Johnson Desk KW failure

Discussion in 'Technical' started by KE7TRP, Dec 30, 2016.

  1. KE7TRP

    KE7TRP Member

    I used the desk for 2 year years on 40 meters. No issues. One day I turned it on and noticed both the plate current and Grid current meters pegged backwards.

    I pulled the RF deck and found the RF parts plate choke burned up on the mid section. I do not think this was the failure. I believe this happened over time. I remember one transmission where I was long winded and smelled something burning about 6 months ago.

    I found the original 4-250A tubes installed. This is serial number 265 so it came with 250s. I believe one shorted and that is why the meters where pegged backwards.

    I installed a new Eaglemade 10 to 160 plate choke. The choke is rated at 10KV and 3 amps. I had it on hand as a spare. Took 5 minutes to install. I also installed two known good Eimac 4-400 tubes.

    The desk is back up and running. I have it on the low taps at 2100 volts B+. I get 800 carrier and 3200 PEP, 120 percent pos peaks and 85 negative peaks as measured on my LP700 Station monitor.

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  2. WD5JKO

    WD5JKO Member


    You dog!.......I am still waiting on my LP-500 Station monitor (#391 down from about 770).........So when did you get yours? Your thoughts?

    Perhaps we could use this to bring up RMS power again...:razz:

  3. KE7TRP

    KE7TRP Member

    I have had it about a month. He told me they are 6 months out for the next batch. I like the monitor. Its really neat. I am not a huge fan of the RF couplers it uses. At high power they can fail and or get warm. So far so good. I have to sensors, One at radio and one at amp. This way you can do the Trapazoid comparison while tuning. It has a really cool AM Mod monitor that also shows O scope, SWR, AVG and Peak power. I forgot I even ordered the thing. I just got an email one day saying it was ready
  4. W5HRO

    W5HRO Administrator


    Wow, Major Zorch !!!

    What are you using for the exciter/driver, a Ranger? I wonder if the middle section overheated because of one of those typical big spurs the Johnson's are known for putting out up the spectrum? If so maybe it found resonance there from one of it's lower products, maybe the origin product.


    Those station monitors look nice, but I noticed one problem. The positive peak scale only goes to 140% :icon_angel:.
  5. KE7TRP

    KE7TRP Member

    I think it really was just to much for the choke. But we will never know. I am using a Ranger 2 as an exciter. I will have to run a test on the KW to see if it has any spurs. I have never heard of this before. Maybe the tube shorted and took the choke out. Funny thing is, there is never any sign. I used the rig, signed off and turned it on the next day to find the problem.
  6. W5HRO

    W5HRO Administrator

    The Johnson VFO's inside of those transmitters are very dirty and produce a lot of byproducts. That and the fact all the stages just have tank circuits which are capacitor coupled all the way across to the final tube grids doesn’t help matters any. My old Valiant was bad about putting out a big nasty one up in the 200Mc range. At night when I was operating on 75-meters my ham neighbor down the street said he was listening to me on his 220 transceiver on SSB (no AM mode). When he first told me I didn't believe him until about a month or so later when I heard Ozona Bob talking about how he tried everything to eliminate it in the one he had. He even tried pounding all the round buss wires flat with a hammer and soldering them back in and nothing worked. Then later when I used mine on the Desk KW it just made it 50X worse :icon_angel:

    Anyway, I cannot remember where the Rangers outputted their junk, but I do know the Valiant was the worst one. Even the old Viking I had a problem and I think it outputted a big one in a spot right dead smack within the TV band’s spectrum. You never know, it could have overheated from just about anything, but it could have also been from one of those VFO byproducts and maybe even from the origin one and that center winding was it's resonate spot.
  7. NewWave

    NewWave Member

    Looks similar to all chokes i have seen before with these problems.
    Highest current was running in the burning section.
    You have made nice spaces between the windings, but if you use thin wire, the spaces will have no function. (To much uH's in one section)
    Use other wire with better isolation and make more small spaces between it..