Johnson kW AM

[John K5PRO]

I picked up a local Junkston Desk KW today. Took a couple of strong men + me to load it onto my trailer. Are there any tricks, mods, suggestions here on this transmitter?
Last week I got the Ranger exciter which needs some resto.

[WQ9E]

John,

Congratulations on acquiring a Desk KW.  I have had mine for about 8 years now and use it frequently.

There were three iterations of schematics with the major changes in the screen and bias supply area.  These were mostly to improve operation for high power SSB, if yours isn't the latest AND you plan to use it extensively on SSB then you might consider updating.  Otherwise I wouldn't bother.

One unusual and very serious problem that can occur is the bottom rails in the pedestal that support the sliding cage assembly can spread apart.  If this happens the "works" will drop to the bottom of the pedestal and the only way to remove it is to cut the pedestal open.  The easiest preventative measure for this is to wedge wood pieces between each rail and the side of the pedestal to prevent them from spreading outward.  When sliding the cage back in place it is nice to have a helper to keep tension on the cables (coax, control) coming out the back to keep them from jamming in the pedestal.  The Harbor Freight 4 wheel carpeted furniture mover is the perfect height to receive the cage assembly if you need to slide it completely out and move it around.

The fans are a potential trouble area, they are old enough winding failures aren't uncommon.  One of mine went up in smoke a few years ago.  These are the same fan motors used in a lot of Johnson gear but all of them are getting old.  I ended up replacing the two pedestal fans with external fans and I built a mount that didn't require any hole drilling.  These are Rotron patriot AC fans that move a lot more air and are quieter than the originals and as a bonus it was easy to fit a Rotron filter to the bottom (intake) fan which greatly reduces dust buildup in the pedestal. I plugged a three outlet short extension into the outlet which powers the Ranger so all fans come on at once.  I have the remaining good pedestal motor as a possible replacement for the two RF module fan motors.  Before adding the filter I found the grid compartment fan picked up a lot of dust on its inlet.

As you know the Desk KW can run far more than the current rated limit. I generally run mine in the low power position loaded a bit more heavily and it produces 350 watts output with everything running easily.

The Desk is wired like a lot of older gear so that the ground wire also functions as the neutral return.  You can lift the neutral off chassis fairly easily but since my Desk KW (and other 240 volt gear in the main floor radio room) is powered off by a separate line with shut off switch to kill power when not in use I left it as original.

I added inrush current limiting for the various supplies in mine.  They are cheap and reduce stress on components.  I also added a small metering resistor in the screen supply and snaked a cable to a small external meter through an existing cabinet opening.  I don't like high power tetrode amps without the capability for screen metering.  In the Desk KW the meter reads cathode current so that meter reading includes the sum of control grid, screen grid, and plate current.

When I got my Desk KW it had been taken apart and the panels (and Ranger) had been newly silk screened.  So basically I put it together like a kit and I built a power cord with two fuse holders so I could do initial testing with small value fuses.  I ended up using normal value fuses when testing was complete and leaving the holders inline, otherwise your Desk is dependent upon your breaker panel for protection.

My Desk KW has been reliable.  The only repairs have been because of the fan and one failed connector in the primary wiring.  I am sure you will enjoy yours!

[KL7OF]

John...congratulations on the desk....
Rodger...Your setup is very nice....I used to own KW desk S/N 13.   Mine had linoleum on the desk top....I noticed yours doesn't ...

[WQ9E]

#13 was right up there at the start of production?  Were you the original owner?  I bet there was a lot of excitement waiting for that one to be produced.

The linoleum for mine disappeared before I got it.  I have been thinking of installing some more but it is on the long list of projects yet to be done. 

I had a Valiant as a novice and a year later acquired just the RF module out of a Desk KW. It was soon traded for a tower and rotor and it was many years later before I acquired a complete Desk KW. 

My best Desk KW story was I rebuilt it in the barn and there was a lot of assembly work to be done.  I replaced part of the wiring harness that had been damaged and also the selector mechanism for the mode switch with a more sure assembly since I didn't like how the cheap original could easily stop in-between selections.  After all of the work and the fact that it had been out of service for over 20 years my confidence level for the first power-up was low.  I had just the exposed cage sitting on the outfeed table for my Unisaw with HV disconnected.  Initial power up was fine and everything glowed that should and nothing smoked or complained.  I then changed the mode selector and when the very loud shorting relay for the modulation transformer changed position I nearly had heart failure.  It is extremely loud and I was already hypersensitive for anything going wrong and I jumped several feet in the air and jerked the plug out of the wall.  I figured out what the noise was but suspended testing for the night after that event.  It turned out fine and that relay isn't very noisy once the cage is in the pedestal. 

My Siamese cat had a similar experience before I discovered the problem with the delay relay in my Viking 500.  In the winter she loved hanging out on top and enjoying the nice gentle warm breeze from the 4-400A inches below. But it turns out that the magnetic delay relay in the 500 power supply loses its delay capability over time and when that happens the antenna switches before the HV decays causing the arc gap to fire and in this case the cat to go into full launch mode.  I cured the problem with a capacitor and isolating diode across the relay coil but since that experience the cat keeps her distance and has found more relaxing heat sources.

[WD8KDG]

John,

You will enjoy playing with the Johnson KW, it is a real kick in the pants! A year or two ago, I sent a bunch of service bulletins to BAMA and are found somewhere on their website, gud reading. I think the first 300 of the KW's left the factory with 4-250A's. Most of the time my KW runs on low power AM. This is somewhere around 305 watts input and about 225 watts of carrier. At that power level the 4-250A's are loafing. Then consider at festers, a 4-250A brings a price of about $20.00, far less than a 4-400A. But if'en you got a pile of 4-400A's..............strap.

The MV 872's got to go..................replace them with 4B32's or SS. Other than replacing the MV toobs, use a Ranger as the exciter and it is just plug & play.

Craig,

[Detroit47]

I got to ask two questions. How much power will a desk key on AM high side. Secondly where do I look for the serial number on mine. I have never fired mine up. It came to me as a basket case. It was totally disassembled with a bad mod transformer. I was able to lay my hands on a stock mod transformer but that is as far as I got. I have read that some folks have taken the B+ off the Mod trans secondary. Ala mod reactor and such. Is this worth the bother? I hope I am not hijacking the thread.

John N8QPC

[WD8KDG]

I got to ask two questions. How much power will a desk key on AM high side. Secondly where do I look for the serial number on mine. I have never fired mine up. It came to me as a basket case. It was totally disassembled with a bad mod transformer. I was able to lay my hands on a stock mod transformer but that is as far as I got. I have read that some folks have taken the B+ off the Mod trans secondary. Ala mod reactor and such. Is this worth the bother? I hope I am not hijacking the thread.

John N8QPC

Hi John,

Hijack away, it is all a learning process. Power: With the set of 4-250A's in my desk, an easy 700+ watts of carrier. Never spent much time twisting the knobs to tweak more. Next time I turn the mode switch, might try to see what the old gal will do. Serial number: It is on a tag attached to the frame which roles out of the cabinet. Suffering from CRS and ain't about to role the guts out any time soon to look!

Taking the B+ off of the Mod Tx seconday.............don't know about that one. Mine seems to play quite well per factory wiring.

Craig,

[KL7OF]

#13 was right up there at the start of production?  Were you the original owner?  I bet there was a lot of excitement waiting for that one to be produced.

The linoleum for mine disappeared before I got it.  I have been thinking of installing some more but it is on the long list of projects yet to be done. 

Roger..I wasn't even close to the being the original owner of #13..It was a real mess when I got it...I traded it to Keith KE7GPO who has done a beautiful restoration on it...I hear it on 75 meters quite often....

[WQ9E]

My experience is the same as Craig's, the Desk KW will easily run 700 or more watts carrier output.  However I run mine at about half that power using the low tap, at 350 watts carrier output I will settle for being S-9 +17 dB instead of S-9 + 20dB  I am sure the Desk KW will cruise along with no problem at full power but it is certainly running cooler and with less stress at the insignificantly different lower level.

Make sure your antenna and feedline are up to the stress of this power level.  I wouldn't try to run the Desk KW on a 120 volt line at higher power and as I recall Johnson states the draw can be up to 30 amps at 120 volts.

I don't recall reading about any problems with the modulation transformer although excessive audio or incorrect loading can kill one, especially if the protective gap hasn't been properly adjusted.  Using a mod reactor was probably as much an audio quality mod as for longevity of the transformer or perhaps someone really wanted to push the power capability of the Desk KW.

[WQ9E]

Roger..I wasn't even close to the being the original owner of #13..It was a real mess when I got it...I traded it to Keith KE7GPO who has done a beautiful restoration on it...I hear it on 75 meters quite often....

There are at least two in this area that were gutted and turned into SSB only amplifiers.  A couple of others were completely parted out.  Even though this was an expensive piece of gear I bet at least 25% have been parted out or modified to the point of no return.

[John K5PRO]

I'll look for that serial #. I have no idea of how many owned this one. The desk is gone cept for the pedistal. Yes, that sheet metal pedistal already has wood chunks in the sides. The casters are missing and the edge they push into is bent so was thinking of a plywood base that fits underneath with casters on it. Maybe that would help secure those side rails too. It has 4-400s. I'll replace those 872s. Me too on the desire for Ic meter, I use tetrodes mostly and screen current metering is helpful. This one has the original manual too. I picked up the Johnson kw marchbox with it.

[W9BHI]

I have serial #91.
Replaced the missing plate iron, solid stated the plate supply and added a mod reactor.
I drive it with a Johnson Adventurer and an outboard speech amp with processing.
More power than one could ever need.
Not worth the effort to use it on SSB.(that's why God made SB-220's)
Great transmitter.
Mine needs some cosmetic work done on the cabinet to finish it.

[Todd, KA1KAQ]

The serial number tag is located between the modulation transformer and tube sockets on the bottom right edge IIRC. Been a couple years since I looked at one. Had two over the years, never had either on the air. The last one now resides at GMS Radio in Ercildoun PA where it should be hitting the airwaves soon.

[W3GMS]

The serial number tag is located between the modulation transformer and tube sockets on the bottom right edge IIRC. Been a couple years since I looked at one. Had two over the years, never had either on the air. The last one now resides at GMS Radio in Ercildoun PA where it should be hitting the airwaves soon.

What a fantastic day, not to mention the best XMAS present my wife ever gave me.  She has given me many great rigs over the years, but this JDK is definitely at the top of the list.      

Here is a picture of the SN Tag on my JDK acquired from Todd, KA1KAQ.  

A lot of the restoration has been done and can't wait to put it on the air which will hopefully be soon.  

Joe, GMS    

[Opcom]

Nice going on the Desk! Can't think of anyone that deserves one more. I seen you lift some heavy stuff but I know how big those desk KW are.. I do remember a frequency monitor you picked up that was too much for me. I have it all cleaned up BTW - have not used it yet tho, or the BTA-250.