MTpockets is a very old build now. At the time of openatv 4.2 there was an old bug going round that was blacking out screens on tuner a.
The fix was an unusual one, but it worked. To fix it all you had to do was turn on Diseqc in tuner config - for both tuner a and tuner b.
This might not be the problem you have got, but its worth giving it ago.
I personally would suggest you update to a more newer backup image like wooshmans openatv 5.3 in the stickies.
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and looking at whooshes thread, you seem to have now installed his build, so I am waffling crap for no reason ;)