Monday, February 23, 2015

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
SET DEAD.  NO POWER - 1
  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