Samsung PN58A55081FXZA pops main fuses

[LameCooter]

I was given this unit since original owner couldn't get it to work. I removed the back, determined that one of the two main fuses was bad. I replaced it with same spec fuse and it immediately popped when powered on.

I have pulled the PS and visually checked the caps and checked them with an ESR meter. All appear OK with caps.

So I notice there are fuses on the other boards in the unit. Can I assume that if the short were on a board other than the PS that the fuse on that respective board would have blown?

In checking components on the PS, is there an order I should go about to troubleshoot?

Thanks much for any assistance.

[stj]

Re: Samsung PN58A55081FXZA pops main fuses

check any rectifiers and diodes on the psu for shorts.

[LameCooter]

Re: Samsung PN58A55081FXZA pops main fuses

Well it appears the rectifier DP808 is bad. I pulled it from the board to test again and it still failed.

Anything else I should look at? The rectifier I pulled is located in A5 on attached schematic.

Thanks for your help.

[tom66]

Re: Samsung PN58A55081FXZA pops main fuses

If DP808 is bad then it's possible that one of the FETs on the heatsink is also bad. Check those too. DP808 alone couldn't pop the fuse.

[LameCooter]

Re: Samsung PN58A55081FXZA pops main fuses

OK, making progress.
Was getting suspicious reading from QP803S so I pulled it and her sister QP804S. Sure enough 803 is shorted out.

Do I need to replace both even though only one is failing? At $10 a pop I'd like to make sure.

Thanks again for all the help.

The forum is great. Think I'll name my next dog BadCaps in honor of you folks.

[tom66]

Re: Samsung PN58A55081FXZA pops main fuses

You only need to replace the failed transistor. It would also be a very good idea to replace the driver transistors QP801/802 and driver IC on PFC_SUB, failure to do so may lead to failure after it powers up.

[LameCooter]

Re: Samsung PN58A55081FXZA pops main fuses

Hmmm.. Schematic lists QP801 as KTD1347 and QP802 KTB985 but I'm having trouble finding sources for those. Know of any equivalents? I'm still Googling...

[tom66]

Re: Samsung PN58A55081FXZA pops main fuses

TIP31A and TIP32A should work as good subs, but they have a metal tab so make sure they are isolated from each other.

[LameCooter]

Re: Samsung PN58A55081FXZA pops main fuses

UPDATE:
So I replaced the MOSFET, driver transistors and driver IC and put it all back together and....

Got a nice little display of sparks. Both of the thermistors shorted out and burned before the fuse could pop. I'm thinking I messed this up somehow.

Any pointers on where to look?

[stj]

Re: Samsung PN58A55081FXZA pops main fuses

foto of the other side of the board please.

[LameCooter]

Re: Samsung PN58A55081FXZA pops main fuses

Here you go.

Thanks

[LameCooter]

Re: Samsung PN58A55081FXZA pops main fuses

UPDATE:
The relay RLQ802S is shorted. Could this be the cause or was it maybe also damaged along with the thermistors?

[LameCooter]

Re: Samsung PN58A55081FXZA pops main fuses

Well, can't seem to edit my posts, but i pulled the relay and it's ok. So disregard previous post. Cant seem to read a schematic properly. Also pulled the bridges and checked them.

[LameCooter]

Re: Samsung PN58A55081FXZA pops main fuses

Any ideas of what to look for? running out of ideas.

thanks

[LameCooter]

Re: Samsung PN58A55081FXZA pops main fuses

LATEST:
Replaced the NTC Thermistors. I also fabbed up the lightbulb test jig and tested the unit with that installed. Now what happens is when I give the unit power relay RLQ802S (for the NTC Thermistor I presume) fires over and over. The light bulb test jigs also blinks. If I unplug the mainboard the clicking stops.

What should I look at next?

Thanks in advance

[selldoor]

Re: Samsung PN58A55081FXZA pops main fuses

If you just connect the power supply with the light bulb test jig what does the light do.
Comes on bright then dims - usually = no short on board
Comes on bright and stays bright = there is a short on that board.

[LameCooter]

Re: Samsung PN58A55081FXZA pops main fuses

If I unplug mainboard and apply power - which I gather is what puts the unit in standby mode - light bulb is off and everything seems fine. Plug in the main bard and the relay starts clicking and test light flashing in unison.

I guess I need to get the unit in standby mode without the mainboard attached and see what happens?

[selldoor]

Re: Samsung PN58A55081FXZA pops main fuses

Yes - try that - I am not sure when the pfc circuit kicks. The circuit diagram you posted and the part you replaced was in the PFC section. Did you replace like with like - did you test the part for shorts before you put it in ( no? not many of us do) Have you checked your soldering for bridges - if we are talking high voltages a near bridge could jump.

[LameCooter]

Re: Samsung PN58A55081FXZA pops main fuses

Just so I'm clear, will the unit go into standby without the mainboard connected? Where exactly should I hook up the resistors to force the unit into stanby?

thanks

[vinceroger69]

Re: Samsung PN58A55081FXZA pops main fuses

as per post 8 if the new transistors have metal tabs you need to use the insulator kits a shim and a washer i beleive this can be causing the issues you have now.