From w4byg at att.net Wed Dec 13 09:51:45 2017 From: w4byg at att.net (Ray LaRue) Date: Wed, 13 Dec 2017 09:51:45 -0500 Subject: [MFJ-Ameritron] Troubleshooting rfi-emi from MFJ 75 amp MFJ-4275 power supply Message-ID: I have the above supply powering a Ameritron ALS-500M amplifier. This HD switching supply is generating RFI on 75 and 40 meters. On 75 meters the interference is mostly heard as I monitor 3980 KHz.??? While it is not severe, it sounds like bubbling water and is bothersome.??? It can also be heard as I tune down the band at various other frequencies. On 40 meters it sounds like a loud hash and can intermittently block out all signals.??? There may be two sources in the supply causing the problems. I wonder if their are others that have experienced this problem with this supply? I have talked to MFJ technical support on several occasions. They have been responsive and tried to be helpful.??? They have attempted to replicate the problem from a supply taken from their inventory.??? They confirmed the problem was an issue, but seemed unable to rectify it.??? They advised the supply is made overseas and they would have to go back to the designers for assistance. For the time being they only offered to send some snap-on ferrites to go on the AC power cable (they did and it did not help). It appears I may have to open this supply up and attempt to locate and correct the problem myself.??? I have good bench test equipment but I have never attempted to locate this kind of a problem. Are there any out there who would like to offer some aid and advice on fixing an issue such as this? You attention is appreciated. Ray, W4BYG