Samsung Plasma PS42A410C1D Failure to launch

[dkneyle]

Stand-by voltage is present (~5V). PS-ON is at about 3.5V at idle, and when the on button is hit it drops to about 0.3V for about ½ second before rising back to 3.5V. No sign of any life such as relays clicking in. SMPS is BN44-00204A for reference.

Thoughts on next steps? Should I be looking towards the power supply or mainboard as the failure?

Not sure of the procedure for force starting on of these power supplies? Any guidance there would be appreciated.

[sordids999]

Re: Samsung Plasma PS42A410C1D Failure to launch

Pictures ?

[dkneyle]

Re: Samsung Plasma PS42A410C1D Failure to launch

Pictures coming Sordis999.

I was just out mucking about with it. The PS_ON repeatedly cycles between 3.5V and 0.3V every couple of seconds as if it is sending the on signal, and re-sets when it doesn't have signs of life. No VS-ON voltage. Seems more and more like the power supply, but without a decent circuit diagram it's very hard to trace the circuit. I think I've worked out which opto-coupler should turn on and thinking I could look across its terminals to see if there are signs of life there.

[dkneyle]

Re: Samsung Plasma PS42A410C1D Failure to launch

This is what the model of PSU looks like.

[dkneyle]

Re: Samsung Plasma PS42A410C1D Failure to launch

And here are the schematics for this PSU.

[dkneyle]

Re: Samsung Plasma PS42A410C1D Failure to launch

I've traced the PS_On fluctuating signal across onto the hot side of the board. It travels through the 15 volt regulator (I think that it what it is) and now I'll try to find out why it isn't pulling in the power relay.

[dkneyle]

Re: Samsung Plasma PS42A410C1D Failure to launch

You know when sometimes in electronics servicing you just go "but that makes no sense?" I was probing for voltages around the series resistor (R803) that feeds the coil of the relay. There was a zap and a flash and I though in the confined space I'd somehow managed to short something. Expecting the worse, I found the regulator feeding this part of the circuit was still operational, although the resistor had jumped from 68 ohms (previously confirmed as correct and OK) to about 300 ohms with some signs of physical damage. Anyway, had another go, and this time I was watching very closely. As I pushed my probe onto the end of the resistor feeding the relay coil there was another flash and zap. And I swear I think I heard the relay pull in.
I pulled back in concern thinking I'd connected the hot side to the cold side or something else. The Fluke meter is definitely on volts scale. And on voltage measurement there should be significant isolation to prevent any current being drawn. Pretty sure I'm connected to the hot side earth, because I'd been using it OK for other measurements around this part of the circuit.
I'm left to conclude there is something very wrong around that series resistor. Did my probing make a poor joint good? I'll replace that resistor and let you know the next installment. In all my years of playing electronics I've never had a voltage meter measurement cause zaps and sparks. Current measurements yes, but not voltage measurements.

[dkneyle]

Re: Samsung Plasma PS42A410C1D Failure to launch

Well to use the old Aussie vernacular, "I'll be buggered". I replaced that 68 ohm resistor and I now have a clicking-relay syndrome. That is getting closer to a solution. My circuit analysis suggests it's not receiving a steady PFC_OUT signal, so the other relay is not coming in. Looks like I'm now onto the PFC circuitry for a solution.

Out of circuit that resistor measures open circuit, so I don't know what was going on with it. I'm sure I measured 68 ohms in circuit. And I can't explain how touching with a meter probe caused arcs and sparks. I certainly had no relay click with that resistor in circuit. With a replacement 68 ohm resistor I now do. Go figure?

[dick_barton]

Re: Samsung Plasma PS42A410C1D Failure to launch

I believe that this power supply was discussed in a recent thread which may be of help to you.

https://www.badcaps.net/forum/showth...ht=bn44-00204a

[dkneyle]

Re: Samsung Plasma PS42A410C1D Failure to launch

Thanks Dick. I think reading that post my next trick will be to change out all those hot side caps.

[dkneyle]

Re: Samsung Plasma PS42A410C1D Failure to launch

Cam someone do me a favour, look at the schematic in Post 5, and tell me if PS-ON should be 0V or 3.5V in this set. My circuit analysis skills are fried after too many years away from electronics. I think it is PS-ON 0V, but need confirmation.

[flocko]

Re: Samsung Plasma PS42A410C1D Failure to launch

PSon should be pulled low Ov (ground). VSon should be pulled high to 5v STby with a 470-1000 resistor, test power board with everything unplugged apart from AC supply from wall socket. if the power board is Ok with PSon grounded you will hear the relay click and all small voltages will come up, only when VSon is jumped will the VS voltage show.

[leccyrob]

Re: Samsung Plasma PS42A410C1D Failure to launch

Hi Guys,
This is probably all finished with your particular faults with this power supply but I've just joined (found this forum whilst searching for info about this) and thought I'd share my findings......

My fault.... Power supply won't switch on properly from cold - just get relay(s) kicking on/off every couple seconds.

The cure.....Tried all small caps for low ESR as suggested here but this didn't cure the fault. I then found a nice, neat little dry joint on one leg of the 680nF square, blue capacitor CS813. This is a sort of 'ground return' on the primary side of the Va/Vs generation circuit.
It did actually look like bad caps to start with as blasting the PSU PCB with a hair-dryer would get the set to work properly.

Many thanks to the fella who provided the circuit diagram here, it really spurred me on to chase this fault, I stopped fixing TV's for a living over 10 years ago, I now do it for friends mostly.

[dkneyle]

That would have been me that put the circuit diagram up! I'm trying to remember the history of this TV repaitr. I'm usually pretty good at posting the final outcome, but that is not what I've done in this case. Anyway, glad to know you saved another one from the dumpster. I'm getting very frustrated with failed mainboards where a ball gate array IC has failed underneath. Not much can be done after that and mainboards don't usually come up as spare parts, or if they do it's an uneconomical repair.

[leccyrob]

Re: Samsung Plasma PS42A410C1D Failure to launch

Well thanks dkneyle,
I worked in UK through the 90's on TVs then moved to Melbourne, Australia where I continued to earn as a TV repairman. It was good that with a bit of brain-power and (usually) only a few dollars of parts (caps, resistors etc) you could fix up a TV worth $500 + and charge nicely for it.
Now the price of a TV has dropped and the ability to repair-to-component-level has also dropped, there's not really any way to earn usefully from this trade.
Still a buzz to 'save one from the dumpster' as you put it...
All the best.