Samsung LA26R71BD - white screen with sound + tcon pic

[DoubleDratt88]

Greetings!
I am trying to determine the problem with a Samsung tv i got free as a needs fixing type deal. 26" 720p model LA26R71BD.
Not usually a tv fixing person but i do have an interest in other areas of electronics so its worth a shot for the price. Here's the basics and info i can give so far on the sick patient tv:

Tv powers on ok, sound works ok, inverter and backlight appear to be ok (I have it setup next to another tv with the same av signal - can see the backlight adjust intensity correctly to suit light/dark scenes or when changing channel etc). I can hear the source change correctly and everything else seems to be functioning apart from the panel itself on which i havn't noticed even 1 pixel move.

Unplugging the tcon board from the main board makes no difference to the above, same results when powered on with tcon disconnected from main board.

Voltages measured on the power supply board seem ok, although the 13v is reading 12.35 which seems a little low but possibly not low enough to be out of tolerance?(10%=1.3, 5%=0.65?)

Capacitors on the psu appear physically ok, i dont have an lcr meter or even capacitance test on my multi to properly test them though so they are not confirmed ok, just look ok.
There is a diode and R2K 5w power resistor in parallel (dm804, rm803) on the hot side of the board and the diode reads continuity in both directions using my cheap multimeter. Something that i thought was a bit odd, but i duno if its anything sinister at all. If other options turn up nothing then i might desolder them and test them out of circuit but for now its just a note.

At this stage I am still unsure if the tcon is the fault at all, only started looking there after reading threads about white/ghosting screens and because it seemed a good place to start for a display related fault. However this screen has no ghosting or double vision or photo negative etc, its just plain and even white across the screen. Menus do not display nor does anything else the only noticable thing is that the backlight slightly changes intensity, so presumably the menus etc are working but not visible. The tv funtions as i would expect apart from the white screen.

I scanned a decent high res image of the Tcon board and noted some measurements taken. If any gurus can make sense of some of this, that would be great.
I figure even if this turns out not to be my problem and this Tcon is fine and ok and my problem is with the panel or main board or cables etc, then the image and notes may serve as a useful reference for other people as a working Tcon board and the voltages that should be expected when testing.
I will put the original high res .PNG of the tcon board up ater too, but right now i am at a different computer.

So anyway I am still trying to determine/isolate which parts are bad, nothing is completely ruled out and i am open to suggestions.

Here's a pic of the Tcon board and the measured voltages at various points:

[DoubleDratt88]

Re: Samsung LA26R71BD - white screen with sound + tcon pic

A quick mention for the arduino!
Like i said i have no capacitance meter etc handy to test psu capacitors and another thing I would like to test is the signal output from the main board GPU to the tcon which requires frequency measurement. GPU is jsut my name for it, samsung call it a subaru svx-p88-mustang or something, but its basically the display processor.
I have a waveform image from the service manual for what should be expected at the outputs of a working gpu.
So basically I'm looking for a decent and easy to construct hardware wise arduino based solutions for both frequency measurement and capacitor testing which both directly relate to testing this tv.

If arduino-doogies can suggest a sketch and hardware setup that might be good for this please drop it in here!
Im looking at a simple transistor pre-amp frequency counter but have not tried it yet, and the capacitance meters are usually not much more than a couple of resistors with the cap to be tested so that's the sort of thing i am hoping for. eg no external ics etc apart from the arduino. The 2 caps that would be first to be tested from the 13v line i think were 1000uf 25v and 100uf ?v.

[DoubleDratt88]

Re: Samsung LA26R71BD - white screen with sound + tcon pic

Update:
I believe i have found the problem! (*although i am still not certain, if anyone could help and share their thoughts, that would be great)

A shorted transformer (TM 801s?)

To my surprise, the rest of the functions including backlight are all seemingly able to power up and operate via the 2 smaller of the 3 transformers on the board. Presumably the smallest 1 would be the standby circuit and I guess the other doing the work for the audio and backlight etc.
I made the mistake of assuming that because the backlight was working ok that the larger of the transformers right nearby would be driving it and all was ok, but i now think otherwise.

I took the psu board out and scanned it and you can actually see what i think is some interesting things.
First off, the heat caused by the transformer has made areas of the board change colour. It's not very visible with the naked eye although u can see some slight browning, but quite clear with the scan and you can see everything around and connected to the transformer cold side has clear discoloration (*note the similarly burned looking rail to IC5801S on the hot side with its legs tied).

Also, the boards measured voltages were all within tolerance (see below) even though 1 of the transformers wasn't doing anything.
The suspect transformer measures 0v when powered on the secondary side, when powered off there appears to be continuity across all of the secondary side. I dont like AC, so it took me a while to test but sure enough there is high voltage on the primary side and zipitty bop on the secondary.

So following the theory that the transformer has shorted the next question has to be why did it short.
Was it caused by its own heat over time etc? or caused by a shorted component on its output.
The parallel diode resistor thing still seems odd too, i would expect to see the resistor value with an open diode but not continuity in both directions with no mentionable resistance.

This is what i am still trying to work out, so that i dont go wasting time trying to replace the transformer only to have the same thing happen again when it gets plugged in.
The transformer has the text:
BORD32-PM
SC-03B-SC-0708D
CLOVER HITECHCO.,LTD

I am thinking about trying to feed in an adjustable 12-13v dc power supply to where the transformer secondary would usually send power just to see what happens. All i have though is a modded pc power supply with adjustable voltage, no fixed current etc but it does atleast have some inbuilt protection/self preservation if there is still a short present. Not sure if i would need to remove the transformer from the board before trying or if that would upset the other side of the circuit?

I would greatly appreciate any pointers as to where i should go from here, feeling my way through the dark so far but its gotta be the hard way to go about things at best.

And sorry! it's impossible to both keep it short and give all the details!
The arduino frequency counter and capacitance meter are easy to search info on, i just used the sketches and schematics already out there and it works but i haven't really had use to test them so far since this has gone another direction.

Be back with an update later.

Here's the backside and top of the PSU board:


also the images are currently hosted off site, my bad. I did it thinking that would be preferable but i will move them soon.

[budm]

Re: Samsung LA26R71BD - white screen with sound + tcon pic

At this point, your power supply is working since you are getting backlighst and sound.
The T-CON is getting the 5VDC to run it, but your pictures does not show the voltage reading at those 3-pin voltage regulators.
What voltages do you get at those DC output connectors to the MAIN board and the inverter board?

"A shorted transformer (TM 801s?)" the transformer will have very low DC resistance, you need to read it impedance/inductance, it appear as shorted when you try to read with the Ohm meter.

[DoubleDratt88]

Re: Samsung LA26R71BD - white screen with sound + tcon pic

Hi, thanks

I didn't include them earlier but i have in my notes some more measurements from the small caps just nearby to the regulators on the Tcon.
C56-5.18
C57-5.18
C58-3.21
C61-4.89
C63-2.44
C60-4.74
C98,99,102,103,104,105,77,79,80 - all 4.83

Sorry its not the actual points you requested, I will get those measurements for you asap. (the tcon and panel are off atm but i'll attach it and get back with results soon)

Im only going from memory for the outputs to inverter because i didnt make notes but think to the inverter all had 23.6 or similar and matched the service manual specs at the time i did it.
The main board to psu i have in my notes as:
13v-12.35
12v-11.76
5.4v-5.24

[DoubleDratt88]

Re: Samsung LA26R71BD - white screen with sound + tcon pic

a few more notes of other things i took and the same ones from the image in easier to read format:
Tcon:
VDA- 4.83 VREF- 4.74 VGL- 0.39 VGM- 2.06 VGH- 0.16 VGHP- 0
V5V- 5.17 V33V- 3.21 V25V-2.44
GM1,GM2- 3.21
GM3- 3.04
GM4,GM5- 2.92
GM6,GM7- 1.54
GM8- 1.40
GM9,GM10- 1.20

STPGM- 0.0v - 1kohm to ground
DET- 0.0v - 1kohm to ground
SCL- 3.23
VIN5V- 5.18
SDA- 3.23
GND- ok
DCK- 0v 1kohm to ground

[DoubleDratt88]

Re: Samsung LA26R71BD - white screen with sound + tcon pic

I hope i have here the measurements you were asking about, if i have made a mistake please feel free to correct me too
The 2 3pin regs measured:
U7-U8 (middle one):
0-3.21-5.16

U9-U10 (lower left):
0-2.44-4.95

I don't doubt you for a second and i ask this only to learn- but why do you think that the transformer measuring 0v on all its secondary side is ok?
I tried all possible combinations of its output pins as well as chassis ground but it reads to me to be deadasadodo?
Curious how the circuit was powering up, i poked about and concluded it was somehow being fed via the opto couplers or smaller transformer. I dunno what im talking about or what that means, but thats what i decided I guess what im asking is, where am i going wrong in my thinking with the transformer?

Heres some specs for the psu outputs I grabbed from the manual also:
13v = 12.7 +/- 7% tolerance
24v = 24.5 +/- 4%
5.4v= 5.4 +/- 5%
Vamp= 9.2 +/- 4%

and heres a link to the place i downloaded the service manual for this model (and also for the LA32, 37):


Will try and tidy this all up later, too much info spread out - sorry about that.

[budm]

Re: Samsung LA26R71BD - white screen with sound + tcon pic

"measuring 0v" What mode of the meter is set to? AC Volt? DC volt? Is it being measured at the out put windings before the rectifier or after the rectifiers?
By the way, the Gamma ref voltages looks bad. So it looks like some thing in the switching power supply on the inverter board is not working and or bad GAMMA IC.

[DoubleDratt88]

Re: Samsung LA26R71BD - white screen with sound + tcon pic

Oops! thanks. doi. I was going about things all wrong and had the wrong settings/location for what i was trying to measure.
The centre pin of the 3 rectifier diodes each reads dc12.5, 23.6, 23.6, so you are completely correct!
The transformer is ok - and i thank you for making it clear where i was going wrong.

Any further suggestions on how to follow up the gamma problem?
I just checked a couple of the Gxx voltages again quickly, and got completely different readings to before.
This time VGL is negative -5.45
GM2 +8.19
GM4 +7.42
GM7 +3.94
GM8 +3.55
GM9 +3.02 etc etc.

Are these the numbers that were off before when u mentioned gamma and are these readings any better?

Thanks for your help

[DoubleDratt88]

Re: Samsung LA26R71BD - white screen with sound + tcon pic

Overdue update-

Never did confirm if the lvds signal was working or not but probed with multimeter voltages on those leads were fluctuating atleast which could be a sign of the signal being present. If i could have scoped it i would have, atleast then i would have had a good idea the main board was ok and outputting the display signals but i never was quite certain and ended up taking a punt.

Found a cheap t-con and thought it was worth a shot to put it in last week.
The tv showed some immediate improvement and displayed some of the image being fed for the first time instead of just pure white, so that was a little bit exciting, but the image still had some wierd grey lines and wasn't refreshing like it should, just sortof had a still frame capture with lots of greyish mess around it that wasnt really changing like it should.

I tinkered with putting some pressure on the connections between the tcon and panel pcb and it did give some result, but again the image was no better it just seemed to have an effect of somesort.

It lasted about 5 mins like this with me checking all the connections etc i could and turning it on again etc - before fading slowly back to pure white, and now the screen is back to showing nothing other than white, and the new tcon is possibly damaged just like the old one.

I assume there is some problem with the panel or panel pcb that has shorted and damaged the t-con boards.

Since i have an idea of where a short might be coming from now i may dig it out another day just for fun to try and work out if theres anything shorted on the panel pcb and follow it back to see where on the tcon has been damaged and if they are repairable. Accessing that panel pcb takes almost an entire teardown though.

So, i've put it away in the corner and all but given up. Sadly I just have to chalk this one up as a loss for now. Truth is it was probably always beyond my know-how and never worth repairing anyway to most people being an "old" 720p 26" and huge bezel - but it was still worth giving it a go and learning a bit on the way.

Besides even if it ends up being recycled now atleast it will know that someone cared enough to "try" and save it..

If anyone local to me needs a power supply or inverter/backlight etc for one of these just shoot me a pm. Thats probably going to be all that's useful in it at the end of it all.

[DoubleDratt88]

Re: Samsung LA26R71BD - white screen with sound + tcon pic

After all but giving up I ended up finding another tv exactly the same model for 10 bucks that was not powering on - no stand-by led even. So had to grab it for the chance.

After a few quick checks the power supply was dud. Fuse was ok, primary side of both transformers had 127vac 50hz, but no output voltages or standby voltage that i saw, which made sense given the missing standby led. Whole secondary(cold) side of board had no voltages. Everything was only a quick test because i had the other power supply in my mind to use anyway.
So swapped the power supply with the one from the original tv and it all works great.

Have the new non-working power supply and other tcons and bits put aside to fiddle with another day. Having a fully working comparison will certainly be a handy aid to have for troubleshooting them. But, that is for another day and since my mum is very happy with her new tv I'm not going to go fiddling with it just yet. That and she already took it home with her so i would have to borrow it back.

So, a happy ending with a working tv -
Although it was the long way around and never solved the original issues.. Maybe there is another update on that in the future but for now it's all been put away again for future tinkering and parts.

Oh and a couple of notes on the differences in the 2 tvs:
The power supplies were rev 1.5 and 1.8. Most obviously the rev 1.8 had mostly red pin1 indicator stripes on the ribbon cables and the other rev 1.5 used mostly blue stripes.
The power supply board also has 2 alternative connectors for the inverter/back light and only 1 is used, the other left empty. Pin layouts are slightly different on each connection. The blue stripe r1.5 version tv used the lower connector when looking at the back of the tv with boards in place and the red stripe r1.8 version tv used the upper inverter connection. Possibly different inverter setups in each but in any case when replacing the power supply- using the same connector that the tv it was going into used originally worked for me even though the the tv the supply came from was using the different connector.

For troubleshooting the new non-working power supply ( BN96-03058A ) elektrotanya has a nice schematic which i downloaded for another day.
samsung_bn7997-1_bn96-03058a-rev1.3_bordeaux-26inch_inverter_sch.pdf

[BangBover]

Re: Samsung LA26R71BD - white screen with sound + tcon pic

The power supply board also has 2 alternative connectors for the inverter/back light and only 1 is used, the other left empty. Pin layouts are slightly different on each connection.

I got an LE26R71B (no backlight). It has same inverter/power board,
BN96-03058A and the 2 alternative inverter/backlight connectors marked as A and S are for different display panels which would have A or S marked on their label. BTW the panel has a manufacture date of 06/16, still 720p specs .