Collins 20v3 help!!

Discussion in 'Technical' started by KC2HGC, Apr 9, 2016.

  1. KC2HGC

    KC2HGC Member

    hi all Steve here,

    I have just finshed converting a collins 20v3 to an 80 meter mono bander..
    the transmitter works well from 3.5 to 4.0 meg ( home brew vfo) when the antenna is hooked to
    the dummie load works very well :icon_thumbup: YEA!!

    How ever when switched to the antenna via the tuner it will sound like a cross between a cox model airplane
    engine & a spark gap transmitter :icon_thumbdown: S&IT!!

    not shore what the couse I tryed other tuners no change the antenna is an alpha delta 80 thue 160
    feed with 450 ohm latter line.
    all the meters look normal volts 2100 ma 275 antenna current is 3.5 !
    But it will work fine on the Dummie load....:icon_eh: ????

    anyone know what it can be?? please get and thanks Steve.....
  2. WD5JKO

    WD5JKO Member

    I'm not familiar with the 20v3 but it does sound like RF is getting back into that VFO. Is there a way to just use a 80m crystal? If so, then go from there. A VFO on the same frequency being transmitted is often susceptible to RF incursion from the power amplifier. Having OWL nearby almost guarantees trouble. A Retro75 XCVR with the VFO option would be ideal as a driver. The VFO runs all the time away from 80m, and beats against a crystal to get you into a ~ 50 Khz segment of the 80m band.

  3. WD5JKO

    WD5JKO Member


    You might describe that VFO in more detail. Some easy to fix gotchas:

    1.) Power the VFO from a power source internal to the 20V3

    2.) Make sure your VFO RF out ground attaches to the 20V3 chassis (exciter chassis) at the same point as your circuit return. Don't ground the VFO chassis here, and ground the VFO RF out coax there. Make them the same point.

    3.) You might consider a Toroidal 1:1 transformer to isolate the VFO ground from the 20v3 exciter chassis ground. Of course that leads into the choice of what coil material, number of turns, drive level, load impedance and all that.

    4.) Run the VFO at 1/2 frequency and have a multiplier stage 2x

    5.) Try a DDS.....Here is one I am seriously looking into:

    6.) Get rid of the OWL in the shack. Any imbalance in feeder current will cause tuner ground current that goes through all your equipment, depending on how your grounding is arranged.

    Also, if you have a Retro75 with the Helper VFO, then that would be ideal; just put inside the 20V3 behind a panel or window.

  4. KC2HGC

    KC2HGC Member

    Ok guys thanks so much for getting back!!!!

    well here are photos of the vfo this is the finished vfo on bench.

    this is the vfo under construction.

    vfo mounted in the 20V3. power conects via four feed thrue caps on the top.

    Power connection and rf in put.

    vfo dial front mounted in 20V3

    Ok as for ground theres a ground wire in the power plug and rf sheld conection now Iam not shore if this
    means anything the box that the vfo is build in has holes in it;It was a antenna swicth..

    Thats the vfo. what do you guys think?? Steve
  5. KC2HGC

    KC2HGC Member

    .1 The vfo is powed from the exciter power supply
    .2 so I will tie the circuit ground to the rf ground I guess at the rf input
    .6 Not shore what OWL is or stands for?? as for shack groung I have a ground rod under the desk and all equement
    conects via ground strap also the shack ground rod gose back to the electric company ground via #6 wire.

    Other stuff, I may need a neutralization cap there is a stub now.
    I took the 50 ohm restor off the 400 plates and put two parasitic suppressor 50 ohm with 3 turns wire
    clead it up a litte but still not working..
    Just a wondering do you guys think that the tuner can be feeding back via the antnna out put RG8 ground sheld ??
    when you think about it this transmitter just had a antenna post that copper tube tied to the tuner??

    well thank for the help 73 for now Steve
  6. WD5JKO

    WD5JKO Member

    OWL is short for Open Wire Line, your feedline to the antenna. I have a similar issue with my Flex3000 SDR station. I have a 15m loop fed with OWL coming into the shack (goes to a Johnson 275 matchbox). When I switch to the loop, and when using an amplifier, I get RF feedback back into the Flex. When I switch to my coax fed dipole, everything is feedback...

    Back to the 20V3, is it possible to use a crystal instead of a VFO as a test? The VFO you have looks pretty nice. I can see a 6SJ7 as one tube, but what is the other? A VR tube?

  7. W5HRO

    W5HRO Administrator

    Hey, that transmitter has an arc suppression circuit that will turn the plate voltages on and off. If it's not set right the whole transmitter can oscillate by turning the plate voltage on and off real fast and everything will start arcing just like a spark gap transmitter :lol:

    See the pic below. Do E108, E109 and E110 have ball or air-gap contacts? If they are what I think they are try making the gaps as wide a possible or just remove them temporally and see what happens.


    "The rf output network is a pi-section followed by an L-section. This network will feed into impedances between 50 and 72 ohms. (Other output impedance values are available on special order.) The tubes and rf output circuit components are safeguarded against short circuits or flashover in the transmitter rf output circuit by an arc-suppression circuit. This circuit interrupts all plate voltages in the event of arc-over in the output circuit and returns the transmitter to the air when the arc is extinguished."

    One or more of the gaps may just be set too narrow or RF could be getting in there and triggering that arc suppression circuit somehow.
  8. WD5JKO

    WD5JKO Member

    Good catch Brian....If that pans out, the instability will remain with a crystal versus VFO. Jim Wd5JKO
  9. KC2HGC

    KC2HGC Member

    Good stuff guys!!!

    I think I may have found it:icon_clap: RF getting in to audio !
    In testing I turned off the mixer board & the orban 9100 and I was abell to load the transmitter into the antenna
    via the johnson match box; A good 475 watt carrier !! 1.1 swr to boot! sounded good on test receiver.
    and NO OSCILLATION!!:icon_thumbup:

    there was a litte ac hum supplys will need to be cleaned up a bit...
    now I need to some how isolate the audio input or some thing like that not shore
    did you guys have this same trouble??

    What do you guys think ??
    Thanks again for all the input Steve....
  10. KC2HGC

    KC2HGC Member

    Here are some photos fo audio chain mixer orban .This gives an idea
    where things are match box is across the room. I did move the mixer power supply it was under the match box desk and it helped some...
    this is the audio out from mixer to orban and to transmitter.

    do you think maybe some toroids will help?? Well 73s for now Tnx...... Steve
  11. KC2HGC

    KC2HGC Member

    Oh By the way Brian have disconected the arc suppression and the cuppling caps off the antenna.
    output comes right off the amp meter..
    And Jim that is a 6AH6 tube and a OA2 ..
    Tnx..... Steve
  12. W5HRO

    W5HRO Administrator

    I wouldn't completely remove those gaps. Maybe adjust them and make them just a tiny bit wider. The one on the output near the RF Ammeter would help discharge corona buildup on your antenna and feedline and might help protect that meter when the other two arc.
  13. WD5JKO

    WD5JKO Member

    I wonder if you are using XLR connectors? There is often an issue called the pin 1 problem. The cable shield should go to the box chassis, but instead of doing that, guitar store box makers attach the cable shield to the circuit ground instead of the chassis ground. This is like telling the RF to go right into the box and modulate the signal ground.

    I was using a Presonus Tube Pre with a dynamic balanced Mic. I had all kinds of RFI when running QRO. As a test I noticed that my microphone was NOT grounded! The XLR into the Tube Pre floats the shield on the signal common.

    The following Rane publication discusses this topic at length.

  14. KC2HGC

    KC2HGC Member

    Ok guys thanks for the good info..
    So heres what we know the transmitter loads fine into the antenna and sounds good with the audio chain disconected.
    Sounding good meaing that the signal sounds ok on the air via the receiver.

    Now here is video of the in&out puts from the orban this is with transmitter transmitting!!
    top trace is the in put from mixer, bottom is on the orban output.
    also the audio chain is disconected so this must be the RF that getting in to the audio??
  15. KC2HGC

    KC2HGC Member

    Ok I tryed to load the video not working..
    anyway here is in&out puts with the transmitter NOT transmitting
    Sorry video not loading bummer:icon_thumbdown:!!! the audio looks fine when not in transmit.

    So that is one hell of a parasitic :surprised: !! Two questions where duz it come from and how to get rid fo it!!
    My guess ac mains ?? need help on this one..

    Tnx.. steve
  16. W5HRO

    W5HRO Administrator

    I you have a video just upload it to YouTube and post the link here and it will automatically embed in your post.