Re: Mad to fix STB?

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On Sat, 09 Feb 2008 11:35:42 GMT, "Mark Kelep"

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Even though they look OK, I would change C2 on the AC side and the
small electrolytics on the DC side. I'd also scrape the copper tracks
around U1 and build them up with solder and wire links. Check the caps
on the main PCB as well. I don't know whether others would agree, but
I'd remove the glue from around the optocoupler at U2. Too many times
I've encountered glue that has become conductive and corrosive, so I'm
uncomfortable seeing any glue bridging the AC and DC sides of your
PCB.

Here is the datasheet for the NCP1200-60:
http://www.onsemi.com/pub/Collateral/NCP1200-D.PDF

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

Re: Mad to fix STB?

"Mark Kelep"

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**  The  SMPS  IC is an NCP 1200 from On Semi.

http://www.onsemi.com/pub/Collateral/NCP1200-D.PDF

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






Re: Mad to fix STB?

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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





Re: Mad to fix STB?
On Sun, 10 Feb 2008 07:24:37 GMT, "Mark Kelep"

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

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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
+5V capacitor (and all the others).

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

Re: Mad to fix STB?

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* Sure is

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* 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)



Re: Mad to fix STB?

: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 shit 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.
:www.spleena.com.au
:there's no add crap here,  I just dont have any other way to put a pic up
:yet.
:Thanks
:Mark
:
:
:


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.

Re: Mad to fix STB?

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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




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

Dave.

Re: Mad to fix STB?

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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.



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