Samsung 81F Dropping Channel

Hi all

Searched but couldn't find anything like this...

My LNT-4081F will randomly drop a HD channel and shift it to another channel. For instance, channel 30-1 (NBC in my area) will suddenly and quickly pixelate and I'll be viewing a different channel, typically 90-1. If I hit the UP channel button on the remote once, it goes to 90-2 which is now the original program that was on 30-1. If I try to get back to 30-1 by manual selection on the remote, it's gone (no longer on channel list)even . After a while, if I stay on 90-2, it will suddenly drop out and then I can get back to 30-1. And, there's no longer any program on 90-2 (missing from channel list). I found out the 90-1 correlates to 8-1 (ABC), which is also gone at the same time.

If I do a channel auto re-scan when I first lose the channel, it will pick up 30-1 in it's correct place (after an excruciating long 25 minutes for the scan).

I thought it could be the signal dropping out but I would think I would just get a "lost or weak signal" message when that happens and the channel would at least stay on the channel list.

Some other things: I have Comcast analog cable service. The shifting channels are "pass through" HD channels (like NBC and ABC). They aren't the occasionally scrambled channels like PPV that sometimes pop up on a blank channel. I have another Samsung LCD TV (ln-t2342) in the house and it doesn't have any problems (at it's at the end of a long coax run with a splitter before it). I also have a Philips DVD recorder with QAM tuner at the same point as the 4081 and it doesn't have the problem either.

So I'm thinking my mainboard may be defective. Any advice?

Yep, I figured I'd have to do that ... just wanted to get a heads up on what I may have to deal with at tech support.

Well I had the main board replaced but the problem of shifting channels didn't go away.

I thought maybe my DVR (Philips DVDR 3576H) could be doing something, even though it should be a straight signal pass-through, so I disconnected the coax and directly connected it to the TV. No luck.

I do have an HDMI cable running from the DVR to the TV too (just for playing back recorded shows and DVDs). I disconnected that but it didn't help either. I now have both coax and HDMI disconnected...let's see if that works.

That was Samsung's last suggestion. They think it could be the signal from Comcast. I don't know much about QAM tuners but I don't understand this. From my very limited knowledge, I would have thought QAM tuners just scan the frequency spectrum and put put a signal into it's respective slot if present. Any QAM experts out there? Do cable signals have enough intelligence in them to dynamically reprogram the TV, for instance putting channel 30-1 into 90-2? I know a STB can do that but not sure a QAM tuner can.

Does anyone know if there's anything else in the TV that can go bad? I was led to believe that the main board has everything on it, including the tuner, so there's nothing else to replace.

This is driving me nuts.

I found out some things that are helping to make sense of this.

Seems that dtv signals include something called PSIP, Program and system information protocol. Amongst other things, this data tells the tuner where to map the signal on the TV. I guess it works like the STB from a cable company. Broadcast stations send this data out so viewers can get the signal where they expect it. i.e. in a market where the station is known to be on channel 4, the digital station ends up there too even though it may actually be transmitted on 10-4. The cable company is supposed to pass it through, although they can map the station where they like to accommodate their channels.

I found a useful website that lists the channels that are available in a zip code.
http://www.silicondust.com/hdhomerun/channels

So now i know why my channels shift to where they do (30.1 -> 90.2). When I loose the virtual or logical channel, it's going to it's actual channel number. Now I just have to figure out why. Is it Comcast dropping or changing the PSIP data? Or is the TV not handling the data correctly? Interesting that my other Samsung TV and my DVR don't have the same problem.