UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - MW hum - higher frequencies only

Skywave · 25th Jun 2021, 3:07 pm

Just a couple of ideas.

This problem - the 200 Hz radiation - could be caused from a building - or buildings near to you - in which there is a device - or devices - that use a switched-mode PSU. Such power supplies are notorious for distorting the waveform of the 230-v. supply and produce lots of harmonics of 50 Hz, usually predominately, but not exclusively, the 3rd harmonic. The cabling to such buildings will almost certainly be underground and possibly radiating those harmonics, therefore making any attempts to locate the source very difficult. The last sentence in post #15 states much the same.

As for the 200 Hz not detectable near the incoming 'tails', but detectable near the fuse box / consumer unit might be due to the tails being close to each other and thus acting as a balanced short line - so phase cancellation is taking place - whereas in the CU, the wiring in it is not so balanced.

Al.

25th Jun 2021, 3:07 pm	#17
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: MW hum - higher frequencies only - 200Hz Just a couple of ideas. This problem - the 200 Hz radiation - could be caused from a building - or buildings near to you - in which there is a device - or devices - that use a switched-mode PSU. Such power supplies are notorious for distorting the waveform of the 230-v. supply and produce lots of harmonics of 50 Hz, usually predominately, but not exclusively, the 3rd harmonic. The cabling to such buildings will almost certainly be underground and possibly radiating those harmonics, therefore making any attempts to locate the source very difficult. The last sentence in post #15 states much the same. As for the 200 Hz not detectable near the incoming 'tails', but detectable near the fuse box / consumer unit might be due to the tails being close to each other and thus acting as a balanced short line - so phase cancellation is taking place - whereas in the CU, the wiring in it is not so balanced. Al.