On 17 jun, 09:28, Franc Zabkar wrote:
On Mon, 15 Jun 2009 16:06:10 -0700 (PDT), b
put finger to keyboard and composed:

I am trying to repair a digital terrestrial set top box which had
bulging caps, would shut down and had no audio. changed the caps, box
works but still no sound. I discovered two small 3 legged smd
transistors (at least I assujme thats what they are) burned,
presumably due to high voltage and/or poor regulation from the faulty
psu.
The only reference visible on the components is a '2'. In the same
pcb there are others marked '2A', so i'm assuming these two might be
the same.

They are on the pcb as Q5 and Q6, and the pcbitself is maerked: HY650
39150 DST VER 1.7 (2004) and it has the Zoran supraTv chip

http://www.elektronikjk.republika.pl/s1.html

2A = 2N3906 / FMMT3906 / MMBT3906 / MMBT3906L / PMBT3906 / SMBT3906 /
YTS3906/ MMBT3906W / PXT3906 / SXT3906

I'd check the other 2x numbers in the above reference.

FWIW, here are service manuals for three LG set-top boxes:

http://www.jordansmanuals.com/Defaul...duct=Set-Top-B...

Maybe they will give you a general idea as to how they work.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Thanks franc. I discovered the two transistors were for muting.
Removal restored the sound. that and changing the caps (C365-100uF,
C143-2200uF AND ONE OTHER 330uF) and the unit seems to work ok. model
in question: Mx-onda TDT MX-STB 5290.

2 of the bad caps were 'CHANG' mentioned hehttp://www.badcaps.net/
forum/showthread.php?t=388&page=5&pp=20
hope this may be of use to others.
-B