Johnson Desk Kilowatt

Discussion in '"Boat Anchor" & Classic Equipment' started by KK4RUT, Jul 8, 2019.

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  1. KK4RUT

    KK4RUT Ham Member QRZ Page

    Hey everyone,

    I have a Johnson Desk Kilowatt, and I need some help with it. The gentleman who had her before did not take car of her very well.

    She has been converted to solid state rectifiers. Everything else looks okay. However because I don't have a 100% complete schematic I am having a problem trouble shooting. I am using a Ranger to drive her.

    My main problem is I go to do a tune up and I have a very low plate voltage. I'm at 150 tops with it on the meter. The grid peaks to 20 fine. However when I go to dip the plate the dip is so small you just about need a magnifying glass to see it. I have hooked up the Ranger straight to a antenna and she transmits fine, it's just when I add the amplifier in to the equation.

    I have only read a operating manual, and I don't know if I am doing something wrong or what. But no matter what I can not get her to go up from 150.

    Does anyone have any ideas, or anyone who has one willing to make a video showing the tune up procedure for me?

  2. WR2E

    WR2E Ham Member QRZ Page

    You meant current, pretty sure...

    I envy you, always wanted one, now it's not to be.

    Good luck!
    WQ4G likes this.
  3. WA2CWA

    WA2CWA Ham Member QRZ Page

    If you're not familiar with working with amplifiers, be very careful. This thing has lethal voltages and a slip of a finger can send you on your way. Safety requirements of the 50's were not as stringent as they are today. The manual has all the preps and procedures to get the amplifier tuned up properly. However, it's 60 years old, and if it wasn't properly maintained over the years all the capacitors need to be checked and/or replaced. Resistors should also be checked. All this stuff ages, moves out of tolerance, just craps out in silence, or craps out when you least expect it with a big bang. And with the high voltages floating around, it could be a big bang.
    W5INC, KK4RUT and WQ4G like this.
  4. WB1EAD

    WB1EAD Ham Member QRZ Page

    Hey Zach..can't help you more than others have said.."be careful..lethal voltages inside"..there is a dedicated website for AM and everything vintage which
    I'm sure wud include this fine's The AM Forum..and if you key it in that way on Google it's the 1st thing that should pop'll have to "join"
    which is just a quick check of ur e-mail ur inquire in the Technical forum and I'm sure you'll get responses there too..a few of the moderators I believe actually have the desk kilowatt and there were LOTS of back articles and hints and kinks postings too on this amp...just another avenue to add to ur searching...GUD LUK I do envy you..73 de DAVE WB1EAD another AMer
    PS: it wud not surprise me if someone had a link to the complete manual either over there...
  5. W2VW

    W2VW Ham Member QRZ Page

    The screen supply is current limited.

    If a small amount of cathode current shows up with no apparent dip the high voltage power supply is most likely not making any voltage.

    How's the overload relay looking?

    Good manual reprints are available online and they contain tables of operating voltages that are very useful for troubleshooting.
    AC0OB and WQ4G like this.
  6. WQ4G

    WQ4G Ham Member QRZ Page

    First, I am glad to see some young people taking up Ham Radio. I was about your age when I got my Novice Ticket. That's a long time ago now...

    Second, I will say you are a LUCKY DOG. I've always wanted a Johnson Desk Kilowatt. Not that it would fit in my Shack - but it's good to have dreams....

    I assume you have downloaded a manual. The typical operating values and typical dial settings from the manual should get you close to where you need to be when TXing into a 50 ohm dummy load. If everything is right with the equipment you should be able to duplicate the settings with similar results.

    As others have suggested any old capacitors and out of range resistors need to be changed as a matter of preventative maintenance. Personally, I would probably go through and update (shotgun) all of the old Electrolytic Capacitors and Carbon Composition Resistors. You would not believe how many old rigs I have been able to bring back to life by just replacing the old Electrolytic Caps and Carbon Resistors. You may also want to clean the relays if you haven't already.

    If the Caps and Resistors are good and you are applying sufficient drive, a very small dip in the Plate Current (Ip) could indicate a weak Tube(s). Also, as the tubes get closer to maximum output the dip will become less pronounced.

    I've been looking at the tune up procedure in the amp's manual. Once you get the Grid Current set the procedure is very similar to the procedure for the exciter (Ranger). Dip the Plate Current, increase Coupling, which increases Plate Current. Then dip the Plate Current. Repeat until the maximum values of Plate and Grid currents, for the tubes, are reached. Grid current, once peaked, is controlled with drive and coupling (of the exciter).

    Hope this helps....

    Dan KI4AX
  7. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    W1TRY and W2VW like this.
  8. N2EY

    N2EY Premium Subscriber QRZ Page

    W1TRY likes this.
  9. N2EY

    N2EY Premium Subscriber QRZ Page

    There is a complete schematic in the "pages" subdirectory.
  10. W2VW

    W2VW Ham Member QRZ Page

    The person who drafted those schematics is still driving around Waseca making lots of right and left turns.
    AC0OB, N2EY and KK4RUT like this.

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