Audio Systems: Bizarre echo home thestre

Question
Follow-up to your last answer...
I have switched ch when echoing on Fox ch. echo always stops, and upon return to Fox ch, echo usually is gone until next broadcast commercial. I have seen echo remain when returning to Fox, but usually it is gone until next commercial broadcast.
I have experimented with using only tv speakers. I have never observed echo using only tv speakers.

I think I need to swap in a different tv and see if fault is present. This will isolate whether issue is tv or receiver.
Are you aware of an audio filter or processing unit I can connect to address the issue?

Answer
Question: Is it possible the echo is coming from the internal speakers and the external speakers being ON at the same time?  Can you listen close to see if that is so?

Trying another tv is a good suggestion.

Also, as an after thought, why not go into the menu and make sure the internal speakers are switched OFF.  Maybe reset the tv to verify the settings.

I think you are making progress.  You should be a system installer and troubleshooter!