Samsung PS50C91HD - No picture, not sure which board is faulty

[illicit]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

I have the same issue as the OP with my PS50C91H - no picture but sound OK.

I downloaded the trouble shooting PDF above and it says to "check voltages after removing all cables from SMPS, if abnormal replace board"

I have done that and the VA reads 207 (as per the sticker on the panel) when all cables are plugged in but reads 0 with them unplugged (C807, C808, C809 & C810)

I have also replaced 3x 16v caps on the x and y boards due to bulging and leakage (I'm very familiar with replacing caps on Samsung LCD TVs)

Any help from here much appreciated.

[illicit]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Anyone?

[tw2005]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

I'd be focussing on the ymain and buffer boards. Check the ymain FETs for shorts mentioned in previous posts. If you have 207v but not when it's all hooked up I'm thinking cct protection and most commonly it's a blown ymain and the upper buffer board is also shorted.

[illicit]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Other way around, when its all hooked up voltages are OK. When disconnected the 207 reads 0

[tom66]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

It may be detecting a fault with Y-main unplugged or maybe you are not testing it properly. Then again I have not heard of such fault detect on a Samsung.

[illicit]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Well basically the fault is the same as the OP - sound but no pic.

- no light emitted from screen
- bad caps replaced
- voltages test OK when everything is hooked up
- voltages OK except VS when all cables unplugged from power supply board (except AC in obviously)

The Samsung troubleshooting PDF said 'if abnormal voltage readout when all cables unplugged = replace board" - I assume the fault is within the power board, otherwise it wouldn't be in the troubleshooting manual?

Or is there other tests I should do to locate the fault(s)?

[tom66]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Where are you testing Vs on the PSU?

BTW, I meant to say I have not heard of it on THIS Samsung chassis. The newer ones do cut power if a board is missing.

[illicit]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Any of the VS pins of the cable connectors and the VS test pin

[tw2005]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Other way around, when its all hooked up voltages are OK. When disconnected the 207 reads 0

SO, you have sound, no picture and All voltages are Ok with it all hooked up.

So from that, Can we deduce power is applied to both the ysus and xsus then from that information? And the panel does not energise?

[illicit]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

If by energise you mean does it 'glow' when on in a dark room? If so. That's a no, can't see any light emitted from screen.

There are a couple of LEDs that flash on the logic board, only when it starts up - relevant?

[tw2005]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

I'm pretty sure it should blink at 1 sec intervals and keep blinking whilst the set runs.

Last time I saw that was when my ymain was bad and a bad upper buffer.

So is there Vs at the y & X main connectors when it blinks and then not when the logic board quits?

[illicit]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Blinks 3 times then stops

[tw2005]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

What about Vs at the ymain?

[illicit]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

207 at the input (cable connector from SMPS) and fuse is ok

[tw2005]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

I hate faults which don't have an ending so i'm tacking onto this one because all of the above applies to my PS50C91HD as well.

My symptoms are as follows.

1. Plug-in standby power available, front indication normal.

2. turn on either front button or remote, blinking front light then locks on solid.

3. From rear of TV (internal) power on LEDS illuminated on X-main, Y-main, CTRL Logic Board and Otic audio out on Main Board glows red.

4. voltages normal.

5. Nil display , but sound and controls normally.

6. No obvious signs of any blown chips,fets or capacitors.

7. Watching the activity LEDs as set is powered on I noticed there was an activity LED as well as the power on LED on the CTRL Logic board. This blinks at 1 sec intervals and quit suddenly after 10 secs or so.

8. With X-main disconnected, same result with the CTRL Logic LED, Connected X-main and Y-main then disconnected, activity LED just kept blinking.

So at this stage I was suspecting faulty Y-main and have quickly checked the FETS which I found an entire bank short cct (Q5500,5302,5303,5304,5306) 1 x 2SK2837 N Type FET, 4 x 80N30W N Type FET 80A, 300V. Q5303 was dead short and shorted the remaining FETs which tested ok once Q5303 was removed.

Thats where I'm at, fingers crossed it was just a protection cct shutdown of the screen.

I could not find a 80N30W TO3P so I'm throwing in an 80N30P in the TO-264 package.

If anyone has an opinion on this I'd like to hear it. I'm assuming the main difference between the 2 would be thermal dissapation and maybe the TO-264 may not be as durable?

I'll post up the results once i have the new FET, hopefully it'll arrive tomorrow.

Well that has me confused. So full Vs on startup and when the logic board quits? So does any of this info I wrote help? The disconnection of the ymain is vague so maybe try disconnecting the main power connector at the ymain and see if you can get the logic card to do what I've stated and then if not retry with the flat ribbon disconnected that goes back to the logic card (and power connector) and see if it stays on.

If it does then I'd have a closer look at the ymain and upper buffer (or both) for shorts.

[illicit]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

How do you mean Logic board quits? It still has an led that is lit constantly, two others come on at start up, one blinks three times then they both go off but the main one stays on.

How/where do I check for shorts?

[tw2005]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Ok, from my understanding it needs to have a "heartbeat" which means it's communicating with the other boards. One indicates power to the logic board and the other is comms.

What you have is matching up with that long blurb I wrote although I don't think there are onboard leds on the x & y boards. Regardless you could try disconnecting the cables at the ymain and see if the logic keeps blinking which will confirm there's a ymain issue.

But there's nothing wrong heading for the ymain fets to check because I do feel that's where the issue is.

I've listed what FETs I had issues with above so if you have a meter, set to diode and test across the pins on each FET or measure the resistance. If it beeps or shows 0 ohms then you've found your problem. I shorted FET will short the remainding FETS in that group.

[illicit]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Disconnected cable to y main, logic led flashes and keeps flashing. Plug in y main, led flashes 3times then stops.

Do I need to disconnect y main to test FETs?

[tw2005]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Well if you can get to the legs from topside, probably not but it's not that hard to get it out just be wary of potentially stored charge in the caps.

[illicit]

Re: Samsung PS50C91HD - No picture, not sure which board is faulty

Found two shorted FETs, both on same heatsink (labelled D5003 & D5001 - all other FETs are labelled with Qxxxx - I assume that's coincidence?)

Looks like I have to remove heatsink to replace FETs, correct?