[600MRG] modified MFJ Antenna analyzer failure

[Edward R Cole]

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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