And you're the recipient of my 20,000th post! I've always loved antennas and antenna systems. Thanks for this opportunity.

If I understand what you're saying, the problem goes away not because there's a problem with the preamp, but because when you unplug the preamp it does not let any signal through which is the same as disconnecting the OTA, which you've already found solves the problem.

The problem occurs when HD and analog stations are on the same frequencies. In the US and Canada, 2 - 13 are the same on all TVs, and channels above that are on different frequencies and may or may not overlap depending on whether they are OTA or cable channels. Look up a table of OTA frequencies and cable channel frequencies to see which ones overlap.

When you're tuned in cable 13 and connect the OTA, if there's a digital channel on 13 it will ruin the cable signal. The only way to get both of these signals into the TV is to use a switch, going from one to the other. That gets dicey if your TV requires you to learn the channels it will allow you to tune in before you can go to them, because you can't learn both sets of channels at the same time.

Go to tvfool or www.antennaweb.org to find out which channel frequencies are used for broadcast stations in your area. Many OTA stations are not on the channel frequencies that match their channel numbers as a result of the way we transitioned from analog channels to digital channels.

Best of luck!