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Author Topic: What's with the mjf-66 grid dip  (Read 414 times)
NB9N
Member

Posts: 68




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« on: June 05, 2008, 06:30:22 PM »

Recently purchased a mjf-66 atachment for my 259b anylizer and cannot see any deflection from any coil!
Called mfj - Said they don't work except in tuned circuits...(or usually)  told me to buy a set from Cedar City Sales.  Did so.  They haven't worked either....  So question is, does anyone know what's the majic to make this work?

Joel/nb9n
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K4VVX
Member

Posts: 61




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« Reply #1 on: June 05, 2008, 08:03:29 PM »

Joel,
Mine works pretty well, but the "null" that you will see is not as "deep" as you will get with a dedicated GDM.  If you spin the freq dial on the 259B too fast, you might miss the null. I estimate it is about 1/2 as deep as my dedicated GDM.

Dig around in your junk box and get an inductor and a capacitor of known value and parallel them together. Use the formula in the handbook and determine the resonant frequency of the circuit and slowly tune the 259B around the freq of interest and look for the null. Make sure to use the correct range coil and to place the coil as closely coupled to the circuit as you can. Good luck.

73, Carl

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NB9N
Member

Posts: 68




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« Reply #2 on: June 06, 2008, 07:50:20 AM »

Thanks for the encouraging word Carl. My question to you is can I simply measure the inductance of a coil alone.  That's what I thought this would do. Or does it only work in a tuned or tank circuit?

Joel
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N0RZT
Member

Posts: 105




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« Reply #3 on: June 06, 2008, 09:33:00 AM »

I haven't tried this myself, but take a look-see at the 259's manual, page 13.
http://www.mfjenterprises.com/man/pdf/MFJ-259B.pdf
It has instructions "To measure inductance".

73,
Chris
N0RZT/8
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K7KBN
Member

Posts: 2756




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« Reply #4 on: June 06, 2008, 05:23:34 PM »

For measuring an inductor or a capacitor by itself, you'd be better off with an LCR bridge/meter.

However, if you have a capacitor, like a silver mica type, with a value you're pretty certain of, you can wire it in parallel with any inductor.  This forms a circuit which will be resonant at ONLY ONE frequency.  When you find that frequency, you know two of the three variables (frequency and capacitance). Now you can solve for L.
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73
Pat K7KBN
CWO4 USNR Ret.
K4VVX
Member

Posts: 61




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« Reply #5 on: June 07, 2008, 12:04:51 AM »

Joel,
Sorry about the delay. 90+% of the time a GDM is used to determine the resonant freq of a combination of capacitance and inductance. As mentioned earlier, for measuring the individual values of a coil or capacitor, a LCR meter is hard to beat. I paid about $75 for mine either at MCM or All Electronics about 10 years ago. Very handy. Nice ohmmeter also.

Yes, you can measure the inductance or capacitance of an individual component with the 259B WITH some restrictions and technique.  You need to look at the section in the manual on this.  The GDM coils are not used for these measurements and there ARE some restrictions on range, etc. If you don't have the manual, you can download it for free at www.mfjenterprises.com .  Consider getting a LCR. It will make scratching through the junk box a lot more fun. Hmmm.....wonder what the value of this is?Huh  Good luck.  
73,Carl.
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NB9N
Member

Posts: 68




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« Reply #6 on: June 07, 2008, 07:37:42 AM »

Thanks Carl!

That was the information that I was looking for.  That will save me hours of needless experimentation and or bugging MFJ company. Unfortunately when I called MJF earlier they only told me to purchase a set of coils from Cedar City Sales which I did and probably for no good reason!  Well, live and learn.

thanks,

Joel/nb9n
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K7KBN
Member

Posts: 2756




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« Reply #7 on: June 07, 2008, 09:12:38 PM »

http://search.ebay.com/search/search.dll?ht=1&from=R4&satitle=lcr+meter&sacat=293%26catref%3DC6

Quite a few LCR meters available...
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73
Pat K7KBN
CWO4 USNR Ret.
N3OX
Member

Posts: 8852


WWW

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« Reply #8 on: June 08, 2008, 08:18:49 AM »

If you're trying to measure the inductance of a coil that's in the reactance range 7 < X_L < 650 ohms at some frequency that the MFJ-259B covers, just plug it right in.

The only ones you won't be able to measure will be ones that are greater than 650 ohms reactance at about 1.6 or 1.7MHz and less than 7 ohms reactance at about 175MHz.

That's a huge range of inductors you *can* measure by directly plugging them into the MFJ-259B and putting it in "inductance" mode.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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