KW Flashbox. Here is the damage

[ke7trp]

Got a new electic screw driver and took the 100 screws out of the kw to have a look inside.  I am a bit confused as to where the arcs came from. 

The left cap marked "tuning" is arced at the fixed plates. The rotating plates are spotless. Did the arc come from the coil to the fixed plates?  Or did it jump that insulator at the ground mount?

Also.. Really odd..  The wire that goes from the meter to the coupler suffered some real damage. The wires are melted together and stiff.  How did this happen? 

The short story is that I keyed up the new box on low power, Flat SWR, I said two words and the tuner flashed over loudly  I let go of the key right away.  We went out with flashlights and found the landscapers unhooked my ladder line from the supports and piled it up on the concrete pad out back.  About 20 ft of ladder line just laying in a pile.  I guess its my fault for not letting them know not to touch that line. 

I am going to scrub this clean and repair the lead.  Maybe Try again with no coupler inline???  The antenna is now fixed and I ran my second KW matchbox the last few nights with the T3 on full smoke.  No issues.  This second KW has no bands past 40 so I would like to repair this one, put it back in line and then tear in to the other and find out why its missing bands.

C

[WA1GFZ]

While you have it apart disconnect that useless PL259 on the output.
I fried mine. I need to pull mine apart one of the feed thrus are loose. I'm going to put lock nuts on them since this is the second time.

[ke7trp]

Ok.. I cleaned the plates with Gun scrubber.  It took the carbon right off and its safe for aluminum.  I used a tooth brush and scrubbed the hell out of it. I also took sand paper and filed down the plate edges where the damage was.

Next, I repaired that coupler wire with solder and heat shrink. 

back on the antenna and the damn thing arced out the second I put 1kw of RTTY through it.  I used one of my big amps that will do 4kw.  I could not even get past a KW and it started to flash. 

WHAT THE HELL?

I pulled the antenna off and its resonant at 3600.  Thats normaly as my wire is long.  Put it back on the KW and had a Brainstorm.. I realized that the intput and the output cap both have several dip spots.  That is you can have the cap just barely meshed, Or half way or out the otherside,  I turned the amp off and found three dial settings using the 259B.  I tested each one inching the power up.  On the second setting, the arc is gone. I Put 1500 RTTY through it over and over no arc.  Then I pushed it hard for a few times. No arc. 

The knobs are off the scale, Time to loosen screws and set them back to the matching settings I used before on my chart. 

So..  Is there a rule of thumb here when setting up a tuner like this for cap mesh???   What takes more power?  I cant really see inside this thing since the window is beaming light in on the tuner, Once nightfall I can look inside using a flashlight.

C

[Opcom]

the less the cap, the more the voltage, sometimes. How odd anyway. Could the knobs have slipped and caused the settings to behave right, but be electrically wrong for the null?  Could be that you were dumping all the jiuce into the tuner and little was going out to the antenna. I found on onee old tuner here that it was possible to tune it wrongly so that the antenna didn't matter but the tuner apparently ate all the power in its losses and had a perfect SWR. It arced over at the roller inductor making a 3/8" arc with only 100W.

[ke7trp]

These things are 55 years old.  THe knobs have been off many times. The shafts are galled up from all the set screw marks.  The trouble I had here was that there are several dips as you rotate each cap.  LOOK INSIDE and pick the dip that uses the most cap. This way, You have more cap to transfer energy.  At least thats my theory anyways.  With the caps dipped and meshed, I can run any power I want.. FULL DUTY RTTY at full smoke and no arc.

C

[Ralph W3GL]

      Acquire a copy of the manual!  It is a double fold sheet of paper...

      "When all else fails, READ THE MANUAL..."

      Don't remember exactly but I think it is available on BAMA.

      Basically, using that MFJ 249 or whatever, on the input to the
      match box, twist tune and match (in that order) for minimum
      vswr on the operating frequency, attach the transmitter in place
      of the MFJ with 10 or so watts out, trim match then tune for
      MINIMUM indicated VSWR on your inline meter... The settings will
      change slightly with application of power to the box...  

      You could have a cooked bridge there on the Johnson from putting
      RF into an open line...

      NEVER apply full power on that box without first going through the
      procedure Johnson calls for in their meager manual.  As long as no
      one messed with the band taps for each switch setting, you shoul
      achieve one setting per band for minimum VSWR

      I keep an MFJ 249 hooked to a coaxial switch that switches back and
      fourth between it and the transmitter feed on my Match Box, never had
      a problem with it as long as there is a matchable load on the box output.

      Also a point to remember, the matching range is limited, not infinite... You
      sometimes have to change feeder length to get the antenna system within
      the Match Boxes range...

[ke7trp]

LOL, Ralph, I know how to tune a tuner.  The arc happend when the ladder line was piled up on the concrete. 

The manual tells me nothing about knob position. I guess I will tune for the lowest SWR and use the dip that has the most meat on the cap meshed.

C

[WA1GFZ]

Put a 6 dB pad on your receiver and tune for maximum background noiise and you will be very close. I've put 5KW through mine into the external 377 ohm dummy load of course.

[Todd, KA1KAQ]

I've been experiencing a similar issue with one of my big transmitters, tunes up fine but the loading cap arcs as soon as I hit it with audio. Weird thing is, the SWR is 2:1 or less, new feedline, and so on. HUZman checked it with his MFJ analyzer when he was down here for Shelby, and the aerial 'looks' fine, resonant at 3800 where it was cut. In fact, it only acts up if I try to tune up in the ghetto area. The same distance away from resonance on the low side, down around 3730 - 05, it works GREAT. We figured it was the Radio Gods way of saying 'Don't Operate in the Ghetto'.  I'll eventually get it working, a cleaning of the cap plates would be a good place to start. Too bad the RF deck is around 200 lbs.

As far as knob calibration, highest number on the skirt (100 on the Flashbox IIRC) should equal fully meshed. 50, half meshed, 0 fully open.

Does every Johnson matchbox out there have those crappy phenolic SO-239 insulators that are always shattered? Or did later models use teflon? I'm in total agreement with Frank, yank those suckers out and replace them with something decent. Type N would be a big improvement.

[W2VW]

I pulled the antenna off and its resonant at 3600. 

How did you measure that?

[WA1GFZ]

Never saw one with Teflon. Delete the one hanging off the output QRO will burn the insulator. You should be able to tune it to 1:1 or need to change the length of feedline ore ant. At QRO it needs to be flat.

[ke7trp]

Using the MFJ259B you can measure doublets.  My wire is somewhere around 135 to 136 long.  CW.


So far so good.  The lesson to be learned here is take a flash light and look into the Tuner to see which dip on the cap has the most meat of that capacitor meshed.  I did this, Then set the knobs for 50-50 on 3880.  Then I made a chart for all the other bands dead center. All the bands fall into the dial scale.  However, On any band that you are going to run max smoke, You need to look in and pick the dip that has the most cap.  Otherwise, you could arc it out.

To me, Testing a tuner like this into a dummy is not a test of anything.  Its when you are trying to match that open wire line antenna in balanced condition do you get those wild voltages.

I ran 3kw key down RTTY over and over.. with no flash this way for a test.  Obviously, This tuner is on the edge. Robert is gathering parts for a massive tuner for 160-80 and 40.  Then, I will use a knife switch to kick the KW matchbox in for 20 through 10 meters on lower powered rigs like the king and the icom. I think for now, using the KW with the new rig is going to be ok on low power.  I have the peak limiter setup to clamp hard at 1500 watts pep so. Sure its not loud, sure its under 100% mod. But I am having fun on the air and I want to play around with the classic xchange contest on sunday!

I mis having the internal SWR meter working on the KW.  The coupler is rated for less power then the tuner is so its probably best its out of line. I am using the bird with the slug backwards for tuneup which works fine.  

The new plate choke should be here from RF parts today, The big rig is torn to pieces right now waiting for that choke. I also put in a brand new ball bearing Dayton blower for the 4-400s. The other blower had some runout and the bearings where making some noise. The dayton is near silient

CHECK THOSE CAPS!!!!!

[Todd, KA1KAQ]

One of the shortcomings I recall about the flashbox is their limited range, not being terribly broad across a given band with a given antenna. With the ant cut for 3600, you may be reaching its limits.

Frank, I've seen a couple of the KW boxes with teflon connectors, wondered if later models used them. More likely the crappy yellow-brown phenolic ones were replaced by someone along the way.

[W2VW]

Using the MFJ259B you can measure doublets.  My wire is somewhere around 135 to 136 long.  CW.

You lost me. The antenna was measured at the feedpoint in the air or the shack?

[WA1GFZ]

I bet the connector was replaced. I bought mine a couple years before they stopped making them around 1976

[ke7trp]

Since I am using the antenna that the KW matchbox was designed to use, I have great range. I can cover any part of any of the bands at 1.0/52 ohms.  I did not have lots of trouble early one setting this antenna up. I was missing 20 meters and could not tune it. I added ladder line and then everything fell in line. There is a chart in the old 50s handbooks and it seems to be accurate.

W2VW. If you have a 259, Get the manual out and read it.  I am not sure what you are getting at but you can make simple tests of the antenna using this analyzer. The length and resonance that the 259 shows matches up if you do the math.  135 or so feet is the CW portion of the band.




C

[ke7trp]

I just got an email from a fellow ham that has the information Johnson left out of the manual!  How to align your knobs. 

Couple tips here..  Stack some books under the front of the KW so its leaning way back. This way, You can access the screws on the knobs without trouble and you dont damage the connectors on the rear. 

Two of my KWs' have flat heads, One has allens.   If you have flat head screws, Get the proper fitting screw driver. If you use one that is to small, you run the risk of sticking the screw driver off center of the screw and the plastic knob. One good turn and you crack the inside of the knob. make sure the screw driver is dead center on that screw before putting pressure to release the screw.

Also. When you put these knobs back on, Pull them about 1/4 inch out from the face. Otherwise, They will touch the cabinet and mark it up.

Here is the info:

Hi Clark,  This information was on the Johnson reflector a few years ago.  I
hope it helps and i hope to run into you on 14286 this fall.
73,
Doug Morgan  KH6U


I wish to acknowledge the post that I received from Bob Sullivan, W0YVA, who
assisted me in realigning the differential capacitor in my two Matchboxes, a
250 and 1 KW units.

Bob told me that the differential capacitor (the one on the right) was
positioned to where the rotor was half way inside each of the two (fixed)
stator sections - the one on the top and the one on the bottom.    The knob
is repositioned on the shaft to read "50" or midscale.  The rotor plates are
positioned to the left inside the stator.

I confirmed what Bob was telling me by removing the cabinet from a Matchbox
that had never been opened.

Incidentally, when the knob on the left should read "100" when the rotor
plates are fully meshed inside the stator.

E.F. Johnson did not include this information in the manual since, I
imagine, they believed that no one had any business inside the unit to
change the settings of the two variables when the tuning knobs were
removed - WRONG!

Thanks, Bob.

Dave, W3ST - W3CRA

[Todd, KA1KAQ]

Bob told me that the differential capacitor (the one on the right) was
positioned to where the rotor was half way inside each of the two (fixed)
stator sections - the one on the top and the one on the bottom.    The knob
is repositioned on the shaft to read "50" or midscale.  The rotor plates are
positioned to the left inside the stator.

Translation: when it's fully meshed, it's at 100 on the dial as I mentioned earlier. 

[WA1GFZ]

Gee the first thing I did with my match box was take it apart and jump out the antenna relay.
Todd make sure you have a good ground on the tuner and you might consider a coax balun between the tuner and the rig.
It sounds to me that the tuner is set up to transform to a higher Z at the rig. a higher Z at the rig will require less load C with higher voltage at the cap.
The tuner will transform a wide range.
A thing you could do that works well for me is tune the rig up into a dummy load. Then connect it to the tuner. Now tune the tuner ONLY to get Best SWR since the rig has been set up for 50 ohms. You have it right when the SWR is low and the rig plate /screen current is the same as with the dummy load

[k4kyv]

Bob told me that the differential capacitor (the one on the right) was
positioned to where the rotor was half way inside each of the two (fixed)
stator sections - the one on the top and the one on the bottom.    The knob
is repositioned on the shaft to read "50" or midscale.  The rotor plates are
positioned to the left inside the stator.

That is the only way to set the knob that makes sense.  At one extreme, one section is fully meshed while the other is fully unmeshed.  That would be at the setting at zero on the dial.  At the opposite extreme, the opposite section would be fully meshed and the other section fully unmeshed.  That would be at the setting 100 on the dial.  At 50 on the dial, each section would be half meshed.  That's the way differential capacitors work.

I confirmed what Bob was telling me by removing the cabinet from a Matchbox that had never been opened. Incidentally, when the knob on the left should read "100" when the rotor plates are fully meshed inside the stator.

That is the standard way dial scales with variable capacitors are calibrated.  The scale reading is proportional to capacitance, and to wavelength.  It is inversely proportional to frequency.  Zero indicates minimum capacitance.

E.F. Johnson did not include this information in the manual since, I imagine, they believed that no one had any business inside the unit to change the settings of the two variables when the tuning knobs were removed - WRONG!

Since the Matchbox was produced in the 50's, more than likely they just assumed that hams of that era would already know how the standard 0-100 dial scales on variable capacitors are set.

[Steve - WB3HUZ]

No need for a flash light. Just start with both caps fully meshed and find the first sweet spot. You'll automatically be at the most meshed setting. KISS.

[WA1GFZ]

That is my method of tuning an unknown load

[ke7trp]

Steve,  Let us know the aparatus you use to see through the metal case or where you born with xray vision?


C

[Ralph W3GL]

    Steve,

    Your KISS advice does not work with the differential capacitor on the flash
    boxes!      Works fine on the left TUNE knob...

    For the right "Matching" knob setting, as stated earlier (at the risk of
    redundancy), set the rotor midway on the left between the stators:
    knob set at 50...

    That way, "0" =  low Z match and 100 =  higher Z match on your open
    wire line.  This also tells you if you can not achieve 1 to 1 which way to
    go on the feeders (add or subtract line) to get to the magic "unity".....

    KE7TRP,

    The response I wrote that got the "chuckle" from you was from what your
     initial posting told me...

     It appeared to me, you were not familiar with the flash boxes tuning procedure.

     As Walt, W2DU stated back in his analysis of the Match Box, that differential
     capacitor in the Match Boxes is just excess  baggage... You can achieve the
     same  results with a normal ganged cap...  

    
    

[Steve - WB3HUZ]

Huh? The caps are at full mesh when set at 100. Now if you went and messed with the knobs, well, you get what you get. But who would go and do such a silly thing?  

Steve,  Let us know the aparatus you use to see through the metal case or where you born with xray vision?


C