[mythtv-users] MythBox unstable when next to stereo/TV

Andrew Casper andrew at andrewcasper.com
Sun Aug 20 17:37:48 UTC 2006

On Aug 20, 2006, at 12:56 PM, William Munson wrote:

> Scott Johnson wrote:
>> I've tried twice now to move my MythBox from my den to the family
>> room.    The box works great in my den.  But when it is next to the
>> stereo or TV in the family room it has trouble booting and  
>> eventually I
>> get a screen full of text that has "DMA" along the left side.  It  
>> looks
>> like some kind of memory dump.  At the bottom of the screen, it  
>> reports
>> that I am out of memory (including Swap space) and some processes are
>> being killed.  There is hangs.  Last night it worked for about  
>> fifteen
>> minutes and I could even start to watch a recorded show.  Then I  
>> did an
>> operation in the MythStatus that required the database and it froze
>> there and every few minutes I could move the mouse an inch.  So I
>> rebooted it and I got the same DMA style errors.
>> I can unhook everything, move it back to the den, and it will  
>> power up
>> just fine.  All my equipment is to record standard def NTSC TV.   
>> The TV
>> is a Hitachi 27 inch tube from 1994 that has worked great for twelve
>> years.  I'm connecting my video card to the TV via SVideo.
>> Hardware:
>> Antec Sonata II Case
>> C19-A SLI Motherboard
>> Celeron D 2.5 GHz
>> Western Digital 250 GB Harddrive
>> Patriot 1 GB RAM
>> Netgear WG311T Wireless 802.11g card
>> Gigibyte Geforce 6200 Video Card with SVideo Out
>> Scythe Ninja CPU Heatsink/Fan
>> Hauppauge 150, 250 (2 Tuners)
>> StreamZap Remote (unplugged for now)
>> Software
>> Fedora Core 4 Following Jarod's HowTo
>> MythTV 0.19
>> Packages are current with a week or so
>> So I figure that it is electrical interference or not enough power.
>> 1) Power
>> If I turn all the equipment off except the Tivo, TV, and MythBox, it
>> still fails.   That doesn't seem like a lot of power to me.
>> 2) Electrical Interference
>> I moved the computer away from all the other equipment to a  
>> distance of
>> about three feet but that didn't help.  The cabling doesn't allow  
>> me to
>> move it any more.  If it is electrical interference, what recourse  
>> do I
>> have?  I can keep the computer away from the other equipment but  
>> I'd be
>> risking the Wife Acceptance Factor (WAF).
>> I'd like to hear from others before I blow another several hours  
>> moving
>> things around, only to have to undo it all again.  It is especially
>> frustrating because I am so close to being finished.
>> Scott
> I would suspect an electrical problem. Even though you will not be  
> able
> to see anything I would try unplugging the tv from power. Dont just  
> turn
> it off, that only makes the picture go away, unplug it. Try booting  
> the
> computer and playing something. You just need to remember the  
> keystroke
> sequence :) If it plays thru your speakers, then there is something
> going on with your tv. Most likly is that there is a ground loop  
> between
> the tv and the computer. Many vintage tv's like your have floating  
> power
> supplies where the internal chassis of the tv is not grounded. Then  
> can
> lead to voltage differences and current flowing down the wiring  
> between
> tv and computer.
> The other likely possibility is that the tv is producing a large
> magnetic field around the coils (poor shielding in the design)  
> which is
> bothering the power supply in your computer.
> The good news in all this is that properly presented to the wife this
> can result in a brand new plasma tv hanging on the wall :)

As a general rule, stereos and TVs won't complain about fluctuations  
in electrical current like a computer will. You might be overloading  
the circuit with everything together and turned on at the same time.  
Computers tend to draw a nice chunk of AC on boot and that's  
typically because they need to spin up many giant hard drives. The  
quick test is to try powering your computer with a 30 foot extension  
cord plugged into a different circuit in another room.

I'd also look at shielding. It is less of an issue with modern  
manufacturing, but it might be if you built your own computer. CRTs  
have LOTS of voltage flowing through them and some pretty big  
magnets. They all lead to weird results when placed next to  
components that aren't shielded or are poorly grounded.

- Andrew

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