"> The switching action of the diodes modulates any RF > present in the mains supply and can produce a 100Hz > hum in certain conditions." Well, 120 Hz hum for those of us in the USA ! We went though an ENTIRE DRILL including several personnel taking to their cars with 12V powered receivers to validate this idea - concern was VERY HIGH that the actual output of these rigs was indeed clean! We were testing some 120 VAC powered transceiver products on the bench in the lab into dummy loads and observed "excessive hum" present on nearby receivers tuned to the carrier frequency when the rig under test was put into tranmit w/o modulation (via use of the CW keying input). Funny thing - the carriers observed on a spectrum analyzer close-in to the carrier via a coupler coming out the antenna port looked 'clean' - but here we were looking directly at the RF so no 60 or 120 Hz components (at least not to the levels to cause the amount of 'hum' we were hearing!) had been 'added' yet. My thoughts were that we were 'hearing' the 'leakage' signal that resulted RF from 'leakage' various leads/wires going into and coming out of the trancsiver - in fact, that's what a spectum analyzer showed when a short coupling loop was used to 'sniff' the area around the power supply connector (these were rigs with tube PAs - so a variety of voltages were required of the PSU and the PSUs in that series of rigs (HW-101, SB102, etc.) was external to the radio). Apparently the RF was being modulatated RF the power supply rectifier diodes - yielding clearly observable modulation 'products'. Jim ----- Original Message ----- From: "Joe Farr" To: Sent: Sunday, June 23, 2002 1:58 AM Subject: Re: [EE]: Hum problem > When building PSU's (especially for audio/radio use) I always place > 4 x 0.1uf caps across the each of the rectifier diodes in the > bridge. > Open up a good quality audio amp and have a look - there usually > there especially if it has a radio tuner input. > > The switching action of the diodes modulates any RF present in the > mains supply and can produce a 100Hz hum in certain conditions. > > Joe > > > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.