[600MRG] modified MFJ Antenna analyzer failure

[pehamel]

John,
First, email mfjcustserv at mfjenterprises.com I believe that they will repair 
the detector.
I have asked them if they will do the mod for me, or make a kit, but I 
believe they are working on an improved model instead.

I repaired (not modified) the heathkit HM-102, the problem was that the 50 
year old composition resistors had changed value.
When I put in resistors of the right value, the original toroid pickup 
calibrated.
The internal calibration is actually a resistance bridge switched in to set 
the meter pots.

I have an MFJ cross-needle meter in line. It reads wrong (absolute power), 
but the reflected reading null matches what the Autek tells me.
Do not use an MFJ 815B. The RF sensor in the "B" model RF sensor uses two 
toroids, not the toroid and capacitance pair circuit.
The second toroid is across the line. Mine got hot enough to melt the 
plastic wire insulation. I now have an 815C sensor inside the case.

Last night my own SWR went to infinity, I will need to go outside and see 
what happened.
As usual, there is always the possibility that some critter got hungry for 
feedline. (Spring is here in south Mississippi).

Pat W5THT & WD2XSH/6
w5tht at arrl.net
Unhappy tubes blush, unhappy power FETs scatter plastic loudly


----- Original Message ----- 
From: "John Langridge" <jlangridge at sbcglobal.net>
To: "600m research group" <600mrg at w7ekb.com>
Sent: Thursday, March 21, 2013 8:05 AM
Subject: [600MRG] modified MFJ Antenna analyzer failure


> 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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