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Author Topic: MFJ 986 Tuner  (Read 2013 times)
EI5GUB
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Posts: 66




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« on: January 20, 2013, 08:53:56 AM »

Hi im having problems with an MFJ 986 Tuner,the roller coaster has 4 spacers along the length of it,these are made from material like circuit board stuff so are supposed to be no conductive,however mine is arcing like crazy between 1 of these boards and the base of the chassis and is buring the material,I put 2 layers of special insulating material underneath it but it arced through both of them,this is all with 500 watts in a Tuner thats able to take a few Kw and also the SWR is fine when it happens,any ideas any one please,cheers Richard
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WB6BYU
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Posts: 13481




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« Reply #1 on: January 20, 2013, 01:41:41 PM »

What load impedance are you trying to match?  What frequency band?
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KE3WD
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Posts: 5689




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« Reply #2 on: January 20, 2013, 02:48:15 PM »

And also, What is the load? 

The antenna used plus the band you are trying to tune might just be developing high voltages in the tuner...
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EI5GUB
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Posts: 66




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« Reply #3 on: January 20, 2013, 02:53:52 PM »

Hi firstly it started on 20 meters using a super loop antenna,now its happening using a windom also,it is arcing like crazy like a small welder,I cant understand how fibre material is able to arc,I sent an email to MFJ to see if they can send me the 2 pieces of fibre and I will rebuild roller coaster and clean the coil and wheel inside as this seems to be a problem,this tuner is 5 years old and has always worked perfect
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WB6BYU
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« Reply #4 on: January 20, 2013, 05:25:29 PM »

Once it starts arcing, you get carbon deposits that reduce the effective voltage
rating, making it more likely to arc in the future.  If the carbon gets into the
fibers under the original finished surface it may be difficult to clean it off.

The load impedance makes a big difference in the power rating of a tuner.  I have
a tuner  built with components from a commercial 200W AM rig, and have seen
the capacitors arc at 10W when matching a radical load (the cord to my soldering
gun on 80m, IIRC.)  You really can't rate a tuner for a given output power unless
you also restrict the impedance over which that rating applies.

Is the tuner matching at the same settings (at low power) as it normally does?  I
wonder if perhaps there is something wrong with the antenna or feedline that is
presenting an abnormal load impedance.  It is also possible to generate higher
than necessary voltage inside a tuner when the settings aren't optimum, even
though the SWR is good.  (It isn't unknown for a tuner to get a match with no
antenna connected - the loss resistance in the inductor is dissipating most of
the applied power.)  I always aim to match an antenna with one or the other
capacitor fully meshed, which should put you very close to optimum efficiency
and minimum voltage across the components.  Of course, you might have just
hit an unfortunate spot in the roller inductor where a stray resonance put a
high voltage point right at the insulator.
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EI5GUB
Member

Posts: 66




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« Reply #5 on: January 21, 2013, 11:10:08 AM »

hi again,the antenna is a home brewed version of Radio works super loop,it always worked well on 80 and 40 with no arcing atall but when I go to 20 mtrs it tunes fine but when the linear is turned on all hell breaks loose and we start cooking,the feeder is new,since this big arc took place last week the carolina windom is now also arcing on 20mtrs,I sent an email to MFJ to see if they will send me replacement fibre parts that are burnt,i read on a few sites that this problem can be common and for rollers and wheels to get dirty which can give eratic swr which mine has done,thanks again
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