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Boatanchor EF Johnson KW Matchbox




 
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Author Topic: Boatanchor EF Johnson KW Matchbox  (Read 695 times)
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flintstone mop
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« on: August 24, 2021, 07:49:04 PM »

Hello everyone. YUP OM MOPMAN is still alive.........75 yrs young now.
Are there any good mods for the Johnson KW Matchbox to get on 160M? I google and see that someone had a mod in an ER mag. I ordered the ER magazine to read what he did.
Did EF Johnson ever release any mods to get this box to 160M? My OWL is 150 feet long and it is feeding a VEE @ 65 feet..........240 feet of antenna wire. Ends are 15 feet high from dirt. Antenna system seems to present itself as a high Z system. An Ameritron ATR-30 will not tune anything on this antenna.
I am using the tried and true K1JJ Monster tuna.
I am experimenting with efficiency of good power transfer on 160M.
Getting ready for AM radio season here.
Thanks
Fred
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Fred KC4MOP
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« Reply #1 on: August 26, 2021, 11:31:58 AM »

Hello Fred,

Many decades ago in my infancy I picked up a Johnson KW Matchbox at a hamfest. I didn't really need it as I was just starting out in amateur radio but it just looked so cool, and it was made by Johnson, so I got it. Cheap. Years later when I actually pulled it off the shelf to use it I discovered that it had already been apparently modified for 160. The paperwork that came with the box included this xeroxed note from Johnson about adding 250pf capacitors across the tuning capacitor sections. Mine has two 500pf doorknob caps instead. Switched in by two vacuum relays. I've never actually gotten it to work decently on 160. With very long wires (400 to 500 feet) I'm able to tune it initially with the capacitors switched in but after a few seconds the SWR starts to climb slowly upwards. I replaced the caps and the SWR still wanders around so I'm thinking maybe some bad solder joints on the relays are causing it? Don't know and this problem got buried on my list of things to do so I haen't investigated it in awhile. So find some doorknob capacitors and vacuum relays and see if you can get yours to work. Maybe I'll see you on 160 if I can ever climb towards the top of my repair to-do list.

Mark - WA2FXM


* KWMatchbox.jpg (1115.45 KB, 2448x3264 - viewed 156 times.)

* MatchboxOn160.jpg (2286.69 KB, 3264x2448 - viewed 159 times.)
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K9MB
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« Reply #2 on: August 26, 2021, 08:52:56 PM »

Hello Fred,

Many decades ago in my infancy I picked up a Johnson KW Matchbox at a hamfest. I didn't really need it as I was just starting out in amateur radio but it just looked so cool, and it was made by Johnson, so I got it. Cheap. Years later when I actually pulled it off the shelf to use it I discovered that it had already been apparently modified for 160. The paperwork that came with the box included this xeroxed note from Johnson about adding 250pf capacitors across the tuning capacitor sections. Mine has two 500pf doorknob caps instead. Switched in by two vacuum relays. I've never actually gotten it to work decently on 160. With very long wires (400 to 500 feet) I'm able to tune it initially with the capacitors switched in but after a few seconds the SWR starts to climb slowly upwards. I replaced the caps and the SWR still wanders around so I'm thinking maybe some bad solder joints on the relays are causing it? Don't know and this problem got buried on my list of things to do so I haen't investigated it in awhile. So find some doorknob capacitors and vacuum relays and see if you can get yours to work. Maybe I'll see you on 160 if I can ever climb towards the top of my repair to-do list.

Mark - WA2FXM


The Johnson note specifies A high voltage Mica capacitor which will have something close to a NP0 curve with temperature shift.
My experience with those CRL doorknob caps is made of a ceramic dialectric that has a -700 to -1500 ppm/degree C shift with temperature.
Putting a pair of these in a tuned circuit with a high current will cause considerable heating, I suspect and when they get hot, these caps drop in value a significant amount. While they may be ok for a non-critical plate coupling circuit where the value is not a big deal, a tuner, by definition has a critical value for resonance and for matching. I would get rid of the door knobs and put some surplus vacuum caps in or some large transmitting micas and maybe it will work, though the L-C ratio cannot be optimal for 160, so losses will likely be high..
There are some nice 160 tuners on this forum history.
The Johnson Tuner is good for what it was designed to do, but they were not hinking of 160 meters back then.
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