Samsung LNT3242HX/XAA T-Con and power supply issues

[dgatwood]

In January, I did a partial recap of my LNT3242HX/XAA because it would not power on. Several caps were swollen, and out of an abundance of caution I replaced every cap on the board that was made by that manufacturer (except possibly some tiny ones that are pretty unlikely to fail). A couple of weeks ago, the set started misbehaving, with the blackest parts of the picture bizarrely showing up as bright cyan. The picture is otherwise normal except for those cyan sparkles in black areas (or cyan over the entire screen if you unplug the HDMI cable).

Suspecting a bad T-Con board, I ordered a replacement. However, the replacement is much worse than the original; everything is bright pink. I'm assuming that the replacement board was simply bad, though the seller claims that all their parts are "tested". I'm not sure what I expected from someone who ships circuit boards wrapped in bubble wrap without an antistatic bag. (What the ****? I'm amazed it passed a signal at all!)

Just to be certain I wasn't missing something else, I checked the power supply voltages with my oscilloscope and noticed what strikes me as an unusual amount of ripple at 120 Hz, with some rails that seem out of tolerance:

• The 5.3V rail is actually 5.4V ± .2V (+5.6% / -1.9%)
• The 12V rail is actually 12.2V ± .2V (± 1.16%)
• The 13V rail is 12.6V ± .4V. (-6.2% / +0%)

Note that these voltages are measured in-circuit, at the input to the main board, so it is possible that the 13V rail is low because of loading. (What does this rail do?) But the 5.3V rail seems too high to me — not out of spec enough to fry anything, but enough to maybe cause subtle misbehavior like the blue sparkles, or cause the T-Con board to fail prematurely.

Is it typical for power supplies in these sets to be so badly out of spec? Am I right in suspecting that the giant film capacitor right after the full-wave bridge is failing to adequately filter the AC here? And is that 13V rail really under so much load that it sags by almost half a volt? Could any of these issues be causing the black noise problem, rather than the T-Con board? Or maybe a defective FWB?

Any other ideas?

[dgatwood]

Re: Samsung LNT3242HX/XAA T-Con and power supply issues

Oh, I forgot to mention that the picture looks normal for the first second or two. And after leaving it unplugged, it looks normal for as much as ten or fifteen seconds before the rising blue wipes out all deep blacks. This makes me wonder if the failure could be thermal in nature, like a bad solder joint somewhere, but I have no idea where to begin looking, and SMT soldering is really not my thing. :-D

[diif]

Re: Samsung LNT3242HX/XAA T-Con and power supply issues

Small caps have just as much chance of failure as large ones, if some were swollen they should all have been changed.

Sounds like a gamma issue. A chip (AS15) that goes faulty on the tcon. There are usually test points to measure and these identify if the chip is at fault.

[neilc6]

Re: Samsung LNT3242HX/XAA T-Con and power supply issues

I have received a few bad used tcons that claim to be tested with definite failure of the AS15 chip. Seller claimed possible shipping damage. I had one seller bubble wrap a tcon and put in a plastic bag mailer which got bent during shipping.

[dgatwood]

Small caps have just as much chance of failure as large ones, if some were swollen they should all have been changed.

None of the small caps look swollen. Of course, that doesn't mean they aren't bad, necessarily.

[diif]

Re: Samsung LNT3242HX/XAA T-Con and power supply issues

None of the small caps look swollen. Of course, that doesn't mean they aren't bad, necessarily.

Small caps don't usually swell due to their small size. If larger caps had swollen then then small ones should have been replaced also.

[dgatwood]

Re: Samsung LNT3242HX/XAA T-Con and power supply issues

Well, all of the obviously failed caps were made by a single manufacturer that I had never heard of, which strongly suggests that these failed because of a bad electrolyte formula (capacitor plague), rather than because somebody at Samsung under-specified the components. They're from the right era for capacitor plague.

As for not swelling, that's true, but usually if small caps fail because of hydrogen pressure, they blow the vent and spew electrolyte everywhere, or else they blow the bottom cap and spew electrolyte everywhere. Either way, it is the venting that results in the capacitance collapse, so no venting *should* mean no failure unless they experienced a dielectric failure because of a massive overvoltage condition or something similarly unexpected.

*shrugs*

[R_J]

Re: Samsung LNT3242HX/XAA T-Con and power supply issues

There is not a lot of electrolyte in those small caps to usually leak or bulge, they just dry up. And they go bad often, and people overlook them, because they "look ok"

[budm]

Re: Samsung LNT3242HX/XAA T-Con and power supply issues

there is not a lot of electrolyte in those small caps to usually leak or bulge, they just dry up. And they go bad often, and people overlook them, because they "look ok"

+10