength)
CAUTION: This is a flammable mixture.
Unless specified otherwise in this service manual, lubrication of
contacts in not required.
5.  Do not defeat any plug/socket B+ voltage interlocks with which
receivers covered by this service manual might be equipped.
6.  Do  not  apply  AC  power  to  this  instrument  and/or  any  of  its
electrical assemblies unless all solid-state device heat sinks are
correctly installed.
7.  Always  connect  the  test  receiver  ground  lead  to  the  receiver
chassis  ground  before  connecting  the  test  receiver  positive
lead.
Always remove the test receiver ground lead last.
8.  Use  with  this  receiver  only  the  test  fixtures  specified  in  this
service manual.
CAUTION: Do  not  connect  the  test  fixture  ground  strap  to  any
heat sink in this receiver.
Electrostatically Sensitive (ES) Devices
Some  semiconductor  (solid-state)  devices  can  be  damaged  easily
by  static  electricity.  Such  components  commonly  are  called
Electrostatically  Sensitive  (ES)  Devices. Examples  of  typical  ES
devices are integrated circuits and some field-effect transistors and
semiconductor  "chip"  components.  The  following  techniques
should  be  used  to  help  reduce  the  incidence  of  component
damage caused by static by static electricity.
1.  Immediately  before  handling  any  semiconductor  component  or
semiconductor-equipped  assembly,  drain  off  any  electrostatic
charge  on  your  body  by  touching  a  known  earth  ground.
Alternatively,  obtain  and  wear  a  commercially  available
discharging  wrist  strap  device,  which  should  be  removed  to
prevent  potential  shock  reasons  prior  to  applying  power  to  the
unit under test.
2.  After  removing  an  electrical  assembly  equipped  with  ES
devices,  place  the  assembly  on  a  conductive  surface  such  as
aluminum  foil,  to  prevent  electrostatic  charge  buildup  or
exposure of the assembly.
3.  Use only a grounded-tip soldering iron to solder or unsolder ES
devices.
4. Use only an anti-static type solder removal device. Some solder
removal  devices  not  classified  as  "anti-static"  can  generate
electrical charges sufficient to damage ES devices.
5.  Do  not  use  freon-propelled  chemicals.  These  can  generate
electrical charges sufficient to damage ES devices.
6.  Do  not  remove  a  replacement  ES  device  from  its  protective
package  until  immediately  before  you  are  ready  to  install  it.
(Most  replacement  ES  devices  are  packaged  with  leads
electrically  shorted  together  by  conductive  foam,  aluminum  foil
or comparable conductive material).
7.  Immediately  before  removing  the  protective  material  from  the
leads of a replacement ES device, touch the protective material
to  the  chassis  or  circuit  assembly  into  which  the  device  will  be
installed.
CAUTION: Be sure no power is applied to the chassis or circuit,
and observe all other safety precautions.
8.  Minimize  bodily  motions  when  handling  unpackaged
replacement  ES  devices.  (Otherwise  harmless  motion  such  as
the brushing together of your clothes fabric or the lifting  of your
foot  from  a  carpeted  floor  can  generate  static  electricity
sufficient to damage an ES device.)
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  circuit
board 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.
SERVICING PRECAUTIONS
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LG 42LH20FD (CHASSIS:LT91A) Service Manual ▷ Download

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  • Here you can View online or download the Service Manual for the LG 42LH20FD (CHASSIS:LT91A) in PDF for free, which will help you to disassemble, recover, fix and repair LG 42LH20FD (CHASSIS:LT91A) LCD. Information contained in LG 42LH20FD (CHASSIS:LT91A) Service Manual (repair manual) includes:
  • Disassembly, troubleshooting, maintenance, adjustment, installation and setup instructions.
  • Schematics, Circuit, Wiring and Block diagrams.
  • Printed wiring boards (PWB) and printed circuit boards (PCB).
  • Exploded View and Parts List.