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Thread: Dentron mt-3000a Tuner

  1. #1
    Beach Bum KA2PTE's Avatar
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    Post Dentron mt-3000a Tuner

    Picked this tuner up at a nice price the other day. Its missing the dummy load resistor and the pc board that has the swr sense circuitry. Curious how hard it would be to
    build a board and populate it with the correct components?

    In this video you can see the dummy load and the other thing, I guess F1 at about 3:10 mins in. Plus the SWR board mounted the same side. He shows the schematic also at some point.

    Was also wondering about building a "protect" mode somehow, where if I could find the right resistor, rigg something up where all the antenna inputs get grounded thru the
    dummy load, as a means to protect the system from lightning strikes.

  2. #2
    'Grumpy old bastid' kb2vxa's Avatar
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    First of all there is no video or link to one, so this is all guesswork on my part. Unless you have a boat anchor transmitter or amplifier with plate and load controls the dummy load has gone the way of the Radiosaurus. Since I cut my teeth on a metal shell 6SK7 (glass tubes were hazardous to mouth parts) I've always built separate dummy loads and tuners, I've never seen a need to combine them. You can find plenty of info on the Internet, plugging "cantenna" into your search engine is a good place to start.

    When using a tuner an SWR meter is essential, so if you don't have a transceiver with one built in you need an external one. Call me Mr. Obvious. (;->) Again I prefer separates, the cheapest CB SWR meter you can find will serve you well. Don't let "CB" throw you, they're good from 160M to about 40MHz. My reasoning behind separates is shielding lacking in less expensive consumer products and accompanying inaccuracies I have experienced especially at higher power levels.

    I never suggest anything I haven't done myself, like when working around anything above 12V keeping one hand in my pocket. That includes the monstrous voltage and current in a lightning bolt, never expect a dummy load to handle them! What were you thinking? The last thing you'd ever want is directing lightning into the shack! Always mount lightning protection devices outside where the common ground stake(s) are located and I for one left unused transmission line ends on the floor and out of the way. That way a strike SHOULD find ground outdoors and not look for one inside destroying everything in its path like what I have seen and once narrowly escaped being part of said destruction. I've always had a deep respect for electricity, it has a nasty bite that in my younger and stupider days nearly took my arm off. Some say real radios glow in the dark, I say 12 volts is for wimps, real radios can KILL you.
    "The universe is under no obligation to make sense to you."
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  3. #3
    Administrator N8YX's Avatar
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    Look around for a surplus Motorola line sampler. These have SO-239s on each end and I've seen them with power ratings to 3KW and beyond. Install inside the tuner and cable up the coaxial feeds, run the Fwd/Ref sampling outputs to the metering circuit of your choice and you're done.

    As far as the dummy load goes, I would install an SO-239 on the back panel and run a coax line to a Cantenna or similar unit. You'll get lots more power dissipation capacity.
    "Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."

  4. #4
    Beach Bum KA2PTE's Avatar
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    Whoops u are right. This is the vid:

    https://www.youtube.com/watch?v=rYFuBw7eSG0

    Looks like 2 resistors probably in parallel to get 50 ohms at about 3:10 into the show.



    Quote Originally Posted by kb2vxa View Post
    First of all there is no video or link to one,

  5. #5
    'Grumpy old bastid' kb2vxa's Avatar
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    Sure, you can put 2 100 ohm resistors in parallel like 2 100 ohm dipoles driven in phase for a lazy H beam, but, there is a big BUT. BUT the Cantenna uses a collection of resistors in a series-parallel configuration for greater heat transfer to the transformer oil in the can.

    Outside of the can or other grounded shield a dummy load for dummies makes a perfectly matched antenna. I found out when a test was heard a few miles away from my basement radio room. I remember a how to article on making a wide band mobile antenna with a 1:1 match from 1.8 to 29MHz in an April issue of QST.
    "The universe is under no obligation to make sense to you."
    Neil deGrasse Tyson

    73 de Warren KB2VXA
    Station powered by atomic energy, operator powered by natural gas.

  6. #6
    Administrator N8YX's Avatar
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    Quote Originally Posted by kb2vxa View Post
    I remember a how to article on making a wide band mobile antenna with a 1:1 match from 1.8 to 29MHz in an April issue of QST.
    Sonny Irons turned that concept into "Max-Comm".
    "Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."

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