Don't know when you posted you problem, But I have a MFJ-986 whish has a similar problem.
I normally work 80 mtrs so have not noticed it on other bands. I run it with a Heathkit Warrior HA10 usually about 500watts.
I noticed it causes my computer to freeze when using a websdr KFS in Half Moon Bay CA. Also my HP Office Jet printer comes alive and starts selecting different menus. With the amplifer not keyed but still passing RF thru the tuner it messes with the printer occationally. With everything staying the same except removing the MFJ-986 tuner and putting my Heathki SA-2040 tuner in place there is no interference. grounding does not change anything with either tuner. Recap... MFJ-986 500 watts interference, Heathkit SA-2040 no interference. I am using ladder line to feed the antenna. Both tuners give same SWR. ?Any suggestions besides connectors?
Thanks, Ken N6BSS
