[600MRG] modified MFJ Antenna analyzer failure

Thu Mar 21 13:37:24 CDT 2013

John,
       You are very lucky that you have only one 50kW AM transmitter and 
it is 20 miles away. I have 4 50kW and one 40kW transmitter within a 
dozen miles of here.

73 Warren K2ORS


On 3/21/2013 12:59 PM, John Langridge wrote:
> Ed,
>
> Thanks again!  Just as I sent my message to the list, I received the following
> response from MFJ so I will be giving them a call:
>
> "must be the schottky diodes d1-d4 then.  those are 2820 surface mounts.
> call jimmie allen, analyzer repair technician, 1-800-647-8324 or 1-662-323-0549
> for more information."
>
> As you say, $70-80 is not bad for a $400 unit unless this happens every 6
> months.. really not sure what happened as nothing was different from a normal
> session.  Its been more humid so static is not what it had been and no storms
> were around... who knows?
>
> Anyway, at my QTH I've never had any problems with BCI even with a 50kw around
> 20 miles west of me.  Interestingly, the FM's and TV's 5 miles south of me seem
> to cause most people around here the greatest problems, but its never been an
> issues for me.
>
> 73 and thanks again!
>
> John /NJD/XIQ
>
>
>
>
> ________________________________
> From: Edward R Cole <kl7uw at acsalaska.net>
> To: John Langridge <jlangridge at sbcglobal.net>; 600m research group
> <600mrg at w7ekb.com>
> Sent: Thu, March 21, 2013 11:43:43 AM
> Subject: Re: [600MRG] modified MFJ Antenna analyzer failure
>
> John,
>
> Coincidentally, I blew the front-end of my MFJ-269B.  I was checking
> the match of a HB coax switch matrix and 26vdc was applied to the MFJ
> antenna.  They specifically warn against applying any dc
> voltage.  There are diodes in the ckt that probably are gone.  I
> debated fixing it myself but decided the possible pcb damage from
> removing surface-mount devices made factory repair a better
> choice.  I wrote them a month ago and here is the reply:
> ------------
> Subject: Re: Repair of MFJ-269B
> From: mfjcustserv at mfjenterprises.com
> To: "Edward R Cole" <kl7uw at acsalaska.net>
>
> we now say $70-$85 parts, labor and return shipping just to cover
> us.  thanks for your business.
> ----------------
> Considering it is nearly $400 new that seems reasonable.  I have used
> MFJ repair once before on this meter (for another issue).  I plan to
> remove the modifications before shipping and add a note not to repair
> the 4-MHz oscillator selection as I will be re-installing my mods.
>
> Use of the antenna analyzer is subject to strong BC signals so it is
> limited where this is not a problem.  I have one nearby station about
> 16-miles south of me on 920-KHz.  There are stations on 550, 600, and
> 750 KHz in Anchorage 65 miles NE but they are not strong enough to
> confuse my measurements.
>
> 73, Ed - KL7UW
>
> At 05:05 AM 3/21/2013, John Langridge wrote:
>> Hey guys,
>>
>> At my station, part of my SOP is to adjust my antenna system before
>> and during a
>> session using my modified MFJ259B.  The mod is the one detailed by
>> KL7UW and I
>> have found that its measurements are "in the ball park" compared to known
>> accurate meters that I have borrowed in the past.  Last night, I had
>> a failure
>> with my analyzer and it has been suggested that the detector has finally
>> popped.  50 ohms loads are now 39 ohms and reactance changes very
>> little from 0
>> even when I know it is much higher.  I have always been very careful to make
>> sure the analyzer is not connected to anything when transmitting and that was
>> the case again last night.  I am hoping someone knows a solution to
>> this problem
>> short of sending the box back to MFJ.  I suspect my mods, which are
>> all crammed
>> inside the chassis, will get wrecked during repair or they may not
>> even want to
>> work on it given it has been modified.
>>
>>
>> While I try to find a solution, I am wondering what methods you guys
>> are using
>> to ensure a match.  How do you know in a pinch that its 50 ohms at the feed
>> point or do you worry about it?  I think we can all agree that a
>> matched system
>> works best and given my low grant ERP, I don't want to waste
>> anything and think
>> that careful attention has contributed to my success thus far. Minor
>> variations
>> probably do not matter too much and the data supports that fact and
>> my FETs and
>> current limited power supply have never blown apart as the result of
>> an untimely
>> mismatch but I exercise an overabundance of caution so I am not experiencing
>> downtime on a regular basis.  I have been pleasantly surprised at
>> just how much
>> abuse the FET's I use can take, however.
>>
>>
>> Here are a few of my options as I see it:
>>
>> I can used the remote control variometer to peak antenna current and I know
>> where it should be but I also have seasonal variations due to foliage.
>>
>> I know about how much current my PA draws for a given voltage when
>> everything is
>> working.
>>
>> I probably have a forward power meter that will allow me to peak the output
>> using the variometer, perhaps peaking that in conjunction with the base
>> current.  Accuracy would not be important on the meter but it may change the
>> system when I take it out of line (it shouldn't but I've seen it
>> happen before).
>>
>>
>> long term I need to look a mod for an SWR meter.  There was one
>> floating around
>> at one point for a simple heathkit meter mod.  anyone have the pdf
>> or a link for
>> it?  I have looked for it in the past but never found it and admit
>> its probably
>> right in front of my nose.
>>
>> I hear good things about the scope match option.
>>
>> adding a motorized coil to my shunt match would allow me to handle minor
>> seasonal or weather-driven variations...all of these are something to think
>> about in the future...
>>
>> Any additional ideas or guidance would be appreciated.
>>
>> thanks!
>>
>> John KB5NJD/WG2XIQ
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>
>
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