Samsung

[Dannyx]

*epic fail in the title there - was a in great hurry and forgot to type the model in* Woops, I'm sowwy It's a Samsung PS- 42c7HX plasma TV.

Good day folks. Well what do you know - it seems like it's time for me to tackle a TV again at last >_> It's one of my all-time favourites - a plasma >_> It's like getting a turd as a welcome-back gift

Simple issue, difficult to pinpoint: has sound but no picture. Display stays completely blank, not even a flash or a speckle on it. I went through the usual routine of checking the power rails first and amazingly (or should I say, unfortunately) they all check out OK, both the high voltage ones like Vs and Ve and the low ones like 5.3v and all that.
Y-sus board, again, amazingly, has no shorts on it, which is probably the death of 90% of the PDPs I've encountered.

I concluded that it's most likely a bad "logic board" as they call it in the manual. This confused me a bit at first, because I kept mistaking it for the "MAIN board" which has the main CPU, FRC and audio amp on it. I'm more used to this "logic board" being called a "T-con board", but here they call it "logic board". There's 3 LEDs along the bottom of this board. When the set is first powered on, all 3 light up steady. After a while (presumably around the same time a picture should come on), the left one goes out, the middle one flashes and the right one remains on. The manual isn't very specific about these LEDs, and it only talks generically about "AN" LED, not individual ones meaning different things. They say it's indeed supposed to be flashing, but offer no info regarding other states of these LEDs and what defect they point to.
Being fairly inexpensive, I ordered a replacement, so fingers crossed it actually works. If not, this could be a bad "MAIN board" after all.

Sucks not having a scope. At least that would allow me to measure between the Y-sus and this logic board to see if there's data being passed around...

What do you guys think ? This could be virtually everything, aside from the power supply: the Y sus./buffer, the X-main, the main board...virtually anything.

[Andrew F. Ali]

Re: Samsung

Basic PLASMA troubleshooting would suggest a defective Logic Bd. Verify for me the Y-Main Bd. is supplying the Logic Bd. with 16Vdc which is usually derive from the M5V source.

[Dannyx]

Re: Samsung PS-42C7HX plasma. No image, sound only.

Verify for me the Y-Main Bd. is supplying the Logic Bd. with 16Vdc which is usually derive from the M5V source.

Wait, so is 15v coming BACK into the Logic bd. FROM the Y-sus ? I know 15v was being generated from 5v on the Y-sus with a booster, but I never thought it actually LEAVES Y-sus. I thought it was for use somewhere on Y-sus itself.

Yes, the fuse that's near the connector is fine and reads 15v or thereabouts. There's no schematic for that Logic Bd. unfortunately and it's tucked under the metal case for the MAIN Board, so I can't measure anything on it, at least not easily and I didn't want to do all sorts of stunts to somehow hoist up the main board OVER the logic one somehow if I'll be replacing the logic anyway...

EDIT: here's the service manual, since I couldn't upload it.

[R_J]

Re: Samsung

I think this logic/control board has auto gen, by shorting pins 3&4 of cn2072 and plug in the tv, The plug going to the main board needs to be unpluged also.

[Dannyx]

Re: Samsung

Yes, the manual talks about an all-white picture showing up if you short those jumpers but offers no additional info as to what conditions it can be done in. Can I omit the main board entirely and leave just the T-con for the test ? Guess I've got nothing to lose by trying it out.
Interestingly, the T-con ("logic") board ITSELF has "PS_ON" and "VS_ON" pins, so it is apparently capable of commanding the set by itself, which would make such a test plausible.

[Dannyx]

Re: Samsung

Ok, replacement logic board (T-con) arrived today but sadly it didn't change anything: same problem as before. No picture at all.

Couple of things I tried:

-leaving just the t-con board in place with a jumper cap on pins 3-4 as explained on page 105 of the manual, but not only it didn't display anything, the power supply didn't even turn on, so my idea about the t-con board being able to control the PSU was incorrect. I tried leaving the jumper in place and added the main board on top to get the PSU to fire up, but still no picture, so I put the old logic board back in for now as this clearly isn't the issue - it's a "drive" issue probably.

-about the Y-sus and Y-buffer: I noticed there's a blue capacitor botch-soldered between a screw terminal (chassis GND) and a Y-buffer plane. From what I've learned over the years, the Y-buffer "floats" and shouldn't be connected to chassis GND ? Would this cap serve any purpose at all or should I remove it ? I doubt it would cause a total lack of picture though.

-there's two small electrolytic caps on the Y-buffer and they both read a 1.8ohm resistance across their terminals and no voltage at all when the set is plugged in. Don't know what to make of that or what they're for to begin with.

-same thing happens on the Y-sus near the white connector leading into the Y-buffer. The negative leg of the small electrolytic cap there (C5012) is not tied to chassis GND, so the supply is already floating at that point and gives me the same low resistance and no voltage when the set is on. I pulled out the Y-buffer entirely and the resistance on the Y-sus side went up, while the "short" remained on the caps on the Y-buffer itself, so could one of the buffer chips be dead ?

-even with the buffer removed, there was no voltage on C5012 near the white connector. The cap just behind that one (C5005) however has 5v on it.

-the top (or left in the high-res pic linked below) part of the Y-sus has some circuitry there which I THINK is a booster. I get 12.7v on C5065 on the far left there. Haven't dissected the Y-sus too much to know what all that stuff does or what some values should be. The TP labeled Vsc_h is dead (also tucked in there).

-there's a high res picture of the Y-sus HERE so you can follow along better