[vdr] Re: burned DVB-s

[Thomas Breuer <tb@tb-electronic.de>]

Hi Roberto,

On 21/10/2002 11:48 Roberto Deza wrote:

> On the thread below, I guy ('xanti') commented to have "a pair of PDF's
> of the DSP pulled, unfortunately password protected" ...

Quite dubious forum and even more dubious postings ...

>> ... schematic of the output stages ...

> I'd like to know its (and many other details :-), but the most close
> approach that I can think is to see how other Digital Video Encoders
> (maybe from Texas) do it.

Without knowing for sure that other designs are similar this would be
quite useless.

[AV7111 reanimation]

> Wooooow!!!, this is as the "recovery" of shorted NiCa baterys, but much
> more miraculous :-).

Yep, AFAIR the total input power during this operation was some 3 or 4
watts. The spot above the affected output stage got really hot and after
that we were very curious to know what would happen on the next boot ...

[Decoupling]

> The analog electronic isn't my preffered, but I think that would not be
> easy to block the DC (without disturb the bandwith and levels) in a video
> output and anyway, a big capacitor could allow to pass enought current/
> voltage to break the chip. Perhaps, a "emiter follower" transistor on
> each output could have been "wise", but the image quality should not
> improve. Anyway, the *fact* is that the outputs are not protected and the
> common ground is a good protection.

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

A capacitor can not be inserted here, but a simple buffer (transistors
as you suggested or opamps). Then at worst only the buffer would be
damaged.

[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. Tracks
on the bottom could be interrupted if necessary. 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!

> ... it could be another way that already I commented:
> The AV7111x is having four Bias resistors (BIAS_RC, BIAS_GY, BIAS_B &
> BIAS_COMP) and four Compensation capacitors (COMP[3:0]) wich are wired
> and mounted on all boards that I've seen (included the v2.1 and v2.2 that
> don't have RGB outputs availables!).
>
> Could be that this Bias and Compensation components have some usefull
> signals to recover a "normalized RGB", perhaps adding a few more parts
> (as OpAmps, etc). For now, I don't have had the need to investigate it
> nor the time :-(, but it also could be investigated on a <v2.1 card, with
> the good point that then one have the propper outputs to compare with the
> "reconstructed" ones (with an osciloscope and a "colours bars" test
> picture) .

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!

Thomas









-- 
Info:
To unsubscribe send a mail to listar@linuxtv.org with "unsubscribe vdr" as subject.