Schoolboy error with solder paste

[signup]

Hi

Was just fixing a psu- pspf531801a (cold joints) and I always use some solder paste on all the joints sucker off and re solder, I put quite a bit around one of the fets and thought nothing more of it as I was going to clean the excess off once I took all the solder away.....I forgot and built the tv back together it worked and left it running for about 20minutes, screwed the back of the tv on replugged it in and KABOOM see pic, I took it back apart and noticed the excess solder paste/now fluid had obviously shorted the fet out on the underside of the board

I cant seem to find any other problems on the board apart from the fuse and fet...What are the chances I have got away with just the fet

[tom66]

Re: Schoolboy error with solder paste

Solder paste? You mean thermal paste?

[signup]

Re: Schoolboy error with solder paste

No, Solder paste, I use it for a few reasons, I get a cleaner finish and seems to help stop me ripping up tracks and stops my solder sucker from jamming up all the time as the solder takes a different form with it on

[tom66]

Re: Schoolboy error with solder paste

I have no idea how you use solder paste to do that, because it is for machine reflow, not for normal soldering. Have you tried using solder wick instead? Regarding the fault, the controller IC may also be bad.

[signup]

Re: Schoolboy error with solder paste

right ok will stop using it as its just bit me on thearse, I will getthe datasheet of the controller ic and check if I have killed that too

[signup]

Re: Schoolboy error with solder paste

Hi

Tom the controller ic seems to be ok, I am getting continuity between Gate and source on the 2 9n50c and the 2 18a fets is this normal

[signup]

Re: Schoolboy error with solder paste

Sorry I tell a lie, if I remove all the fets from the board I get continuity between Source and Gate on the 2 18a Fets and continuity through Source and Gate on one of the 9a fets, the one that blew up DOES NOT show continuity through Source and Gate

This is measuring on the board pads not the legs of the fets out of board

[tom66]

Re: Schoolboy error with solder paste

What do you measure on the board itself, is it open between source and gate?

[signup]

Re: Schoolboy error with solder paste

On the 18a fets I am getting 9.3ohms and the 9a that blew is open, the other 9a fet is 10.4ohms

Thats with components removed from board

[tw2005]

Re: Schoolboy error with solder paste

just some info hopefully some help for you.

QX801,802 are on page 4.

repair kits for these, http://www.ebay.com.au/itm/SAMSUNG-P...item1e7845b33b, there's a heap of kits , may give some clues what parts to check out.

Impossible to be sure just the kit will fix it, could have other issues. I would not be too hard on yourself, real chance it already had other issues, bad joints are real common.

I have seen one powerup which had intermittent shutdowns, few minutes latter it went bang and smoke from the FETs.

I've only had one go at one of these which I was lucky with, just needed reflowing on the heatsinked parts.


Closely relate PSU, one of 4 I think which are all suitable to power the 50" Samsung Plasma of these series.

https://www.youtube.com/watch?v=dVaC2KVVLvs

This is a 42" version , also very similar design https://www.youtube.com/watch?v=za3EixgxBkk

Photo c/o SJ

[signup]

Re: Schoolboy error with solder paste

Cheers Tom

on the link it tells you which smd resistors are gone

rx802 = Value=200 Reading=11ohms
rx833 = Value=200 Reading=11ohms

rx803 = Value=200 Reading = 4.8kohms
rx832 = Value=200 Reading = 4.8kohms

rx803 and rx832 are the ones that come off the fet gate that blew and are a little charred on top

I am guessing the diode is the three legged jobby that is next to the two resistors rx803,832

[tom66]

Re: Schoolboy error with solder paste

If those resistors are gone it's more than likely the controller IC is too.

[signup]

Re: Schoolboy error with solder paste

I have just aligned the PSU back in situ and the burn mark on the rear of the screen is not near the fet that blew, but there is a massive black mark under this (pic)

[signup]

Re: Schoolboy error with solder paste

Here is the controller IC datasheet, I cannot find any shorts on it should I test it in a different way

[tom66]

Re: Schoolboy error with solder paste

It may not be obvious. It might only fail during operation leading to more blown FETs. The only way to test it for sure is to power the controller IC up with 15V DC or so and observe the waveforms with a scope.

[signup]

Re: Schoolboy error with solder paste

Right cheers Tom, just to make sure the viper 22a is the controller ic, its the 8 leg ic that is below the transformer on the picture in post#13

[tom66]

Re: Schoolboy error with solder paste

No...that's the one for the standby supply. You'd be looking for a little 8 or 14 pin SMD on the bottom of the board most likely.

[signup]

Re: Schoolboy error with solder paste

Now im confused, the burn marks were right under the standby ic/transformer.

The only other ic on the underside of the board are (A393F) Board position ICQ803

The only other ics I can see are circled in the pic

[tom66]

Re: Schoolboy error with solder paste

Probably the IC on the little stand-up board next to those devices.

[signup]

Re: Schoolboy error with solder paste

Theres is two, would one be for the 18a fets and the other for the 9a fets