[vdr] Re: burned DVB-s

["Reinhard Walter Buchner" <rw.buchner@freenet.de>]

Hi,

>The "common ground" is a simple wire that *permanently*
>interconect the metal chasis of all devices (in your case, the
>PC and your TV)

>and is not need that this wire  will be connected to a real Ground.

--> but it WILL be connected!

Doing this is going to give you a ground loop and some hum (=
dc current) to your system. It isn`t possible to connect ALL metal
parts of your system w/o connecting it to the mains ground
(therefore also to neutral via all the wiring in your house, which
means you can pick up stray voltages due to different loading of
the three individual phases of the electric companies input power)
at the same time.

Your PC (and or VDR) system is the first problem. It has a
grounded "Schuko" plug, so this will be your first connection to
the house ground. (which means audio and video circuitry of the
DVB are also connected to the mains ground).

The second (3rd) ground will be your DVB Antenna (F) connection,
if it is VDE approved ;ö)) So you already have a (2) ground loop(s)
there. I know of no "Mantelstrom filter" specifically for DVB
circuitry (and it wouldn`t work, as you need DC for your LNB,
motor or whatever).

To avoid these problems, NO true Hi-Fi equipment has a
grounded 3-prong plug (2 pole Eurostecker, only) The only
way you can avoid ground hum in a grounded system is to use
TOSlink for the audio. This of course will ONLY work, if your
amp and the rest of the system has a differential ground for
it`s video and audio circuitry. VDE demands a special isolation
class for 2 prong equipment.

Removing the mains ground is a bad idea (if your SMPSU goes
bad, you may have live current on your chassis, which WILL kill
a lot of components (maybe including you).

I also can`t see how disconnting the video cable will kill the
DVB card. As long as you have ANY connection between the
PC and the HiFi system, (audio, Lan, serial, actually anything
connecting the VDR box to a PC) you will have a ground
connection. While your system may hum, when changing the
connections (due to ground loops = a few mV),  the few mV
shouldn`t kill the DVB circuitry, but there is no way your are
going to get a few VOLTS on the actual signal line. (remember
the amp or TV is an INPUT cicuit --> FET or OP-Amp
technology, so there can`t be any voltage on the line from the
video amp, etc) If this was a problem, I am sure the RCA (cinch)
plug would have been changed in design (i.e. shorter middle plug
and / or longer shield connector), so that the ground would always
be connected first.

I also would *guess* that the AV7110 circuitry (be it in the
chip or on the card's PCB) has two protective diodes, which
should supress any spikes going to plus or minus spikes (
similar to a meter protection circuit).

As to adding a "safety device" in form of a decoupling capacitor
in series to the video line: This is a bad idea, because the
capacitor is really a RLC "circuit" (has R, L and C parts). This
can introduce all sorts of interesting junk into your video line.
If you are really unlucky (maybe due to external loading such
as the amp or TV, you could actually make a self resonant
circuit ---> VERY bad). This is most likely the reason why
Technotrend didn`t do this.

In an audio circuit a decoupling cap also presents a problem, but
not as severe, because you are not dealing with multiple signals
(meaning sync, Y, C, etc) on a single line (as are present on the
FBAS output).

Sergei: Please check the DVB card for supressor diodes on
or around the video circuitry leading to the output. If you are
lucky, you just shot one of these and can be replaced by
anyone with SMD soldering skills. Of course, if they are
built into the chip there is no real hope. The PDF I have
of the AV71xx doesn`t mention this (or at least I didn`t find
it while breezing through the manual).

One should also realize that DVB and other hardware is of a
very complex nature. Sometimes things just go *bad* simply
due to the complexity and high scale integration. Think of the
thousands of transistors envolved in a "simple" AV7110 chip.
If just one dies (for whatever reason), you are out of luck.
(although I hope you can solve the problem at low cost ;o))

Greetz,
Reinhard








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