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Knobs fell of my E.F.J. Matchbox (Small one)




 
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Author Topic: Knobs fell of my E.F.J. Matchbox (Small one)  (Read 3086 times)
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N8ETQ
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Mort


« on: August 29, 2014, 11:15:29 AM »




    No mention in the Bama manual where to set the
knobs 0-100 for either cap.  Searched here a bit
but couldn't find any hints.  Looks like most guy's
leave the cover off, I'm not so I marked the shafts
for full Mesh on C1 and a 50/50 split on C2.

    Should that setting correspond to 0 or 100 on
the knobs?

TIA

/Dan
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Ralph W3GL
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« Reply #1 on: August 29, 2014, 01:22:14 PM »


As both caps rotate 360 degrees it really makes no difference for your setting the knobs however, as I recall, C1 (ie the tune knob) was set full mesh with the dial set at 100.

The differential cap should be set at full mesh on the lower segment with the knobs dial set at 100.  In this setting you can tune for match setting and record on a tuning chart for your antenna or go the other awy (set the upper segment full mesh) and again record match points for your set-up, whatever flips your switch...




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73,  Ralph  W3GL 

"Just because the microphone in front of you amplifies your voice around the world is no reason to think we have any more wisdom than we had when our voices could reach from one end of the bar to the other"     Ed Morrow
N8ETQ
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Mort


« Reply #2 on: August 29, 2014, 01:36:39 PM »


  Thank You Ralph,

    I'll try it that way. I ran through all the bands a couple times
and was getting "Sweet Spots" off the dial markings. I never
had or even used a tuner (Sans the GRC-19 set) here. Same
with the OWL.

    Just want to be able to reset it per a chart once I get
the darn knobs on right. I'm sure the info would be in the assembly
manual but it's not on Bama AFAIK.

    Feeding my old 75m dipole "Worm Burner" with HB OWL.

Thanks again,

/Dan






As both caps rotate 360 degrees it really makes no difference for your setting the knobs however, as I recall, C1 (ie the tune knob) was set full mesh with the dial set at 100.

The differential cap should be set at full mesh on the lower segment with the knobs dial set at 100.  In this setting you can tune for match setting and record on a tuning chart for your antenna or go the other awy (set the upper segment full mesh) and again record match points for your set-up, whatever flips your switch...





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Ralph W3GL
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« Reply #3 on: August 29, 2014, 01:55:44 PM »


Your welcome...

One word of warning with the small MB...  Always tune your antenna with LOW POWER
applied to the box.

The coupling between the two sections in the differental cap is FIBER and WILL arc over if you hit it with any power over a few watts if not properly matched... It is a job putting a proper ceramic coupling, requiring total disassembly and modification of the shaft of the cap...   

Those boxes actually have limited  range but are great for properly designed antennas.
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73,  Ralph  W3GL 

"Just because the microphone in front of you amplifies your voice around the world is no reason to think we have any more wisdom than we had when our voices could reach from one end of the bar to the other"     Ed Morrow
N8ETQ
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Mort


« Reply #4 on: August 29, 2014, 02:59:30 PM »



   That worked!  Every 50kc point on 75 and all
the 100kc points on 40,20,15 and 10 hit the
dial marks. Easy to reset. Only the top of 20m
and the top of 15m run just over 100 on C2.

Thanks again Ralph,

73

/Dan
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N8ETQ
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Mort


« Reply #5 on: August 29, 2014, 08:15:13 PM »



   One has to wonder what "Full Mesh" really means
on a split stator cap...
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Ralph W3GL
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« Reply #6 on: August 29, 2014, 08:38:26 PM »



   One has to wonder what "Full Mesh" really means
on a split stator cap...

Full Mesh =  Max Capacity.
Open = Min Capacity

Split Stator Cap = Two variable caps in series on a common shaft, both rotors
at the same potential.

Tracking variable caps = 2 single section caps with the rotors connected with an insulated coupling

You have seen the "Diferential Cap.", You describe it...

Okay?

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73,  Ralph  W3GL 

"Just because the microphone in front of you amplifies your voice around the world is no reason to think we have any more wisdom than we had when our voices could reach from one end of the bar to the other"     Ed Morrow
N8ETQ
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Mort


« Reply #7 on: August 29, 2014, 10:06:32 PM »



  Hey Ralph,

     I must be slow... It's a differential cap.. 2 stators one
rotor.. As one set of stators gets max mesh, the other is
going less..  The way I see it Max Mesh is equal mesh on both
stators, In this application they are in series as is
the other one but on the opposite phase..  Maybe
someone can "Chime In" on this....

     I'll have to bust out my "Fancy Dancey" DMM and dick
with it.

     I can already tell my SP200LX likes the 300 ohm "Native"
RX output very much..

"The coupling between the two sections in the differental cap is FIBER and WILL arc over if you hit it with any power over a few watts if not properly matched... It is a job putting a proper ceramic coupling, requiring total disassemble and modification of the shaft of the cap..."

     Yeah, Used my SB-104A at 25W for "Chart generation".
I'll hit it with the ART-13 and the 610 later. My 610
works nice at 275W carrier. That should max it out but
it looks new and I can pre-set-it... I also have a Dentron
MLA-2500 freshly converted over to Svetlana 4CX400's
so I can "Stress Test" it! If it fries, it fries. I'll
go back to resonant antennas.

Thanks again,

/Dan
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Ralph W3GL
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« Reply #8 on: August 29, 2014, 11:39:55 PM »

The small match box like you have will tolerate a KW in SSB but don't try any more than 300 or  so watts output through it with your plate modulated transmitters..  

There should not be a problem running at "legal limit" output with AM through your SSB amps in AM mode.  The peak voltages in the box using this mode are much less than with true high level modulation.  I've been using this method with the small MB for over 25 years with no real problems.

The caps in those boxes are rated at 3kV and will stand off arc-overs when a perfict math is acheved but don't push it too far on the bands where you might not be able to get the match down to less than 2 to 1 match.  

Of course, the AM activity is on the high ends of 15 and 10 meters but 250 or so watts up there does a good job most of the  time and the difference at the receiving end is very small between 200 to 300 watts...

Oh, I almost forgot, there is a file on this forum "Examining the Johnson Matchbox ATU" written by Walt, W2DU (SK) and he covers the differential caps operation in the MB's.   Check it out, it might give you an insight into its operation...
   
"Reflections and Other Thoughts from W2DU"  a good read for all users of this forum...
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73,  Ralph  W3GL 

"Just because the microphone in front of you amplifies your voice around the world is no reason to think we have any more wisdom than we had when our voices could reach from one end of the bar to the other"     Ed Morrow
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