[MFJ-Ameritron] Troubleshooting rfi-emi from MFJ 75 amp MFJ-4275 power supply

Wed Dec 13 09:51:45 EST 2017

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.ï¿½ 
While it is not severe, it sounds like bubbling water and is 
bothersome.ï¿½ 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.ï¿½ 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.ï¿½ They have attempted to 
replicate the problem from a supply taken from their inventory.ï¿½ They 
confirmed the problem was an issue, but seemed unable to rectify it.ï¿½ 
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.ï¿½ 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