Samsung PS50Q91 Plasma won't turn on

[psmedley]

Hi Guys,

Arrived home from a week's vacation and found our approx 7 year old Samsung Plasma tv - model PS50Q91 wouldn't turn on.

The red standby light was flashing and there was a weird almost popping sound with each flash.

Removed the back from the TV and found 3x 10V 2200uF capacitors that were obviously failed. Replaced these and now the standby light remains solid red and the weird noise had gone.

However, on turning the TV on, the startup chime played, however, the screen didn't light up and the TV powered off with a flashing standby light.

Sorry, I don't (yet) have photos, but wondering if anyone has any opinions on probably causes for failure? If it's likely to be more failed caps on the PSU I'll have a crack at repairing, but if it's a failed sustain board or something it will end up in the trash

Cheers,

Paul

[tom66]

Re: Samsung PS50Q91 Plasma won't turn on

The most common fault on these sets is a failed Y-main plus Y-buffer board. You can identify the failed buffer board because usually the first or second IC from the top will have a burn mark on it.

[psmedley]

Re: Samsung PS50Q91 Plasma won't turn on

Uggh I got the model number wrong in the above.

The actual model is PS50P91.

Major difference is the Q91 was HD, but not full HD. The P91 is full HD. I used to have a Q91 that I was trying to fix. The buffer boards look quite different on this TV, although it may be partly due to the heatshields over them.

I was planning to take some photos this afternoon but started drinking instead

[psmedley]

Re: Samsung PS50Q91 Plasma won't turn on

Would I be correct in thinking that if I disconnect the boards other than the main/input board and the PSU, and try and turn the TV on, that if the TV then completes power up, then the problem lies with one of the other boards?

If it still fails to power up with just main/psu connected, then the problem is with the main or PSU boards?

[tom66]

Re: Samsung PS50Q91 Plasma won't turn on

Yes -- should do - though worth noting this would not apply to some newer plasmas which will shut down if board faults are detected or boards are missing (Samsung sets in particular)

[psmedley]

Re: Samsung PS50Q91 Plasma won't turn on

Confirmed. With all boards unplugged from the PSU, except for the main board, the TV powers up, startup sound plays, then it powers down and 'reboots' and the cycle continues...

[tw2005]

Re: Samsung PS50Q91 Plasma won't turn on

I've been trying to upload a training powerpoint but good ol telstra keeps timing it out due to the size.

i wonder if it'd worth trying the test patterns jumper with the LVDS isolated from the main?

[tom66]

Re: Samsung PS50Q91 Plasma won't turn on

Confirmed. With all boards unplugged from the PSU, except for the main board, the TV powers up, startup sound plays, then it powers down and 'reboots' and the cycle continues...

Yea, that indicates a main board problem or a power board problem, although it's much more likely to be a main board problem.

[psmedley]

Re: Samsung PS50Q91 Plasma won't turn on

Yea, that indicates a main board problem or a power board problem, although it's much more likely to be a main board problem.

Photos of boards/TV are attached. Sorry for the quality, lighting wasn't great, and after recent knee surgery I couldn't move the set by myself.

I just did some voltage checks whilst the TV is powering up. Most voltages from the PSU appear in line with the markings, however, Vs is measuring 0V instead of the 200 or volts it should be! Va is 65V in line with the label on the panel.

This would suggest the problem is with the PSU as Vs is not coming up?

[psmedley]

Re: Samsung PS50Q91 Plasma won't turn on

Just been reading some more old threads - PS_ON is only around 0.6V whilst the TV is trying to power up, is this what should be triggering Vs to come up in voltage?

[newtothis]

Re: Samsung PS50Q91 Plasma won't turn on

Usually there is a pin for VSon , Also the PS_ON pin should be reading above >2.5v. This indicates a main fault. Tom is correct in his thinking

[psmedley]

Re: Samsung PS50Q91 Plasma won't turn on

Will take a look for vs_on when I get home.

PS_on was around 3.3v whilst in standby mode and dropped to 0.6v whilst it was trying to start up. Does this still suggest a main board issue?

[psmedley]

Re: Samsung PS50Q91 Plasma won't turn on

OK I'm officially an idiot.

I found the VS_ON pin this afternoon on an empty connector. I then noticed a ribbon cable behind the PSU.

Seems I forgot to make one connection after replacing the original 3x failed capacitors. After making that connection, the TV turns on and gives me a picture

Thanks all for the suggestions

[tom66]

Re: Samsung PS50Q91 Plasma won't turn on

Haha yeah I've done that myself... We all make mistakes. Glad to hear it's fixed.

[tw2005]

Re: Samsung PS50Q91 Plasma won't turn on

OK I'm officially an idiot.

I found the VS_ON pin this afternoon on an empty connector. I then noticed a ribbon cable behind the PSU.

Seems I forgot to make one connection after replacing the original 3x failed capacitors. After making that connection, the TV turns on and gives me a picture

Thanks all for the suggestions

Those air gaps will get you every time

[psmedley]

Re: Samsung PS50Q91 Plasma won't turn on

Worst part is I could have avoided buying a new TV, as the initial repair (3x capacitors) was correct.
However, the new TV uses around 400W less than this TV, so this one's off to ebay