CHINESE COLOR TV KIT - SERVICE HELP - LA76810 - LC863532C
USED ICs: AT24C04/08 (System Memory) – LC863532Cxxxx (Main System Control) –
LA76810 or LA76818A (Horizontal_Vertical_Chroma_VIF _Sync-Seperator Jungle) –
AN17821A (Audio Output) – LA78040 (Vertical Output) – BSC24-0144014K /
BSC-09N20E LOT_ (Line Output or Fly-back Transformer)
Possible SMPS faults and repair procedures
Set Dead - Condition -1
Set dead - Condition - 2
When
switch ON, there is no power light or any other indication. Sometimes, the Power LED may light up, and
shut down atonce.
The
power supply section circuit to this kit has short circuit protection. By anyway, any short circuit occurs at the
main board, the power won’t switch ON.
The main cause found to this circuit is with shorted Horizontal Output
Transistor [2SD1651C / 2SD2499; (Collector to Emitter or Collector to Base;
short). The best way to troubleshoot is,
just de-solder out the horizontal output transistor from the PWB, and just
switch ON the set, and see whether the Standby Light is ON. If it does, make sure that this transistor is
damaged.
Before
replacing this transistor, just check for any other reason for its failure. The C435 & C438 should be checked with a
capacitance meter for its capacity. It
will be best to replace both of them at the same time, along with the
horizontal output transistor. Even loose
solder terminals at these capacitor terminals will surely make the H-Out
transistor get shorted. Check VD435
& VD436 and C437 too. Without
checking all these components and its soldering thoroughly, never switch on the
set with a new H-Out transistor.
The
next possibility for H-out transistor failure is; it’s loose seating screw with
the heat sink. Another one is with
shorted winding [Horizontal section] of the deflection yoke. Other is HOT [Horizontal Output Transistor;
otherwise called as LOT BSC24-01N4014K / BSC25-09N20E. Check all the solder terminals to the leads
of this Line out Transformer [LOT] too, for any loose soldering.
If
deflection yoke winding has any short (this short can’t be measure by normal
technique or with a multimenter); it should be replaced before switch on the
set. You can switch ON the set without
this deflection yoke connection; but; must unplug the CRT base card from the
CRT base; otherwise there is a chance to make a dark spot at the center of the
screen due to accelerated beam of electrons inside the CRT, without any
scanning. It will fall straight at the
center of the screen itself, provided the LOT and H-Out transistor as well as
the SMPS is OK. Be sure about this, when
you switch on the set without deflection yoke connection. This is true to any CRT based televisions;
irrespective of its brand and size.
If
the horizontal output transistor is found damaged, check the load resistor R551
2.2Ohms 2W W/W for open. In most cases,
this won’t happen to this kind of circuit, as any short circuit occurs at the
secondary stage of the SMPS will clause the SMPS to shut down instantaneously.
In
practice, this is the most venerable fault that can occur to sets fitted with
this kit. 90% of it I did faced was
it.
If
the LOT is defective (Internal short in its winding), the horizontal output
transistor will heat up within seconds.
To make this sure, just switch ON the set, and see whether the standby
light lights up. If it does, just switch
it OFF after 30 or 40 seconds, and unplug the set from AC mains wall socket,
and feel the heat of the horizontal output transistor with bare fingers. It can be slightly warm, but not too
hot. This effect can due to a
damaged deflection yoke too.
It
is advised to re-solder all the terminals of IC LA76810 / LA76818A;
because
this IC will heat up to an extent while the set works, and repeated
heating and
cooling can make its solder terminals to expand and shrink; and make any
loose solder terminals, so microscopic to see with naked eye. It is be
best to suck out the solder from
each terminal; using a solder suction pump and re-solder it by applying
fresh solder,
without making any solder bridge short in between. Be very careful.
After soldering the IC terminals, clean it using Medical Quality
Turpentine or Rectified Spirit.
In
most cases, the main system control IC found OK. Do not do any unwanted checks and meter prods
touches to it; when power is ON. This is a very sensitive
IC; can be damaged easily.
The
SMPS circuit to this set is quite simple related to STR based ones, and is easy
to follow. I’ve here described some
service experiences of mine with this type of kit. There might be more to describe for
others. Service experiences to person to person
will be different indeed.
SCHEMATIC
CLICK ON THE SCHEMATIC TO ZOOM IN
SET DEAD - Condition - 2
And
now; let’s come to the second stage of no power possibility. Its main power regulator [SMPS] section
circuit. There are two stages to this
section, a primary stage power [input] and a secondary stage [outputs]
How
can we check it?
Let’s
take the primary stage AC [input] section first.
AC power is the input to this section and passed through a line filter
[L501], a full wave bridge rectifier section consists of 4xRL 207
diodes, the smoothed by a main filter capacitor C507; and the DC voltage
is applied to the primary
winding of the SMPS transformer, the other end of the winding is
connected
directly to the collector of main switching transistor 2SD2498.The
emitter
of this transistor is pulled back to ground via a resistor of low Ohm
value (usually be a wire-wound fusible type.
The
transistor 2SD2498 acts as a high speed electronic switch, at about the rate of
40 to 45KHz. The oscillation to its base
to work at this frequency is generated and supplied by two other small signal
transistors. In simple, this SMPS
consists of 3 semiconductors. Let’s go
straight to the fault/s that can occur to this circuit.
Assume
that the set is dead, No power light, no other response on switch ON. As we checked earlier, take out the
horizontal output transistor from the circuit, and then switch ON the
set. Check whether the standby LED light up or not. Still the set is dead. Let’s continue.
The
first thing to check is whether there is any electric charge accumulated
is stored in the main filter capacitor C507 (Main Filter). It should be
checked first; because, if it is
fully charged by any way; the voltage across its two terminals might be
up to
375VDC. In this condition, by chance if
we try to measure any resistance parameters in this primary circuit,
with multimeter set to Ohms or other resistance measurement range; the
meterwill be get damaged instantaneously.
There is no doubt about it. So,
the first thing is to check this. Set
the multimeter to measure 1000VDC and measure the voltage across this
main filter capacitor. Make sure that there
is no voltage at all; before you proceed.
If some voltage is present; say at least 10VDC, it should be
discharged. Discharge it well.
In
most cases, the fault I found to this stage was with a blown fuse. But, if the fuse should blow up, there
should be sufficiant reason. The fuse is not
used there for none. It is a protection
fuse. If a protection fuse should fail,
surely there will be some fault (short-circuit) to the circuit. Without rectifying it; if we put another
fuse in place, and switch On the set, we are actually inviting danger, and damage
to more component. So; if fuse is found blown
up, check and find out the reason for it.
There will be a reason. It's sure.
See the SMPS the circuit. [Given below]. A 3.5Amp (Glass Cartridge or Ceramic type) fuse is used in
series with any one of the AC inputs.
Check
the resistance across R501 first, It
should be more than 200KOhms. In most
cases, the resistance measurement at this point might measure almost Zero Ohms; means a
short circuit. If so, de-solder out one
terminal of C501 and then measure the resistance across R501. If it is found High; (No short circuit), be sure that the C501 is
damaged, a direct short happenened inside it; and it might be the reason for the failure of the
fuse. But; never stop there. Check all the other capacitor/s ; out from circuit; means
let it be there; but de-solder out one end of it and
measure. If found OK, solder it there
again. Before measuring these resistances,
unplug the degauss posistor connection. If the degauss possistor used
is a three terminal one, unsolder it out from the circuit, till all
other work has been finished. Solder it into circuit after the service
work has finished. Otherwise, it will get damaged [ RT501.]
Till
there shows an extreme low resistance, check the main power
switching [SMPS] transistor V513.
Measure its junction resistance [Collector to Emitter]. It should be
very high up to the rate of
Megohms. If it is damaged, this
transistor will show a dead short across the rectified DC voltage
[measure in Ohms]. If so; unsolder it out from the circuit, then
measure the forward and reverse resistance of each of the main rectifier
diodes.
Here
one point should be keep in mind. These
four rectifier diodes are venerable to get shorted due to any voltage spike or
a slight lightning stroke. Even one among them
found damaged, replace the four of than at the same time, otherwise there
is a chance to get the others to be damaged by time.
If
the main switching transistor if found damaged [Shorted]; surely the resistance
R510 too have opened. Check and replace
it if found damaged.
In
my opinion it will be best to replace other two small signal transistors 2SC3807
& 2SA1015 too. It won’t cost
more. Replace the Zener diode
VD519. Never leave it there even for an experiment.
If
you have done all the checks stated above, and found the primary side [Live
side] of this circuit OK, now it is time to check the secondary section. Normally, secondary section circuit won’t
have any problem at all. The only
problem I've found is with a busted photo-coupler. So I advise you to replace this photo coupler
and its control transistor [V533 - @Sc1815] too.
Now
trace out the B+ [110V] point. Disconnect
other loads from this line, and connect a 100W Filament type bulb [Filament
type bulb only; not a CFL or any other].
Unplug
the AC fuse FU501 from its holder. Solder
two separate wires at both end of the fuse holder, and connect these wires to another bulb
holder. Insert a 100W filament type bulb
there too. This is a temporary
protection procedure. Now the filament
of this bulb acts as a fuse of high resistance.
Let all the other
connections, except the B+110V connected.
No problem at all.
Now
plug the TV to AC mains wall socket, and switch it on.
The
first bulb connected across the fuse holder will light up bright at first, the
bulb connected across the +B will light up faintly indicating that there
is a low DC voltage. The the bulb connected across the fuse
will light up brightly for a moment, and will get dimmed, while the bulb
connected at the B+ will stay lighted constantly. It should be.
Now measure the voltage across C561.
It should be constant. (at about 110VDC). Keep in this
condition for at least half an hour. If
no abnormality is noted, you can surely connect the load to B+ voltage, and can
remove the bulb and holder from the fuse bracket, and insert the correct rated
fuse. The SMPS of your TV is now OK.
SMPS SCHEMATIC 
