Posting Comment for "Fixed the MFJ-207 FM Problem"

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27th June 2009 17:45

Alan Yates wrote...

Tom,

Here's an email I replied to Don Montgomery about the same question:

To: Don Montgomery From: Alan Yates Date: Thu, 4 Jun 2009 23:35:39 +1000 (EST) Subject: Re: Question About MFJ-207 Your manual must be different? The diagram in mine has some voltages noted, I notice the online one doesn't. I couldn't achieve the exact voltages specified, but the main thing is that TP1 and TP2 should be basically the same under all conditions, if they are vastly different or there is any kind of AC on them it means it is operating open-loop. TP1 is the set point voltage and TP2 is the detected voltage, if the loop is closed TP2 should track changes in TP1 and not vary over changes in load and frequency. Essentially you pick the largest TP1 voltage you can without running out of range across all frequencies, then back off a little. Check after loading the counter output too, it isn't well buffered. How hard the oscillator is driven does effect its output purity, but it is pretty hard to make it stay clean right across the full range of the instrument. Once that is set, recalibrate the SWR scale with a known load, say 2:1. Because the bridge detector port isn't terminated in 50 ohms the voltage will vary for 100 and 25 ohm loads... Not much you can do about that. Sounds like your loop is oscillating even worse than mine was. The little bit of AC feedback calmed it down. There is nothing special about the values I picked, and worse units might need more feedback. Regards, Alan

25th June 2009 19:29

Tom wrote...

I have read this nice review and the comments, and I can confirm the 'quirks' which also exist in my recently purchased MFJ-207.

The "FM" problem was also in my unit, so I have carried out the modification with the resistor and the capacitor to decrease the ALC loop gain.

It does work, I now have a 'clean' oscillator signal after the mod. However, I do not know how to adjust the ALC pot (R12?).

The schematic diagram is very bad printed, so I cannot read all the details on it.

Alan, where did you find that adjustment procedure ("After this addition I reset the ALC loop control point to that indicated in the manual")??? It is not in my manual.