Sharp LL- T17A4-B lcd Monitor

So far, no luck. ESRd all caps on video board. All good. Bad news is that monitor rarely straightens up anymore. I got it to display correctly only once this evening, so no chance of trying to reproduce symptom via freezing components. Power supply voltages appear nominal and clean. No effect from flexing components. Reseated all cables on the video board.....

It looks like the horizonal rows are not being addressed, as the current display is a series of fine vertical lines of whatever color is predominant in that area of the display. Still some strange horizontal scrolling effects when first powered, but they only last a few seconds...two or three horizontal bars scroll down the screen, then disappear.

Any help here? Is this toast [beyond my ability to fix...]? My scope (20 mHz) is not fast enough to look at the video timing....

jak

Has this one gotten everyone as stumped as it has me?

After three days of occasional attention, the symptoms still present as before. I occasionally see the beginnings of a legible image, with bits of recognizable video scrolling down the screen. Usually these break back up into a screen full of vertical lines again; but occasionally I'll get an 'almost' image first.

Some help, even if it's just commiseration, would be appreciated. So far, not a single reply.....

jak

jakdedert wrote in news:ipT4k.4859$3F5.2098 @bignews2.bellsouth.net:

You have my sympathy. What makes you think that the video signals should be much higher than 20 MHz?

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With a fixed video input signal, you should be able to figure out what signals should be present on the leads to the LCD. A giant X painted on a board and captured with a camcorder or regular point and shoot camera connected to the video input of the monitor whould give you a starting point. Do you have a schematic?

That's an interesting concept. No, I don't have a schematic; but pulled up another Sharp lcd service manual on the web hoping it was similar.

No such luck, but some of the diagnostic procedures spec'd a higher bandwidth scope than mine...which I use mostly for audio bench work.

Thanks,

jak

On Thu, 12 Jun 2008 12:04:02 -0500, jakdedert put finger to keyboard and composed:

FWIW, that monitor came with a 3-year warranty.

If that is not an option, then I'd be hooking up your scope to the horizontal and vertical sync inputs and following them through. It may help if you told us which chips are used, eg Genesis Microchip.

Have you considered that the video source may be just outside the LCD monitor's range? I sometimes find that a 75Hz refresh rate is right at the limit before the monitor blacks out. Try a safe 60Hz refresh rate.

What about the OSD? Is it stable? Pull the RGB cable and watch the "No Signal" OSD window, or whatever your monitor reports.

- Franc Zabkar

Thanks Frank, for your attention.

It contains a (socketed) Winbond W78E56P-40. I pulled up the data sheet for that, but it's beyond my experience to decipher how this generic processor is employed in the circuit. The other large scale (100 or so leads) chip has a logo that looks like a backward capital 'N' with numbers MST9111. Google drew a blank on that one.

Changing video resolution makes no difference (refresh was initially 60 Hz anyway). The OSD, when accessed while 'in symptoms', is just a blue smear up the entire height of the display. Any video input to the display is likewise; all columns and no rows. Each column of pixels is uniform in color from top to bottom, although that color changes with the video content. Occasionally horizontal bands will appear, either a lighter or darker shade...sometimes only a pixel wide, other times wider.

(I wonder if I could record and post a short video somewhere?)

For the last day or so, I've not been able to get any sort of legible display. I hooked it up to a spare computer, on screensaver, next to my desk, hoping to catch it actually working...turn it on occasionally for short periods.

You might be able to tell that I'm not too familiar with the innards of lcd displays . This board appears to have several layers, making visual tracing of the circuitry difficult, although I should be able to trace continuity from the input pins to the various sm devices.

I'm afraid I'll be almost totally occupied with other business for the rest of the week; but I'll eagerly check back here for any insight into this....

Thanks again,

jak

On Tue, 17 Jun 2008 00:38:41 -0500, jakdedert put finger to keyboard and composed:

That's the microcontroller and firmware:

It's a single-chip LCD Controller for SXGA LCD Monitors by MStar Semiconductor:

The PDF contains a functional block diagram of the chip but no details. :-(

There are no datasheets on the MStar web site, either, only an email address:

FWIW, the MST chip appears to be the one that interfaces to your DB-15 connector. The OSD appears to be generated within it as well. I'd be looking around there. Maybe check its supply rails, caps, crystal?

I'm not sure what would happen if the host microcontroller (Winbond) were to fail. If the monitor still responds to the front controls, then I would think that the Winbond chip is working, or at least not totally brain dead. I wonder if you would still get anything on your screen (eg an error OSD) if you were to remove the Winbond chip. Might be worth trying ...

I would compare the OSD in the presence of H&V sync input with the OSD in the absence of an input. In the latter case the MStar chip would have to rely solely on the crystal for its reference, whereas in the former case it would be generating a pixel clock by using its internal PLL to multiply the H sync frequency.

Youtube?

Ditto. Maybe we can both learn something. :-)

- Franc Zabkar

Thanks again, Frank. As detailed, I've no time for bench work this week, but will mull over the above while mapping out my next steps. Thinking it over, it seems obvious that the lcd is getting no horizontal information. I need to study how that's normally supplied to an lcd monitor, then look to where it may have disappeared. Unfortunately there's little generic lcd troubleshooting information on the web. It seems like usually the power supply fails. Most lcd info I've accessed is either pretty dense engineering data...or specific fixes for various models, which usually involves replacing one or more caps in the smps.

In fact, I've repaired a couple myself with that very problem. I was hoping this would be one....

Perhaps we'll write that chapter in the repairfaqs. ;-) OTOH, I have little interest in pioneering...mostly want to get my monitor working so that I can send one off to college with my daughter.

jak