Cynascan DS551LT4 backlights working but no video or menu

[freakaftr8]

Re: Cynascan DS551LT4 backlights working but no video or menu

I would say that you have a shorted LCD panel on that side. Sounds like your classic symptom that's failed LCD gate driver.

If you can access the panel boards start running down-the-line checking capacitors for shorts.

[dicky96]

Re: Cynascan DS551LT4 backlights working but no video or menu

OK I have got a little further with this.

I got to the panel PCBs

I could see short circuits across all of the larger and and most of the medium sized capacitors on the panel board that was dragging down VHDD and VCOM. I removed the first of the larger capacitors and this made no difference to the short. I measured the capacitor our of interest and it reads about 11 uF.

I then noticed on the PCB this capacitor is marked as DUM6 (I also have DUM2 to DUM5 which also read short circuit) but this is because, looking with my jewelers loop, both ends of these capacitors go to the ground plane on the PCB!! I'm guessing DUMx stands for dummy? but wtf is this all about other than wasting capacitors????

On a similar topic what are these funny little metal bridges with a rubbery thing inside GA205 etc (see pics) they don't seem to do anything useful either

Anyway I then removed the first of the medium sized capacitors C204 and the short went away. This was confirmed by measuring the capacitor out of circuit, it is short.

I now have VHDD 8.25V and VCOM 7.10V with both TCON cables attached

YEsss! I thought.. I left the capacitor out of circuit just for now (I figure it is probably just a decoupling cap) to see what the screen does now. I didn't bother to clean the flux just yet either as you will see on the pic, I'm not being unprofessional I wanted to see if I fixed it... but all is still not good.

The fault has changed at least, and I now have vertical lines down both sides of the screen instead of just a dark screen with backlights only. If I remove the TCON ribbons in turn then that side of the screen goes blank white and the other has the vertical lines.

I still do not have any image. On the good working monitor they have a blue 'no signal' box on screen with no video input when there is no input so I expected to see that at least.

One of the flex cables has a little bit of damage by the way, and using my jewelers loop I can see one track inside it is broken, but this track goes to one of the pins on the panel PCB that has no connection so I don't think that is causing a problem

At least I am getting somewhere thanks to you guys, but what next?

Rich

[freakaftr8]

Re: Cynascan DS551LT4 backlights working but no video or menu

The more I get into this the more I understand that these drivers are Daisy chained serial. I would replace that missing capacitor with a good substitute. If the signal is lost in transition with the one that's missing it may not have the feedback circuit to carry the information on to the other drivers. This is just my theory of course. Other knowledgeable people might be able to chime in.

[dicky96]

Re: Cynascan DS551LT4 backlights working but no video or menu

Yeah no worries I can do that. If I pull one of the other ones I can measure the value. That doesn't really explain to me why I have nothing but vertical stripes on both sides of the screen though - and plain white with the relevant TCON disconnected

Rich

[freakaftr8]

Re: Cynascan DS551LT4 backlights working but no video or menu

Do the panel driver boards have ICs on one side only? These are attached to the panel circuit board on one side only. If so, daisy chained info needs to go from one to be passed to another, and also I believe the capacitor closes the circuit it's driving as a whole..

[dicky96]

Re: Cynascan DS551LT4 backlights working but no video or menu

No ICs on the panel boards just a few capacitors etc

[budm]

Re: Cynascan DS551LT4 backlights working but no video or menu

https://www.badcaps.net/forum/attach...5&d=1513535647
There is embedded IC on each one of those flexible brown cable between the tab bonding and the LCD glass panel, they do go bad because they run hot.

[dicky96]

Re: Cynascan DS551LT4 backlights working but no video or menu

Yes Bud I am well aware that there are chips on the flex cables. But it seems strange that they would all fail and I would not get some image on some part of the screen

On your advice I previously checked that there was data on the LVDS cable and most of the twisted pairs had some data. Now I fixed the short on the panel PCBs and have VHDD and VCOM how can I determine if all those vertical lines (and no image) on the screen are due to a faulty TCON board or some other reason?

BTW I honestly do not understand why this Dynascan has more than one fault. I thought fixing the short would have sorted it :-( However the damn thing was about 6000 euros new so the owner would like it fixed if possible

Rich

[freakaftr8]

Re: Cynascan DS551LT4 backlights working but no video or menu

The only real way of knowing is swapping the tcon. You said you have another one of these tv's? Also scope ready t's between the two.

[dicky96]

Re: Cynascan DS551LT4 backlights working but no video or menu

Unfortunately the other one is a different model DS55 LT6 and it is in daily use

OK thanks, I will have a look though if I can see signals coming out of the TCON board - I know there are signals getting in to it on the LVDS though of course I don't know the integrity of the data

[dicky96]

Re: Cynascan DS551LT4 backlights working but no video or menu

I spoke with the nice guys at Dynascan and they are sending me a TCON board early in the new year. Will let you know if that fixes it.

Rich