A remarkably interesting fault.
By Bob Tait VK3XP
Recently I decided to upgrade my UHF TV antenna. Whist connecting the coaxial F connector to the antenna I got a small tingle.
I also noticed broadband 50Hz QRM on 40 and 80 metres.
Where was this coming from?
There were a number of possibilities. I recently had a new metal roof put on the house; could have one of the roofing screws pierced a lighting circuit?
I isolated the lighting circuits, but no change.
The roof was 108 volts RMS when referenced to ground, as was the TV mast and antenna.
What was the common denominator?
When I turned off the circuit breaker which supplied power to my TV and multi-media system the fault disappeared.
Next step… start from scratch and unplug everything: HDMI, power and antenna leads.
I then reconnected the TV and the UHF antenna. All ok. As most of the equipment uses two wire power supply cables, I was measuring between reference ground and the chassis of each piece of equipment. I then connected the audio system. Again, all OK. Then the hard disc recorder and the HDMI link. Still OK.
Then I connected the Blu-ray Player power and measured between its chassis and ground and there it was. Suddenly I had 108 V RMS with reference to ground. The faulty unit was found.
What we had here was a daisy chain effect from the Blu-ray Player to the HDMI cable to the outer shield of the TX coax.
This fault was there even when the Blueray Player was turned off, so I concluded that it was a faulty common mode filter unit on the power side of the switch. That Bluray Player (Model BD-C6600) is now binned.
And my QRM on HF is gone.