An unnamed householder in Aberhosan, Powys, was unaware the old set would emit a signal which would interfere with the entire village's broadband.
After 18 months engineers began an investigation after a cable replacement programme failed to fix the issue.
The embarrassed householder promised not to use the television again.
The village now has a stable broadband signal.
Openreach engineers were baffled by the continuous problem and it wasn't until they used a monitoring device that they found the fault.
The householder would switch their TV set on at 7am every morning - and electrical interference emitted by their second-hand television was affecting the broadband signal.
The owner, who does not want to be identified, was "mortified" to find out their old TV was causing the problem, according to Openreach.
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