[CQ-Contest] hum

[Lawless, William]

I've encountered hum twice with 2 DVKs:  MFJ-432 & recently MFJ-434.

MFJ-432 was a ground loop through the 12v supply line from an Astron supply feeding rig (IC-706 MK2) and MFJ-432.  Solved by using a plug-in-the-wall AC/DC adapter to power DVK.

Recent hum was lack of ground on MFJ-434, which has a ground terminal (screw with wing nut).  Grounding solved problem.  FYI - I was using both MFJ-432 & MFJ-434 in series... both worked OK.

Makes me wonder if MFJ-432 couldn't perhaps benefit from an earth ground to its chassis??

Tips:
1. Run DVKs off the internal back-up battery... if no hum, you know its power supply related.
2. Record message with no connections other than microphone.  Play through radio & listen.
3. Compare using the built-in mic; if no hum this way but hum with tip #2, then it's in mic.
4. Simplify connections and nibble away.

Bill  KA1UQ


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