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Old 02-09-2020, 10:54 PM   #21
momaka
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Default Re: Viewsonic vx2252mh

Hi SolomonMan,

White screen issue typically means either LVDS connection problem or T-con board power supply problem. Since you already more or less eliminated the LVDS connector as the problem, that leave the T-con board power supplies. So let's get you on the right foot with some basics on that.

The T-con board generates several voltages (i.e. contains internal power supplies) that are used for driving the TFT, along with powering the drivers and LVDS transcoder IC.

The first/main voltage is typically a 3.3V to 5V supply from the main board (sometimes called "Vcc", "Vdd", "3V3", or "5V1" on test points - the notation varies between monitors and is not consistent.) This is what connects to your "F1" fuse and powers everything else on the T-con.

The second voltage is generated (through a boost regulator) from the first/main voltage above and is typically in the range of 10 to 15V. Most T-con boards call it "AVDD", but again, the notation may vary. You will find this rail usually near a round inductor and a large diode.

The 3rd and 4th voltages are generated from "AVDD" mentioned above via charge-pump method. Many T-con board often refer to them as "VGH" and "VGL" (though sometimes also "VG_ON" and "VG_OFF".) VGH is typically +15 to +32V depending on the monitor, and is what is used to turn "On" the transistors in the TFT panel. VGL is a negative rail usually in the range of -5 to -10V. It is used for driving transistors Off in the TFT panel.

Apart from the 4 voltages / power supplies on the T-con mentioned above, there may be a few more for various other functions. Usually, the "big chip" on the T-con is what decodes the LVDS signals from the main board, and this chip may require 1.8V, 2.5V, or 3.3V (or a combination of these). So depending on its requirements, you may see up to 3 more dedicated voltage regulators (typically linear ones, like 1117 or 1084) near that chip.

Worth noting here, at least from my experience, is that these (usually linear) supplies rarely fail. The ones that fail most often are AVDD, followed by VGH and VGL. In fact, if AVDD fails, VGH and VGL cannot be generated. The cause of the failure for these supplies is usually "shorted" ceramic capacitor(s). And by shorted, I've had some ceramic caps show as much as 30-40 Ohms. Either way, when something goes wrong with these supplies, the fuse almost always blows.

So with all of that said, see if you can find any test points on your T-con that mention names similar to what I noted above. If you can, set your multimeter to the lowest resistance scale (200 Ohms) and measure between those test points and a ground. Make sure the T-con is well grounded before this, so we get accurate measurements. After that, post what results you get here. (I'm willing to bet there is a shorted/bad ceramic cap on the AVDD rail.)

If you can't find the above test points, please post another picture with more detail around that "AUO" chip that is seen on this picture:
https://www.badcaps.net/forum/attach...1&d=1581047773

I would also like to request a more detailed picture from the area around the fuse / LVDS cable.

An alternative way to finding if the issue is caused by a shorted cap is to first measure resistance across all of the larger ceramic caps. Any that show less than 100 Ohms, note/mark them down, as one of them could be bad. (You will find that many of these larger ceramic caps are in parallel, so if one reads a certain resistance, so will any other caps in parallel with it. This means if you stumble across a ceramic cap that appears to be shorted, it may actually be another ceramic cap on the board elsewhere that is making the one you are currently measuring as showing shorted. So just keep that in mind when doing measurements.)

Quote:
Originally Posted by SolomonMan View Post
I have seen a couple that I am not familiar with...U, B, O, and I believe a T somewhere on the boards?

U - looks to be ICs

Is there a standard Legend on these identifiers?
"U" prefix is indeed used for ICs (through some manufacturers will also use "IC" instead.)

"B" is typically used to mean "Ferrite Bead... or simply, an inductor with a very tiny inductance.

"O".... not sure I have seen that before anywhere. Perhaps you mean "Q"? In that case, this would indicate a transistor (BJT, MOSFET, IGBT, and etc.)

"T" is used for transformer.

"L" is inductor.

Last edited by momaka; 02-09-2020 at 11:00 PM..
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Old 02-10-2020, 08:57 AM   #22
SolomonMan
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Default Re: Viewsonic vx2252mh

momaka,
Thanks for the help!

I have included the scans a little higher resolution than the last...I did notice my solder job on the capacitors, that loosened themselves off the board when I tried to enable the fuse again, is horrible and needs to removed and started over. Also looks like the one capacitor is in very bad shape (Funny I did not see any of that till I upped my scan resolution) - another thing learned. Also the before image C11, if you zoom in on it, looks like their is a lighter dot on the capacitor?

I included a before picture and the current picture...**Shakes head**

Is there a way to know the capacitor specs to purchase/locate another?
Not to mention the fuse size?

I do have a Hot Air/Solder station...I was looking for my solder paste...but probably going to have to purchase another...I do have some other TCON boards from TVs I recently cleared out of my garage/Office...Crack Panels...maybe I can source/practice from/on those?

After looking things over...I will not attempt the repair until I feel confident on the other junk TCON boards on a R&R (Remove and Replace) situation.

I have included the image of the front of the TCON board. With your previous description I did locate the Main,Second,Third and fourth Voltages (circled them with marker)...their names and ohm readings are below I am fairly certain the measurements included the bypassed fuse (good - 1ohm resistor);

5V1 - (O-L) - Over Load
AVDD1 - 26.5 Ohms
VGH1 - O-L
VGL1 - O-L

On the measurement (alternate method) of the Ceramic caps...Are these when they are in circuit?

Please describe this process with some more detail...I am still learning...

Again thanks for the help,
Chris
Attached Images
File Type: jpg 20200210_081825.jpg (825.7 KB, 3 views)
File Type: jpg 20200210_082209.jpg (1.04 MB, 3 views)
File Type: jpg Before.jpg (615.7 KB, 5 views)

Last edited by SolomonMan; 02-10-2020 at 09:00 AM..
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Old 02-10-2020, 09:02 AM   #23
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Default Re: Viewsonic vx2252mh

Sorry,
the After Pics of the Left of TCON did not come thru...

Thanks
Chris
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File Type: jpg 20200210_082303.jpg (1.06 MB, 6 views)
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Old 02-10-2020, 07:19 PM   #24
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Default Re: Viewsonic vx2252mh

Quote:
Originally Posted by SolomonMan View Post
I have included the scans a little higher resolution than the last...
Thank you! That actually helped quite a bit.

Quote:
Originally Posted by SolomonMan View Post
Also looks like the one capacitor is in very bad shape (Funny I did not see any of that till I upped my scan resolution) - another thing learned. Also the before image C11, if you zoom in on it, looks like their is a lighter dot on the capacitor?
Yup, I see it too. Can't tell if it's just a spec of dirt/dust or a chipped off piece... or is it a bubble? If the latter, that ceramic cap could be bad/shorted.

Quote:
Originally Posted by SolomonMan View Post
Is there a way to know the capacitor specs to purchase/locate another?
That's one question I never quite managed to answer fully.
When I first encountered this issue on an LCD monitor, I just desoldered some ceramic caps from a dead video card and the monitor worked. But obviously, that's not the most proper way.

With that said, since most of these larger ceramic caps filter AVDD, VGH, and VGL voltages, I would say get ceramic caps rated for at least 25V. To be safe, it might be even a good idea to go with 35V-rated ones.

Capacity-wise, just measure the physical size of the old caps (probably easier to do in mm) and see what is the largest capacity caps you can get in that size that are also rated for 25-35V. I suspect you will find those to be in the range of 1 to 4.7 uF. Maybe 10 uF at most.

If you do have T-con boards from other dead TVs/monitors, then it should be fairly safe to re-use the ceramic caps from those, as all t-con boards tend to have similar voltages.

Quote:
Originally Posted by SolomonMan View Post
Not to mention the fuse size?
The fuse size is not that important, but the current rating is. Was there a letter or number printed on the old fuse? Most t-con board usually have 2 to 3 Amp fuses on them, rated for 16-35V. So if bad comes to worse and the old fuse isn't marked, get several fuses: maybe one or two rated for 2 Amps, one or two for 2.5 Amps, and same for 3 Amps. Start with the smallest fuse rating (2A) and go up / keep replacing if you end up blowing the fuses - that is, >after< we discover the cause of the fuse blowing up.

As for physical size of the fuse... just look for SMD fuses that have same or similar physical size as the one currently in there. That should make it easier to solder.

Quote:
Originally Posted by SolomonMan View Post
I do have a Hot Air/Solder station...I was looking for my solder paste...but probably going to have to purchase another...I do have some other TCON boards from TVs I recently cleared out of my garage/Office...Crack Panels...maybe I can source/practice from/on those?
Yup, feel free to practice on those board or get parts from them, if they don't have much value to you for anything else.

That said, you don't actually need a hot air station. In fact, for a job like this, I highly prefer a soldering iron - one with a chisel or bevel tip that is at least 3 mm wide. The easiest way to remove these ceramic caps is to put flux on them, then add a blob of fresh solder to the tip of the iron and touch the ceramic cap on both sides at the same time. Heat for a few seconds and the ceramic cap should give away and stick to the soldering iron's tip.

For soldering the ceramic caps back on (or any SMD component for that matter):
1) clean pads on the PCB with solder wick or apply flux on them and heat/clean with a clean/wiped soldering iron tip (i.e. no solder applied to it.)
2) put flux on pads again and place SMD component / ceramic cap on top.
3) Hold component with tweezers and heat the pad and component lead on one side.
4) repeat the same for the other side of the component (or until all leads are soldered, in case of multi-pin SMD components).
5) For "2-legged" components, heat the first component lead/side again. If you've successfully soldered one side, the component will not move when you touch it with the iron again.

Quote:
Originally Posted by SolomonMan View Post
I have included the image of the front of the TCON board. With your previous description I did locate the Main,Second,Third and fourth Voltages (circled them with marker)...their names and ohm readings are below I am fairly certain the measurements included the bypassed fuse (good - 1ohm resistor);

5V1 - (O-L) - Over Load
AVDD1 - 26.5 Ohms
VGH1 - O-L
VGL1 - O-L
AVDD1 is "shorted" to ground. It's not a proper short-circuit, actually... but for this circuit, it practically is. Since 5V1 does not read a low resistance, that means the forward diode (D6, I think) after the boost inductor is OK.

More than likely, you have a shorted ceramic cap on the AVDD1 rail somewhere. Measure resistance on all ceramic caps near AVDD1 test point, and see which ones also show that 26.5 Ohm resistance. Chances are, you will see at least several of these. It only takes one shorted cap for all of them to appear bad. But more than likely, it will be just ONE shorted ceramic cap somewhere. From what I can trace, it looks like C9, C10, C11, C12, and C87 should all be connected to AVDD, so check those first. C27 and C28 appear to be connected to either 5V1 or perhaps V33_1 (you can confirm this by measuring resistance: one side on those caps should measure less than 1-2 Ohms to ground... or as low as your multimeter will measure for shorted leads... and the other side should measure 1-2 Ohms to either 5V1 or V33_1, if these caps are connected to one of these rails.)

Once you find all of the caps that are connected to AVDD1 and appear to be "shorted"... unfortunately, there is no easy way to tell which one of these is the culprit (it will likely only be one!) Sometimes, you might get lucky and the ceramic cap might have darkened the PCB around it. But don't count on it. Generally, the only 100% safe method I know is to remove each cap from the board that measures the same 26.5 Ohms "short-circuit" above, test its resistance off the board, and if it doesn't show short-circuit any more, move on to the next cap on the board until that AVDD rail no longer measures low resistance anymore. If it still does after removing all ceramic caps that measure the same resistance, then the issue might be with another component connected to the AVDD rail - but I haven't found that to be the case yet. The driver ICs used for generating the AVDD rail are usually pretty robust and won't easily blow up or kill the boost driver transistor(s). On that note, your AVDD boost transistor appears to be Q3 in section "B"... close to that L2 round inductor.

Quote:
Originally Posted by SolomonMan View Post
On the measurement (alternate method) of the Ceramic caps...Are these when they are in circuit?
Yes.

Quote:
Originally Posted by SolomonMan View Post
Please describe this process with some more detail...I am still learning...
You measure all ceramic caps while in the circuit and note any that have a very low resistance (that is, you do this only if you can't find any issue with the 4 generated rails mentioned above - which we did with AVDD.) Then you can remove any caps that measure very low resistance from the board and see if they show the same low resistance off of it too. If not, then that ceramic cap is probably good and you can move onto the next one.

I would say anything below 30 Ohms is highly suspicious, and anything under 100 Ohms is somewhat suspicious. These aren't hard numbers, though. And worth noting is that these apply to t-con measurements only.

Now, if you are working on laptop or desktop motherboards, or video cards, that's a different story. Particularly on video cards, the GPU chip usually has a very low resistance - sometimes as low as 1 Ohm. Any ceramic caps connected to that (and believe me, there will be many many) will also show the same 1 Ohm resistance across, making one conclude that a ceramic cap is shorted. But usually, that is not the case, and the low resistance across those caps is from the GPU chip. So keep that in mind if you service other PC components.

Last edited by momaka; 02-10-2020 at 07:42 PM..
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Old 02-11-2020, 01:36 PM   #25
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Default Re: Viewsonic vx2252mh

momaka,
Thanks so much for the information....I will practice a little bit this weekend (getting over the flu) and then try and get those caps replaced.

I will let you know what I find/end up.

Thanks
Chris
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Old 02-11-2020, 07:04 PM   #26
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Default Re: Viewsonic vx2252mh

Sounds good. Hope you feel better soon, too.
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