I have run across an unusual problem.

I live in an area of poor reception and use a rotator mounted rooftop Yagi with mast mount preamp and split the feed to a tuner and a TV. Probably 50' of RG6 including the preamp and power unit before the splitter, 3' to the TV and 30' to the tuner. The system has worked well to date with a number of different tuners and receivers.

I recently acquired a fairly inexpensive Quad FM4 that I suspect is in need of alignment, but which does work well enough for stereo reception. The problem is that TV reception is markedly degraded when the tuner is connected to the antenna even if the tuner is not powered up. I've been disconnecting the antenna at the tuner in order to use the TV, but this is a poor solution.

I'm not knowledgeable enough to understand why this happens or how to cure it. Is it an indication of a tuner problem? All advice appreciated.

If the antenna input on the FM is 300 ohm, maybe a matching transformer would isolate it from the TV (while not trashing your reception).

Can only guess that there is some sort of ground (leaky capacitor/tube/whatever) at the tuner that is bleeding off the signal but is overcome by the amplification within the tuner when it is operating.

An easy fix until you can troubleshoot is to use an F-connector switch at the TV rather than a splitter. This will let you isolate the device not being used and will drop your input input signal loss from 3db (half) due to the splitter, to about 0.2-3db. Here is a really cheap one from PE. http://www.partsexpress.com/pe/pshowdetl.cfm?&Partnumber=180-090&CFID=4530082&CFTOKEN=89698882 There are others that allow one to switch between more than two outputs.