Switching power supply repair advice needed

kolevbg · Joined: 14 Mar 2008 Posts: 5 Location: Coppell,TX, USA

I have an SPS from a very late model LCD TV (Viore) which looks fairly simple in design and is yet giving me troubles.

It is supposed to produce 5V standby and given a STB signal it is supposed to enable 5V and 12V which come from the same source (flyback) where the 5STB come from, as well as 24V for the backlight which come through another flyback.

When I turn the TV the green status light blinks forever and upon checking the power board I find that it's because the 5V STB itself is pusling.

It does it regardless of whether it is connected to the main board or taken out.
A quiet tsk-tsk-tsk sound is coming at about 1Hz, just as if there is no sufficient load.

I tried to load it manually with a 470 Ohm resistor and it dropped sharply to about 2V, but continued to pulse.

When the pulsing 5V STB is feeded to the STB input pin the 5V and 12V show up pulsing distorsions as well. The 24V didn't come up at all.

Same happens when a stable 5V is passed to STB input from outside source.

I forgot to mention the board is brand new. It's apparently a lemon.

All electrolytic capacitors look perfect , no dust anywhere whatsoever, replaced the feedback opto-couple and yet no result.
Found a zener diode at the 5V output and replaced it - same effect.
There's also a TL431 used to power the opto-couple and I replaced that without success as well.

Don't know where to look anymore...any advice from professionals for a more systematic approach to finding the bugger are much appreciated!

jts1957 · Joined: 24 Nov 2008 Posts: 2476 Location: Far, Far Away

"Flyback(s)" are used only in CRT equipped sets.

You probably mean "transformer." Smile

Capacitors can "look" perfect, yet still be defective.

kolevbg · Joined: 14 Mar 2008 Posts: 5 Location: Coppell,TX, USA

Yes, sorry...still learning the lingo...apparently what I should have said is "transformer" , not "flyback".

Anyway, in the meantime I made some progress and am now facing the following problem:

I have a miniature 6 pin (2x3) power driver IC which is labeled "62847" and I need to find out what it is and, hopefully, the data sheet.

Any help is appreciated!

krytron · Joined: 05 Sep 2009 Posts: 28 Location: Europe

Is there any logo on the chip, which could help identifying it?
There are sites containing electronic manufacturer's logos, so if you look it with a magnifying glass you could be at least identify the manufacturer.

Once I had few blown resistors with strange code and a logo. Didn't know the specs until I found that the logo belongs to Futaba company, then downloaded small document from their website and I found full specs of the resistors from the code on them (fusible, flameproof, resistance, etc.).

kolevbg · Joined: 14 Mar 2008 Posts: 5 Location: Coppell,TX, USA

Nope...it is so small that those 5 digits are all that could fit...looks more like small SMD resistor matrix than an IC...either very slightly larger or - more likely - exactly the size of corpus SOT-346 - 2.9 mm x 1.6 mm .

kolevbg · Joined: 14 Mar 2008 Posts: 5 Location: Coppell,TX, USA