LG 50PG1000. Bad YSUS? Strange image behaviour

[Jurki]

Re: LG 50PG1000. Bad YSUS? Strange image behaviour

You are an Lucky guy !

I sold mine with that "small" failure for 130 eur to restaurant.

[jbfr_54]

Re: LG 50PG1000. Bad YSUS? Strange image behaviour

Hi All,

Replacing these Buffer IC's is a nightmare specially, (as I do), if you use conventional soldering tools.

You have to work with patience and very carefully, using a correct magnifying glass.

Before starting, you should consider the following :

1) There are several Y-buffer IC's vertically arranged on board(s), each IC driving a corresponding horizontal image stripe.
2) The first driven IC is usually the top one, each following IC being driven by its upper neighbor.
3) The input IC pads are usually located at the center of on one side of the IC oriented towards the connector(s) going to the Y-sustain board. All other pads located on the four sides are outputs going to the panel connectors (easy to check following the PCB tracks).

Identifying defective IC's :
There should not be any short between pins of the panel connectors (corresponding to outputs of the buffer IC's) and all common pins of the input connector of the buffer board.
Use an ohmmeter to check. An analog (needle type) multimeter is easier to use.
If one ore more pad show a short, the corresponding IC is bad.
If a resistance value is displayed, try to invert probe wires, value should change.

Removing silicon glue above the IC pads :
It is essential to remove as much as you can this glue before starting to desolder. You can use a special silicon removal paste (plumbing toolpaste).
Clean carefully with isopropyl alcohol after process done.

Desoldering :
1) Desoldering the IC's is quite impossible without hot-air because of the center thermal pad located under the IC. I personallly use a hot gun (paint stripper) through a metal cache to avoid small other components to melt or desolder. You can use solder flux to ease solder melting.
2) NEVER APPLY STRONG FORCE WHEN REMOVING THE IC FROM THE PCB TO AVOID DESTROYING PCB TRACKS. REMOVING MUST BE VERY EASY IF IC IS HOT ENOUGH.
3) Clean very carefully IC pads on PCB removing as much as ou can solder residue in order to have a flat surface under the IC. Use solder flux and desoldering wire wetted with flux. Try to avoid sliding the desoldering wire perpendicularily to the pad tracks, but it is OK in the direction of each pad track.
4) Check that no other component has been unsoldered when the hot-air was applied. If so put them back on place.

Soldering new IC :
Repeat the following steps process for each defective IC one by one, starting from the one immediatly below the last correct image stripe (or the one at top).
1) Make sure all pads on the PCB are clean and without unnecessary solder left.
2) Make sure IC pads are not bent and that IC pads are having the same height from IC bottom surface in order to correctly lie on PCB tracks.
3) Place the IC in position on the PCB, checking the four sides for PAD to TRACK correct placement. Maintain it with one finger
4) Put solder flux over the IC pads and a bit of solder on the iron end.
5) Apply iron on a corner pad of the IC to solder it, check correct IC placement on four edges and proceed with an opposite corner pad.
6) CHECK VERY CAREFULLY IC PLACEMENT OVER PCB TRACKS then proceed with the two other corner pads. Check that pads are soldered and isolated from neighborhoud.
7) Pour solder flux again above pins and apply soldering iron and solder all over the pads. This must result in shorts between pads but allow correct soldering of the pads to the PCB tracks.
8) Use desoldering wire wetted with flux to remove unnecessary solder between the IC pads and PCB tracks. ALWAYS SLIDE THE DESOLDERING WIRE FROM CENTER OF IC TO OUTSIDE (in the direction of the pad) TO AVOID BENDING PADS OR MAKING SHORTS.
9) Inspect carefully the pads one by one from being shorted to next ones or not soldered. IT IS VERY IMPORTANT TO MAKE SURE INPUT/CONTROL PINS ARE CORRECLY SOLDERED TO THEIR TRACKS AND CORRECLY ISOLATED FROM EACH OTHER. Using a jeweller's magnyfier is recommended.
10) Clean solder flux thoroughly as much as you can with isopropyl alcohol.

Soldering the thermal pad :
This pad is located under the IC and must be in good contact with a large PCB pad for thermal dissipation purpose. The PCB is usually having holes at the pads location. I use this to solder the thermal pad from the solder side of the PCB by doing the following :
1) Pour flux in the holes
2) Using a very hot iron, pour solder in the holes and keep it melted several seconds to allow solder to reach and attach to the pad.
3) Clean as much as you can solder flux.

Checking the result :

1) Pads soldering can be verified using an ohmmeter between each IC output pin and the corresponding panel connector. Each IC output pad must be connected (zero ohm) to ONE and ONLY ONE pin of the connector.
2) After this final check and when you are sure your input pins are safely connected, you can put your buffer board back to the TV and verify the result as following :
3) Assuming the image above the section driven by your IC is good, An image stripe corresponding to your IC position should be displayed.
-If all black, you input pins are not connected.
-If correct, you are lucky, ready to proceed to the next IC.
-If the image is all fuzzy, check your input pins.
-If image can be identified but is not all correct, consider the following :
>Black lines correspond to unsoldered pads.
>Uncorrectly displayed set of consecutive lines (mixing colors/content) indicates shorts between IC pads.
4) Unmount the buffer board and use solder flux and desoldering wire to achieve correct pad's soldering, try again.

The above figure shows opened and shorted pins.
Working on the IC solders and repeatedly checking this way should allow you to get a good result.

Proceed this way for each IC to be replaced.



Finishing your work :

After you are having a good image all over the screen, you should keep your TV under test for several hours to make sure of the following :
1) Your replaced IC's do not heat more than original ones and are all at the same temperature (you finger can stay on it but be carefull about the high voltage on pads when chenking this).
2) No defect appear on image due to continuous thermal conditions applied on poorly soldered pads.

Then, when everything looks OK, clean very efficiently the PCB and the IC's with alcohol and apply Silicon glue (I use plumbing silicon joint paste) all over the pins and pads of the IC and PCB to avoid dust and humidity to reach the pads in the future. This prevents future shorts or arcs between pads.
Let it dry for several hours before appliying power again for fianl vérification.


Sorry for the length of this post, I hope my english is clear enough. It is the result of my successfull work on several boards.

Just hope it will help.

I keep ready to answer any question.

Regards. Jean

[electronicsurplus]

Re: LG 50PG1000. Bad YSUS? Strange image behaviour

Looks like you have a Shorted Panel