Update on Now you see it, now you don't
Thanks to everyone who responded for the useful information.
I probably tried to simplify the situation in my original post, and led some down the wrong path.
I know that after the transition, some of the stations which were on UHF migrated to the upper VHF band. In this area channels 7, 9, and 11 are the ones I am aware of. Since I was on the fringe area for analog TV, there was a VHF antenna in my attic (not on the rotor) which I left pointed to Boston and fed into the distribution system through a combiner. Channel 7's signal is solid, 9 and 11 are marginal since they are in Manchester, NH and the antenna is pointed at Boston.
Yes, I am in a hilly area but fortunately I am on the top of a hill. I can see Boston buildings from my roof.
Thanks for those who mentioned tvfool. That has been the best source of information that I have seen on the internet on available channels, direction and expected signal.
When I was trying to characterize the signal loss problem, I tried every configuration including directly from the UHF antenna to one of my converter boxes with no splitters, combiners or amps at all. I did lose some of the weaker channels with this configuration, but basically I had the same work-for-a-while, die-for-a-while operation that I described in my original post.
As to correlation with a time of day, there was not an exact one. Signals would be good from a few hours to a couple of days and then go out for a few hours to a couple of days. The majority of the time they seemed to die around 9-10pm and come back in mid morning.
As Murphy's law would have it, everything has been working fine for the last week. Weaker channels come in sometimes and sometimes don't as is to be expected, but the stronger ones are always there.
Possible causes that I have thought of include:
1) Multipath effects with sometimes re-enforcing, sometimes canceling,
2) A strong interfering signal on a close frequency causing saturation of the pre-amps.
3) Some signal fiddling by the stations while kinks were being worked out of the system.
If anyone has any suggestions of other things to try if another major signal dropout occurs, let me know.
Thanks again to all who responded.
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