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Author Topic: MFJ-644 Problem  (Read 442 times)
K2QPN
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« on: November 23, 2008, 05:51:05 PM »

I have a new MFJ-644. Eveything works fine until I the MFJ-644 gets grounded to the radio (IC-756ProIII). The ground can come from the audio input cable or from a common 12V power supply. With the ground I get nothing but a large growling noise on the mic input to the radio. Without the ground, the rig audio is fine.

Any ideas?  73  Bob K2QPN
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W8JI
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« Reply #1 on: November 24, 2008, 03:27:18 AM »

Bob,

If you only connect it to one radio at a time and one microphone that floats from ground everywhere but the plug and one ungrounded floating power supply does it work??

73 Tom
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K2QPN
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« Reply #2 on: November 24, 2008, 05:49:13 AM »

If I use an isolated power supply, connect the mic, connect the cables to the radios - no noise. Any ground to either radio, with an audio cable or thru the power supply, - noise. I tried a new power supply - no help. Switching a Heil headset thru the Aux connectors - same problem.

Here is the weird part. If I purposely ground the MFJ644 to the radio using a clip lead - noise. If I touch the clip lead to the MFJ644 rear panel - noise, case bottom - noise, front panel - no noise. And the case is one solid u shaped piece. Weird. I have tried lowering the transmitter RF to zero - noise.

All the other switching seems to work fine.

73 Bob K2QPN
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K0BG
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« Reply #3 on: November 24, 2008, 06:08:59 AM »

Aren't ground loops fun to deal with?

Alan, KØBG
www.k0bg.com
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W8JI
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« Reply #4 on: November 24, 2008, 07:06:52 AM »

I looked at the schematic and I see a fundamental design flaw in the unit. It connects everything together with one common ground to the case. That is always bad news.

Now to be fair to MFJ, OTHER SO2R switches do the same stupid thing!

If audio equipment was designed correctly, the ground of each audio line would be isolated from all other grounds at every point except ONE.

I haven't found a SO2R box yet made properly.

So, you have two choices:

1.) Nearly perfectly bond all your cabinets of all your gear to ONE common point, and FLOAT all power supply ground paths from ground even if that means getting rid of the safety ground pin!

2.) Isolate all the audio lines with 1:1 siolation transformers.

You can make it work either way. If it is any comfort I haven't found any SO2R boxes built correctly, but most people are luckier than you.

You also COULD have an open ground lead someplace or a miswired ground in one of the jumpers in the SO2R box, but the whole idea of all the SO2R boxes I've seen is silly.

There should be only ONE path to chassis ground on any audio shield. This is why commercial gear floats the shield from ground at every single point except one.

73 Tom
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