Re: Mad to fix STB?

"Mark Kelep"

The app notes indicate it runs damn hot when operated from a 240 volt, AC supply.

The internal ( switching ? ) voltage regulator runs from the rectified AC - about 340 volts and draws around 1.8 mA

An 8 pin DIP dissipating 600mW continuously will darken a phenolic PCB.

If the DC rail tests OK on a DMM and scope - then it ain't your problem.

...... Phil

Thanks for the replies, the board certainly does have that familiar burnt stink thats for sure.

I checked all output voltages with a CRO and DMM. Where it should have been 12vdc it was, with no ripple. However where it should be 5vdc, it was 5.75vdc (still no ripple) and finally what should have measured 3v3 (3.3v?), it was only 3.05vdc with no ripple. All was measured with unit in standby mode and on (yes it still starts up sometimes) with very little diff. in either mode. I should mention that when connected up to TV, diagonal interference lines are noticable but only when screen is mainly white. It never used to be like that. I haven't changed any caps yet btw. Any other ideas would be great, before I chuck it.

Thanks again Mark

On Sun, 10 Feb 2008 07:24:37 GMT, "Mark Kelep" put finger to keyboard and composed:

Could that be because this rail is derived from a 7812 regulator (located above C7)?

I suspect that the +5V rail is the one that is being regulated via the optocoupler at U2 and the TL431 (?) at U4. If so, then I'd replace the

- Franc Zabkar

:Hi, :I have a mate who has a Wide screen SD TFT TV, but wont be convinced of any :benefit by getting a digital STB, yep he's still using the old reception and :it looks shabby. :It cant even seem to get wide screen format (not sure if that's the only :reason why though) :Anyway, my old Teac DV-B420 slowly s*it itself, frozen screen, wouldn't :respond to remote or front panel commands unless I plugged it out (including :aerial) then it did work until it became so slack wit these re-boots that I :bought a new one. :But After looking in the old unit, it was clear that the eight pin chip had :been getting bloody hot so I wonder if I can fix this thing and give it to :Timmy the slackarse mate, for nothing. :See Pic here. :

Apart from suggestions by Phil and Franc, some of those electros at the top right of the pic look a little suspicious (bulging top), so I would be ESR testing or just replacing them. While your oscilloscope testing on stand-by seems ok the real test should have been performed while operating at full load.

As far as the darkened pcb caused by the controller IC, I haven't seen that problem before, but I haven't encountered that particular IC device yet. On the more common UC384x series I have never seen it unless the IC is severely overloaded. I would replace it anyway.

• Sure is

• U4 is a TL 431 yes! There is no obvious bulging on any of the caps but i'll change them and see what happens.

Thanks again Mark

(remove x v z and a b c)

At under $30 for an SD STB these days, it's a no-brainer! Seeing will be believing.

Dave.

Yeah yeah, it was only a matter of time before someone responded with that one in this 'throw away' world. I reckon i'll wait a bit longer and replace a few caps etc before I add to the land fill though

Mark.

Finally got around to replacing caps (as many as I could from spares I had lying around) and it's now working as good as it ever did. Thanks to those with the very helpful advice.

Mark