[vdr] Re: burned DVB-s

[Roberto Deza <rdeza@unav.es>]

Hi Thomas and all,

At 21:04 21/10/2002 +0200, Thomas Breuer wrote:

>Quite dubious forum and even more dubious postings ...

Indeed, many of the forum threads are not too legal and many of the 
posters don't answer the questions; however, I put it only to show that
other AV711x PDF's could be "around".

[schematic of the output stages]
>Without knowing for sure that other designs are similar this would be
>quite useless.

Maybe there aren't much different ways of make an output stage of this
type (constant current, fixed voltage values, low impedance, etc) and
with some reference design one could figure if the AV7111x match with it.
Well, not relevant as I don't have found anything close :-(.

[Decoupling/Protecting the outputs]
>As I described, the outputs are constant current outputs. Try yourself:
>insert a small 100 ohms potentiometer between one of the outputs and the
>75 ohms termination resistor (either the one in the TV or, if not
>connected, a normal 75 ohms resistor to GND). Then take a look at the
>video signal (preferably a test picture) across the 75 ohms resistor
>with a scope and change the potentiometer value from 0 up to about 80
>ohms. The amplitude will stay constant! ...

I believe you :-).

>... So inserting a resistor would at
>least limit currents through the clamping diodes without affecting the
>output signal.

Yes, the current will be limited but I don't have clear if the AV7111x 
outputs have internal protection diodes; in your case, the "reanimation" 
could have "vaporized" the faulting transistor (or its metal tracks) at 
the output.

Measuring the voltage between two *un-referenced* AC devices, many times
I've saw 70~90 V AC, and this *voltage* will break electrostatically many
semiconductors.

About "my way" to protect my devices, I don't want to enter on polemics
about it; I only say that it works to me.

I can tell that on my (old) home there isn't third Ground wire on the 127 
Volts installation; my dish is near my window and have two more floors 
over it, so also isn't specifically grounded. The only time that I had
some "hum noise" (AC ground loop) was when I connected the community FM
antenna to the HiFi chain (wich is connected also to the DVB card); the
total cure was to put two small capacitors in series with the two FM 
antenna terminals.

[AV7111 RGB outputs]
>> On addition to tap the RGB signals on the BGA mounting_PCB tracks or my
>> crazy idea of "in the balls" (drilling a hole, with big precision, on the
>> board PCB), ...
>Hitting the correct balls might be possible, but it is a multilayer PCB,
>so you might hit an inner supply layer and end up with a short.

Sure, but one could over-drill, fill with epoxy or ceramic compound and
redrill after cured. I agree that is too risky and not to be done "at
hand".

>...If the layouts of the
>inner and top layes were available, this might be more easy than
>contacting the thin tracks on the AV7111-PCB. These are less than 4 mils
>(< 0.1 mm) wide!

 About the layout (~coordinates) I offered a high resolution scan of a 
X-ray picture, of a v2.1 board (now, it could be also of a v1.3).

[extracting RGB/CVBS of compensation components]
>Well, before I applied the brute force method to the dead card, I
>checked all pads and vias around the AV7111 to find at least h and v
>sync signals, but as far as I can say there is nothing!

Very likely, but it could work on a non_damaged card and solve the 
problem of rev>=2.1 cards :-).


Best regards:
--
Roberto Deza Asensio
Universidad de Navarra
Data Procesing Center
rdeza@unav.es


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