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.