Samsung PS42C96HD no glow, Vs=0

[senndogg]

Hello!
-this old plasma i am talking about was working very well in the evening and next morning there was no picture, just audio.
-after opening the case 5-6 caps were visibly bulging. no other visible signs.
-fuses were ok.
-bulged caps were in the 5V, 12V and 18V rails. they were replaced hoping that this would be the culprit.
-the tv acted the same way. after measuring, i realized that:
Va 63V
Vs kicked for some milisecs to over 200 and than 3-8V.
-it acts the same way if you disconnect either Ysus or Zsus.

where do i start now?
thanks!

[senndogg]

Re: Samsung PS42C96HD no glow, Vs=0

the Ps ON and Vs On from the main board should be 3V right?

[senndogg]

Re: Samsung PS42C96HD no glow, Vs=0

every output from the main logic is 0 instead of 5v. the indicator led in the logic doesnt even lit, so i suppose the board is gone. maybe the bulky caps in the 5.3v rail have shorten its life.

[senndogg]

Re: Samsung PS42C96HD no glow, Vs=0

after removing the jumper in the 3-4 pins (had left it in test mode) led in logic board started to blink. normal?

[senndogg]

Re: Samsung PS42C96HD no glow, Vs=0

board part no is LJ41-05136A and all voltages seem ok. the main chip gets hot too. but there is no 5V output to turn on the Vs.

[tom66]

Re: Samsung PS42C96HD no glow, Vs=0

Vs-ON is active HIGH, PS-ON is active LOW.
Logic board LED should blink even if power board does not function.

[senndogg]

Re: Samsung PS42C96HD no glow, Vs=0

i understand. but its the logic board that triggers them. i replaced the 5.3V input with another known source and the results are the same. maybe the Vs is switched off because of an error. i examined all buffers, fuses and transistors, everything is ok as far as i see.

[Ltank]

Re: Samsung PS42C96HD no glow, Vs=0

If you disconnect the scan boards then jumper the SMPS. Will it power up then? Have you checked for shorts on Vs voltage on the scan boards? A short would shut it down and you might see the Vs breifly.
This would at least eliminate the P.S. A 5 digit ohm meter or a Leekseeker will help you find low resistance shorts without having to unsolder components that is very time consuming.
Ltank

[senndogg]

Re: Samsung PS42C96HD no glow, Vs=0

So you are assuming that there might be an internal PSU error which shuts the Vs? Hold on!

[senndogg]

Re: Samsung PS42C96HD no glow, Vs=0

a steady 204V!
with the boards disconnected, for sure.

[senndogg]

Re: Samsung PS42C96HD no glow, Vs=0

so there might be 2 possible scenarios:
1. error in the logic board (maybe program) which refuses to turn Vs on.
2. error everywhere else, opened FET, gone IC or anything else which can not be detected by the Ohm test.

i have a strong desire to connect the Z and X main to the hacked Vs but i am afraid of the consequences. i dont want to cause extra damage.

[tom66]

Re: Samsung PS42C96HD no glow, Vs=0

Make sure Vs is <10V (unplug, maybe discharge with 230V lightbulb) then connect the Y and X boards. If there is still a short, the Vs will not rise above a few volts and should shut off quickly.

[senndogg]

Re: Samsung PS42C96HD no glow, Vs=0

Done:
the Vs is now 204V across the x and y main, and the screen doesnt give a single sign of life!
Which are the next steps? Or the display is simply gone?

[tom66]

Re: Samsung PS42C96HD no glow, Vs=0

The control board LED flashing or not?

[senndogg]

Re: Samsung PS42C96HD no glow, Vs=0

flashing.

[Ltank]

Re: Samsung PS42C96HD no glow, Vs=0

What is the resistance on the power pins to the Ysus and Zsus? I would expect to see several K ohms across the pins. Some times the Zsus power is from the Ysus board. The board with the lowest resistance should be the culprit. Sometimes each board power is from the SMPS on two different plugs.

[senndogg]

Re: Samsung PS42C96HD no glow, Vs=0

Both drives at the >1 Mega Ohm range.
they are powered from the same rail from the psu on 2 plugs.
but if there is Vs across them, shouldnt the screen glow?

[tom66]

Re: Samsung PS42C96HD no glow, Vs=0

Screen will glow only if priming ramp, -Vsc, Vset and Ve are present. Are they? The priming ramp can only be checked with an oscilloscope and 10X probe.

[senndogg]

Re: Samsung PS42C96HD no glow, Vs=0

please help me measure some of the voltages. just to have an idea whats going on. thanks.

[tom66]

Re: Samsung PS42C96HD no glow, Vs=0

Do you have a scope?

VSC voltage is marked on board next to test point VSCAN. Expecting around -190V there, exact rating written on plasma panel.