Samsung ps42-b451 plasma - clicking

[Liam.25]

Hi guys

Plasma TV - ps42b451
Psu - bn44-00273d

Just received this plasma TV this morning. I've worked in a few of these before, the fault is that it's not powering up. Clicks 5 times then stops, all caps seem ok, fets 801 and 802 seem ok.

Does exactly the same with y-sus and z-sus disconnected.

Cn801 connection to main board-
Ps_on is at 3.3v
Std5v is at 5.2v
No other voltages because psu won't power on.

Cn802 connection to logic board -
Vs_on at 0v jumping to 3.3 whilst trying to turn on (clicking)
Ps_on also at 0v jumping to 3.3 whilst trying to turn on (clicking)
Gnd
Gnd
D5.3v is at 5.2
D5.3v is at 5.2

No visible cracks on solder anywhere

I need to figure out if psu is at fault or the main board is stopping this from powering on.

Any help guys??

[tom66]

Re: Samsung ps42-b451 plasma - clicking

What do Vs/Va do?

[Liam.25]

Re: Samsung ps42-b451 plasma - clicking

[Liam.25]

Re: Samsung ps42-b451 plasma - clicking

Vs and va are showing 0 volts
Thats if I'm testing in what I'm thinking is the test points situated near the va and vs adjustment pots

[Liam.25]

Re: Samsung ps42-b451 plasma - clicking

I believe they are the correct test points.
Vs is 0.7volts
Vs is 0 volts

[tom66]

Re: Samsung ps42-b451 plasma - clicking

Ok disconect Y-main, X-main and recheck Vs/Va while it clicks

[Liam.25]

Re: Samsung ps42-b451 plasma - clicking

Both va and vs are the same with both boards disconnected

[tom66]

Re: Samsung ps42-b451 plasma - clicking

Well, looks like main board is sending PS ON and VS ON signals but the power supply voltages are not coming up. I would suspect the power board, further tests would be a good idea. Check all fuses on the power board, and check the two FETs for Vs/Va tract for short circuits.

[Liam.25]

Re: Samsung ps42-b451 plasma - clicking

All fuses are ok. What tests can I check on the fets?

[tom66]

Re: Samsung ps42-b451 plasma - clicking

You can check for shorts, but since the fuses are ok that's unlikely to be a problem.
Consider replacing blue poly cap near Vs transformer. common fault with many types of SMPS. Also check output diodes aren't shorted (Vs/Va side)
BN44-###### number on PSU?

[Liam.25]

Re: Samsung ps42-b451 plasma - clicking

All diodes seem ok.

However if I disconnect the main and the logic board from the psu. Then short ps_on with gnd on the cn801 output to main board, the psu powers on to a point where I have the 15v and some more 5v active on the cn801 to main board output.

Remember this is with all boards disconnected.

[tom66]

Re: Samsung ps42-b451 plasma - clicking

You can try jumpering VS-ON to 5V using a low value resistor (100 ohms or so)

Note, Samsung uses active-LOW for PSON (signal goes to 0V to turn on) but active HIGH for VSON (no idea why)

[Liam.25]

Re: Samsung ps42-b451 plasma - clicking

Power board part number is bn44-00273d

[Liam.25]

Re: Samsung ps42-b451 plasma - clicking

so I ruled out the logic board and main board by tricking ps_on and vs_on with resistors and the psu still was clicking away.

But further inspection on the psu, I think I've found the culprit. On the underside there is a semi conductor at "pcm801s" with a slight burn mark on.

So I've ordered a replacement.

Anybody seen this fault before?

[tom66]

Re: Samsung ps42-b451 plasma - clicking

No, because that just looks like flux or conformal coating. It's not failed. It's an optoisolator/optocoupler. They are safety critical devices, so if replacing it make certain to get an exact replacement. They rarely fail especially in any "burny" manner, as this might lead to an isolation failure (very, very bad.)