SLow loss of Gain in Set Top Box

Most likely cause is connections....
Where the connector is soldered to the pcb.
Not much you cac do other than re-flow all the joints...
Trying to faultfind anything like that is impossible unless you work in an RF lab* and practice those Dark Arts of which I shall not speak.
Del

*Or you are a radio ham with a shed ful of kit.

Dear Del,

Assuming one has some inkling into the Dark Arts, into what in particular would one begin to delve?

Would there be a specification sheet which one could peruse?

Maybe a drop in voltage somewhere that one could check or observe?

These tuners come as a package - what would be involved in changing a tuner? - can replacement tuners be obtained?

??????

Many set top boxes run very hot. I had to return one once because I was afraid it would set my shelves on fire!

Such heat will cause an RF amp to degrade over time. You might try adding a heat sink to the input RF amp.

Disclaimer: I know nothing about RF but:-

Are there any aluminium electrolytics (metal canned capacitors) in the design, they dry out over time especially if they are running hot, so in combination with with

WoodwardDL's answer at #4 this might be a problem.

For quick and dirty calculations cheap electrolytics are often spec.'d at 2000 hrs. at 85ºC. Their life improves two fold for each 10º C lower you run them, so you'd expect them to last about a year at 55ºC two years at 45ºC etc.

I don't think I'd expect to find them anywhere in the signal path but if they're being used to smooth a power supply rail they might still produce the effect being seen.