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Author Topic: Versa Tuner II Capacitor  (Read 5973 times)
KD2AJN
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Posts: 24




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« on: August 07, 2012, 09:24:38 AM »

Hello everyone,
This morning my MFJ-941E Versa Tuner IIk started to make a mean buzzing noise. When I opened
the case, I found the variable capacitor on the 'Antenna' knob was arching between the plates. I wanted to
know what the model number and specifications were for this cap, so that I can order a new one. It is
the farthest to the right knob.

Thanks,
Joe
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K8AXW
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Posts: 6994




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« Reply #1 on: August 07, 2012, 09:50:25 AM »

Suggest you simply contact MFJ and tell them about the problem and where it's located.  I'm sure they will sell you an replacement.  

If the plates aren't melted perhaps you can clean the plates.  Then you should determine why the cap is arcing.  Are you exceeding the tuner specs?  Are you trying to load an antenna with an SWR so far out that the tuner rating is exceeded?

It's always a good policy to try to determine why a component fails so that when you replace it, you don't ruin the new one.
« Last Edit: August 07, 2012, 09:55:33 AM by K8AXW » Logged

A Pessimist is Never Disappointed!
WB6BYU
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Posts: 17790




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« Reply #2 on: August 07, 2012, 12:33:18 PM »

I'd first check your antenna.

If the tuner had been working fine with that antenna, and you haven't change output power
significantly or done something different recently, then the most likely cause would be that
something happened to the antenna or the feedline that caused the SWR to jump, resulting
in higher voltage across the capacitor that it can't handle.

Usually if you fix the antenna, the tuner will still work properly without replacing the capacitor.

It might also be a bug, metal shaving, or other contamination that lodged on the capacitor
plate.  In any case, unless there is obvious damage to the plate that you can't file down
smooth (sharp points will arc at lower voltage levels), replacing the capacitor is rarely
required.


Think of the capacitor voltage rating like circuit breaker:  when it trips, it is rarely the
circuit breaker that is at fault.  And when you do find the fault and clear it, you don't
have to replace the breaker unless it was damaged in the process.


If you do have to replace it, just call MFJ and get a direct replacement.  Or, if you want
to try to find one at a local hamfest, look for a 208pf variable with the same plate spacing
that will fit in the available space.
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KD2AJN
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Posts: 24




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« Reply #3 on: August 07, 2012, 03:34:00 PM »

The antenna I'm using is a an MFJ g5rv antenna, rated for 10 - 40 meters with a tuner. The arcing does not occur on 20 meters or 15 meters and gets great SWR. This tuner works 40 meters OK on my random wire, without any arcing. Could it be that mfj's claim of 10 - 40 meters isn't true? ground connections and ladder line are all attached tightly.
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WB6BYU
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« Reply #4 on: August 07, 2012, 04:57:02 PM »

Quote from: KD2AJN

Could it be that mfj's claim of 10 - 40 meters isn't true?



Have you used the tuner before on 40m at the same output power where
you are having a problem?  If you have, especially with the same length
of coax, and didn't have a problem before, then we need to understand
what has changed.  It is more likely that something outside the tuner case
changed than something inside (unless you've had the case open while
milling aluminum or brass next to it, or poking the capacitors with a
screwdriver...)

How long is the coax feedline from the end of the twinlead to your tuner?

How much power are you running when the capacitor arcs?  How far do
you have to reduce power to get it to stop arcing?


If this is the first time you've operated on 40m at that power level, then
the problem is more likely due to an impedance that the tuner can't handle
at that power level on that band, given the way you are adjusting the tuner.
That's a big sentence because it contains a number of possible factors to
consider.

A tuner matches an impedance, not an SWR.  That is, the tuner settings, and
the voltages across the various components, will be quite different when matching
10 ohms or 250 ohms, even though the SWR is 5 : 1 in both cases.  And there
often are multiple settings that will match the same impedance, some more
efficiently than others.  The impedance at the tuner will vary widely from one
band to the next, and also with different lengths of coax.  So just because
it can get a good match on one band, or with one length of coax, doesn't
mean it will do so under other conditions.

(And just because you can adjust the tuner for a low SWR doesn't mean that
the antenna is necessarily working:  it isn't at all uncommon to be able to
match a feedline when the far end isn't actually connected to anything at all!)


Certainly a good first step is to inspect the capacitor plates closely at the point
where it is arcing to see if one of the plates is bent or there is some debris that
is causing the capacitor to break down.  Anything that makes the one plate close
to the plate on one side of it than the one on the other side reduces the minimum
separation, and therefore the breakdown voltage.


Another factor is how you adjust the antenna.  Wherever possible I look for a
match with one capacitor fully meshed.  Sometimes that isn't possible, but always
keep one of them as close to maximum as possible to get a match.  You can get
a match at other settings, but that will mean a higher voltage across the capacitor.



Quote

...ground connections and ladder line are all attached tightly.



So the ladder line is attached directly to the tuner, rather than using a length of
coax?  That's good for efficiency, and allows operation on a couple extra bands.

But it brings up another question:  can you hear signals on the antenna?  Have
you made contacts using it on other bands?  Did you connect the jumper wire
between the single wire port and one side of the balun output connections?
Without that jumper wire, the tuner isn't connected to the feedline.  (It's a
common occurrence.)
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K4JJL
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Posts: 1011




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« Reply #5 on: August 09, 2012, 07:28:25 AM »

Check the antenna connections on your G5RV.  I had a Van Gordon G5RV break at the top where one wire of the twinlead connects.  It made my Dentron SuperTuner arc, as well.  It would still tune up, but would arc when I put more than 50 watts into it.
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K9YLI
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Posts: 1262




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« Reply #6 on: August 12, 2012, 07:41:23 AM »

I have had  one of the two twin leads break at the feed point several times. (160 meter version)  it would still load some places as an end fed long wire..
when you fix the broken wire (if its broken)  , get a 8 or so inch piece of  3/8ths  rubber  automotive hose,  use several  black  zip ties , tie it to the  center insulator.
then more zip ties and tie up the twin lead so there is a small loop, to take the strain off the  wire connections.  Won't break the wires again that way..
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KD2AJN
Member

Posts: 24




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« Reply #7 on: August 14, 2012, 07:37:11 AM »

Quote
Have you made contacts using it on other bands?
Yes, I worked London and Poland on 20 meters yesterday. Today I am going to open the tuner again and
clean the insides and try to repair any warping on the plates of the cap. I will try to post back again tonight.
Thanks for your help.
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KD2AJN
Member

Posts: 24




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« Reply #8 on: August 21, 2012, 07:22:04 AM »

OK, I cleaned out the capacitor and found it had a warped plate. I bent the plate back to shape. Today I keyed up on 40 meters and it tuned beautifully. Thanks everyone for the replies, I suppose I should have examined the cap a little bit further at first.
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W8JI
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« Reply #9 on: August 22, 2012, 05:41:02 AM »

OK, I cleaned out the capacitor and found it had a warped plate. I bent the plate back to shape. Today I keyed up on 40 meters and it tuned beautifully. Thanks everyone for the replies, I suppose I should have examined the cap a little bit further at first.

The single most common cause of a failure like yours is adjusting the tuner wrong. Read the manual of the tuner. If it is an updated manual (I rewrote the section on tuning many years ago, and they were supposed to get it into all manuals) it should tell you to use the maximum possible capacitance that allows a match.

Whatever you do, do NOT tune for maximum signal or noise without regard to capacitor or inductor settings. Follow the manual, and preset to a starting value and tune using maximum possible capacitance that allows a match.

Voltage across the capacitors increases with less capacitance and decreases with more capacitance. In one case you could tune and handle 500 watts with that tuner, and with the SAME load by using more coil and less capacitance, you could make the tuner arc at 50 watts.

When you sand or file on a variable capacitor, the voltage rating will never be full again. This is because the plates are micro-polished when originally manufactured. You can easily knock 30-50% of the voltage rating off once you get a sharp edge or burr on the plate, even at microscopic levels.

73 Tom



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