Johnson Adventurer help

Discussion in '"Boat Anchor" & Classic Equipment' started by K5DH, Feb 13, 2020 at 1:13 AM.

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  1. K5DH

    K5DH Ham Member QRZ Page

    What function does R13 (2,700 Ohms) perform in this oscillator tank circuit?

    upload_2020-2-12_19-11-5.jpeg

    I'm planning to modify this beast to operate on 80-40-30-20 meters (sacrificing 15 and 10). When I re-tap the oscillator coil, do I just leave the resistor there?
     
  2. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    It's to prevent the tube from saturating too far....theoretically giving a slightly cleaner waveform. Not sure how it would affect a different tap.
     
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  3. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    upload_2020-2-12_16-48-10.png
    Right from the horse's mouth.
     
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  4. N2EY

    N2EY Premium Subscriber QRZ Page

    No, that's not its purpose. Sorry.

    R13 serves two purposes:

    1) In transmitters like the Adventurer, the grid current of the amplifier is adjusted to the correct value by detuning the tuned circuit.

    In the Adventurer, on 80, 40, and 20 meters, the 6AG7 oscillator is capable of producing far more grid current than the 807 needs. The resistor dissipates some of the RF so that the tuned circuit needs to be detuned only slightly.

    2) The resistor helps reduce the chance of the 807 stage oscillating, since it is not neutralized.

    Best bet for 30 meters is to leave it in the circuit.

    73 de Jim, N2EY
     
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  5. W8AAZ

    W8AAZ Ham Member QRZ Page

    Have you tried tuning it up with a 10 Mhz crystal ? In the 7 MHz position? See if the tuning range of that position can cover 30M? After all, alot of novices got burned with those rigs tuning them to the second harmonic, I have heard. I have one of these rigs but would have to dig out a crystal. Got a 10.00 somewhere but it is not FT 243 style. I may have an adapter. I would consider that worthwhile as if you hack the radio with that mod, you might end up losing resale.
     
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  6. K5DH

    K5DH Ham Member QRZ Page

    KL7AJ and N2EY: Thanks for the info. And yeah, I missed the text in the manual (duh). I'll leave the resistor in place when I re-tap the coil for 30m.

    W8AAZ: I have several FT-243 crystals in the 30m amateur band, but I haven't tried to load the rig up on 30m in the 40m position. It's no big deal to re-tap the coils properly for 30m, and I have no interest in using the rig on 15m or 10m, so I'll just rewire the bandswitch. I'm not worried about resale value, 'cuz this beast has no resale value. It came to me as a cruddy, trashed-out, $20 junker with a shorted power transformer. The cabinet and front panel had been poorly spray-painted "battleship" gray and the panel had been re-labeled with gray Dymo label tape. It was UGLY. With nothing to lose, I tore the rig down to the bare chassis and completely rebuilt it. The vintage Stancor power transformer I used is much more stout than the original and I now have +750 VDC on the 807 plate. The rig works well on 80-40-20 but crappy on 15-10. I figured it would be more useful to me if it would also work on 30. When the weather warms up, I'm going to paint the cabinet and panel, probably a nice hammertone gray, and use rub-on lettering to label the panel, protected by clear-coat paint. Should be a neato little rig when I'm done with it.
     
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  7. N2EY

    N2EY Premium Subscriber QRZ Page

    How did you redo the power supply? The classic "economy" supply that gives two voltages?

    73 de Jim, N2EY
     
  8. K5DH

    K5DH Ham Member QRZ Page

    Yep, classic ciruit. The power transformer HV winding is 280-0-280 VAC @ 280 mA. I ran the two outer windings through a bridge rectifier to a capacitor-input filter for the PA supply, and ran the CT through a choke-input filter for the oscillator supply. Plenty of filter capacitance in both supplies to keep it clean. In both stages, I come off the plate supply with screen dropping resistors. I also wired the unused 5 VAC secondary winding in series "buck" with the primary winding to bring the 6.3 VAC heater voltage down to a reasonable level. Works well and used all junkbox parts.

    Did I mention this thing was UGLY?

    upload_2020-2-13_17-40-55.jpeg

    And it had a fried power transformer?

    upload_2020-2-13_17-41-17.jpeg

    It took hours to clean up that burned spot on the chassis.

    upload_2020-2-13_17-50-18.jpeg

    Here's what it looks like now. Quite a few changes from stock. Added a 3-position rotary switch for Power/Spot/Send. Also added T/R relay with muting contacts.

    upload_2020-2-13_17-41-59.jpeg

    upload_2020-2-13_17-42-14.jpeg

    upload_2020-2-13_17-42-36.jpeg

    No guilt at all. Nobody else would have bothered, and it would have ended up in the landfill.

    I sold the original (working) panel meter to a fellow who was actually restoring an Adventurer. No sense in that going to waste!
     
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  9. W8AAZ

    W8AAZ Ham Member QRZ Page

    Well no shame about the meter. Alot of people replace that dang jiggly thing with a properly damped vintage meter movement and it makes tuning so much easier! I got mine down and found the HC-6 10 MHz crystal. First I tuned up on 40M into a load to make sure everything was humming. Then I tuned up with the 10 crystal in. Got enough grid drive in the 40M position but it was with the cap at minimum. Then I tuned the output. Got nearly as much output as I get got on 40 but with the plate cap at about 86 or so, getting close to minimum. Then I checked in the 20M position to see if max cap would tune down but no luck. So you can estimate your taps to probably be maybe a little closer to the 40 than the 20 but I would try right in between for starters. I was right on 10 and you will be tuning a bit higher in practical use.
     
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  10. N2EY

    N2EY Premium Subscriber QRZ Page

    Well done! And a much better design than the original! Of course, in those days, they didn't have inexpensive silicon diodes of suitable rating, so the economy power supply would have cost more and taken up more space.

    If my math is right, the oscillator is running at maybe 250 volts on the plate. While that's quite a bit lower than the original, it's more than adequate for the bands you are interested in.

    Wow! Talk about making a silk purse! Nothing I can see to be guilty about.

    I've used the elevated-transformer trick myself. The pillar-on-the-back-so-you-can-work-on-the-rig-upside-down is a favorite too.

    The cheap moving-iron meters were pretty standard for the times - the Adventurer, DX-20, DX-35 and many other bottom-of-the-line rigs used them. They were about 15-20% of the price of a D'Arsonval meter back then.

    Thanks for the photos.

    73 de Jim, N2EY
     
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