Help Needed - Thomson 27LCDB03B not working

[Acrantophis]

Let me start by saying hello

Now on to the problem.

I've been handed a Thomson 27LCDB03B that was reported as not always wanting to turn on. Anyway upon taking a look at it I've not been able to get it to power up at all, no power led, nothing.

Hunted high and low for a schematic and finally tracked one down and now I've done a little poking around.

I can confirm that I have 230VAC at PSU EMI board and a DC voltage of aprox 321Volts across the 100uF 450V caps, from there on the voltages appear to vanish. I don't appear to have a VCC voltage at pin 7 of U6301, nor do I appear to have any voltage present on the output stage of the transformer T6301 (pins 7 & 9).

I do have an electronics background just not in TV repairs so I was wondering if there is anyone on here who could work through this problem with me?

Thanks in advance.

[astracat]

photo of psu diagram required b4 can help, but prob start up resisters or cap

[Acrantophis]

 PDF service manual I have is now attached below.

Board manufacturer and Part N#; DARFON 48.B0090.001

Unfortunately I don't have a camera to take a picture of the board but there is an image of one here:- http://shop.sjstv.co.uk/ekmps/shops/sjstv/images/48.b0090.001-2444-p.jpg

[astracat]

hi think you should check these first
   r6303.6304,rb301,c6311,u6301,also voltage onto d6310 cathode,what volts on q6302base connection
regards
astracat

[Acrantophis]

Astracat, I may have been mistaken originally. Upon checking U6301 (VCC) pin 7 I discovered that I actually have 8.42Vdc when measured back to the ground of that device.

Q6302 voltages as follows:-
Base = 9.6vdc
Collector = 185vdc
Emitter = 9.57vdc

D6310 voltage measured at the Cathode back to Gnd is 0.57vdc

Visual inspections of the components listed show no outward signs of defect/failure, these also appear to verify as OK when tested with 2 seperate multi meters.

U6301 proving a little difficult to test. When I try and see what is happening with the aid of a scope I end up tripping my bench supply. I think that this is probably due to the 0V of the board not beign a true GND, I will try again tomorrow with our portable one tomorrow which should be back from calibration.

I don't know if the above helps at all or if you have any further suggestions but thanks so far for your help.

[astracat]

hi
those resisters tend to go high in value ,dont think ic will fire up till about 9volts on it .should check why theres less voltage there also check caps to that supply.roughly 9volts there ic fires up then transformer takes over from then on to keep it running
regards
astracat

[Acrantophis]

Replaced the electrolytic caps C6203/C6310/C6311/C6312/C6313/C6314, I had read somewhere that these can fail, I had checked them and felt that they were good but given that we are talking pennies I figured it was cheap enough to replace them. No joy, still only just over 8V on VCC (pin 7 U6301).
Double checked the resistors R6301/R6303/R6304 by removing them from the circuit, happy to report that these all verify as OK and within their specified tolerences.
This repair is still a work in progress I had limited time available today and no time tomorrow so any updates will be posted on Friday assuming you are able to help me further.
 

[astracat]

hiya
think prob further back on pfc board check r6201a,b,c,d,fuse on end of d6201also r6212
g/luck
astracat

[Acrantophis]

TR6201 is a Metal Oxide Varistor, it could be faulty I suppose but I'm not sure as I'm able to measure a voltage after it within the circuit. A quick google search has failed to throw up any specs on the device in question though so if I was to replace it I am not sure what I would need, any ideas?
SCK-056L

Thanks again.

[astracat]

ok check to make sure u7401 has power as that circuit supply 12v then inturn from u6401 for 5v  both on audio psu board check caps on there as well.
astracat
ps assume you checked tact switch on tv?

[Acrantophis]

OK you lost me in terminology; ??Tact switch?? Do you mean the On/Off push button on the side of the TV, if so this is known to be good, checked the switching continuity of it as one of the first probable causes.

I have been testing the PSU PCB as an individual item leaving the TV at home and working on the board at work during my lunch breaks, I do not know if this will give me the correct results or not but I don't appear to have any voltages at the various connectors CN701 to CN703 which supply the volatges to the Main PCB/Audio PCB/Tuner PCB/etc.

It could well be that there is an issue with the MOV as I have a low ohms reading across it which I think is wrong because typically these should have a high resistance reading at a low voltage and a low resistance at a high voltage, assuming my memory is still serving me correctly and I have correctly identified the device.

Anyway I will investigate further tomorrow (Friday) and provide an update.

Thanks again for all your guidance.

[Acrantophis]

I might have discovered the cause, or I might not have. You never can tell with anything electronic until you actually replace it. Anyway I opted to lift pin 7 of U6301 to isolate it from the emitter of Q6302, upon measuring the voltage  at the point where pin 7 would solder to I started seeing +15Vdc so it could be U6301 is faulty or something it is supplying pulling the VCC low. Anyway, I'm off out for a beer tonight so will look into this more tomorrow.

[astracat]

hi
that ic is a self starting oscillator ,sounds like you found problem suggest change it first
g/luck
astracat

[Acrantophis]

  Hi five Astracat! Together we've cracked it, appreciate the help.

Changing U6301 has resolved the problem, TV now back up and running. Just to confirm for anyone looking to fix the same power supply showing the same symptoms and they should subsequently stumble upon this post. The dc voltage to pin 7 of U6301 should be around 14V dc when working correctly. If you have the skills to lift the pin 7 re-measure the voltage at the PCB sniff point and see if it goes up to 15V which supports the IC failure as I found.

[astracat]

hiya
glad its sorted well done
astracat