Desk Kilowatt any mods ?

[Detroit47]

Hi guys I am new to the group. I thought I would ask the gurus if there are any mods. That I should do to my Desk and Ranger before putting them on the air. And while I am at it would I be better off using my 32V2 as a driver with the padding network as opposed to the Ranger. I am just getting on the air finally. Nothing fancy just a 75 meter dipole. So I am open to all advise.

 73 Johnathan N8QPC

[WQ9E]

Johnathan,

In my folder of Desk KW stuff I have service bulletin 3 with 5 pages.  I don't know if there was a bulletin 1 and 2 for the Desk KW or if these bulletins were for other Johnson rigs.  This bulletin may have been from the Bama website but if you don't find it there I can email it to you.

There are 3 different iterations of the Desk KW circuitry as I recall, the changes were in the screen and bias regulation circuitry.  I believe the changes were to improve performance as a linear amp on SSB so if you are using it only for AM and CW I would not worry about making any changes to these circuits.

With the Ranger, make sure you change the 18 K dropping resistor in the VFO to one with a higher wattage rating (10 watts would be a good conservative choice).  You can change it through the left side (your left viewed from the front) without having to take the entire VFO apart.

With the Desk KW, clean out any dust (particularly from the screened inlet on the bottom of the RF deck) and lubricate the fan bearings.  Even so these fans are likely to be showing their age and I had one go up in a smelly cloud of smoke a few months ago.  I am now using a pair of external fans with a filter on the intake side for the pedestal.  Check and clean relay contacts as needed, pay close attention to the contacts on the plate relay.  Neutral is tied to ground inside the Desk KW (common for the era), if you wish you can go through and provide separate neutral tie points above ground-I did not do this for mine.

You must provide an antenna relay that has a fast make and a slow break as indicated in the instructions, you don't want to hot switch the output of the Desk KW.  There are arc gaps designed to contain the damage from this type of event but you don't want to rely upon them; however do adjust them properly before operating the Desk KW.  Also check to make sure that the overload relay is properly set.

On 75 meters I find that the Ranger provides too much drive even with minimum loading so it is necessary to reduce drive to the Ranger final on this band.  Since the RF and audio are separated at the Ranger you are not modulating the Ranger final so running with low drive is not a problem.  The Desk KW was designed for the old 1,000 watt input AM limit which is now over the legal limit since it produces around 700 watts carrier out (at least on the lower frequencies) with a theoretical peak output of 2,800 watts fully modulated.  I usually run mine in the AM low position where it produces around 300 watts carrier output and runs cool as the proverbial cucumber.  If you do run it close to the rated limit make sure that all of your feedline/antenna hardware is up to snuff as high power high level AM will demonstrate (generally with Pyrotechnics) any shortcomings of your hardware.  If you run it in the AM high position but with lowered RF input then also make sure that you reduce the audio level appropriately.

I made a few changes to mine:  the plate relay was in really bad shape and it already came with SS replacements for the 872 rectifiers so I removed the existing rectifier filament transformer and used this space to mount a pair of SS relays and their heat sinks for controlling primary power to the plate transformer.  I added inrush current limiting for the 4-400A and 810 filaments, the screen supply, and the plate supply.  I also added a resistor in series with the screen lead with a small external meter across this so that I can monitor final screen current.  I really think that every high power tetrode final should have screen metering.  Even though the 4-400A screens are pretty tough compared to many modern external anode tetrodes the screen current still is the best indication of tuning for tetrode amps.

Finally, exercise real caution with this beast.  When I was a young "johnny novice" one of the old timers had one of these and he was neutralizing it after replacing the finals.  This is done with a long screwdriver through the front (behind the tuning logging dial) and he was flipping the HV off to make adjustments.  The relay stuck on and he was thrown across the room (the insulation on the screwdriver apparently wasn't up to 2500 volts) but fortunately survived with no long term consequences.  It made a big impression on me!

Enjoy the setup, it is a classic.  I first had mine paired with an HRO-50T1 like the Johnson ads but the selectivity left a little to be desired for AM so I am now using my Ranger/Desk with an SX-88.

73, Rodger WQ9E

[WV Hoopie]

Johnathan,

Roger has touched on most of the points. I'll second the motion to stick with the Johnson Ranger for the exciter. This little rig is made for the Desk KW, mates up just fine and has great audio.

On to the arc gaps: The gap on the mod tranny is set at 0.050" and I don't see any changes in the service bulletins. But, G-100 in the RF plug-in unit was increased to 0.100" in service bulletin #3 dated 9/19/55.

From the information at hand (service bulletins) the first Desk KW's to leave the factory, serial numbers 100001 through 100300, more than likely had 4-250A toobs as finals. Later units left the factory with 4-400A toobs. I've been able to find 4-250A's for about ten bucks each at festers. So running on Lo-Power, 305 watts inpoot, they loaf along. Even turning up the wick a pair of these are still running within specs. Hard to find 4-400A's at that low price, but if you got'em.........they are plug n' play. The switch to 4-400A's was more than likely due to use as a linear for SSB.

As to fast make and slow break, already mentioned, get the Johnson KW matchbox and don't fool around. If no one has butchered the tuna, the relay in it is ready to go. Might want to replace the electrolitic that is across the relay coil. Damn thing is dried out after all these years anyhow. Gud tuna and it will handle full scrote from the desk. Just build the antenna correct, the matchbox doesn't have a wide matching range.

Funny Roger should mention the HV plate relay. Someone took an ignition point file to mine, almost no contacts were left, all filed away. Worked fine on low power, full scrote and they started to arc&spark just a little. Potter and Brumfield still makes the same relay.

872 MV toobs are to be cursed! Damned things spit, arc, spark and other unmentionable things. I replaced mine with 4B32's. Life is good.

So, last but not least, get the manual. Download it from BAMA if nessassary. Read it. Then go though and check all the bias settings and neutralization before anything else. Check every nook and cranny for mods, loose wire/connections, poor solder joints of past owners, etc. This will save $$ in the long run.

73's
wd8kdg
Craig

[Detroit47]

Rodger thanks for the advice. I have bulletin #3 also, and I was looking it over. I had written a letter to Johnson in 1993. Asking if they had any old literature, and they sent me some in house assembly instructions for timing the gear train on the Pi network. I will share them with you if you like. I have gone through the whole amp it was a basket case when I got it. I replaced the original fans with new stuff the old ones were slow.  I recapped the low voltage stuff. The Hi voltage caps are OK I brought them up slow.  I put some Penta graphite 4-400’s in the broadcast ones.  On 75 meter into my dummy load it loafed at 700 watts on the Bird. The previous owner was an 11 meter operator.  The guy took good care of it but was clueless as to what it was. It came with an extra mod transformer. But alas I have only the pedestal and no desk. As an after thought how would the johnson electronic tr sitch work with it. I am just going to run a staight dipole wiith a  1 to 1 balum. I was going to run it with an HRO 50t but I decided to use my 75-A3 instead. With the 6kc filter the audio is awesome.  I would like to know just how many of these beasts are left. The last of the 402 that were made was born in 1957. Once again thanks for the advice my next project is a Johnson 500 ala basket case.

73 Johnathan N8QPC

[WQ9E]

Hi Jonathan,

You are welcome and I would appreciate a copy of info on timing the gear train; hopefully I will never need it but you never know!  That little Johnson TR switch probably creates as much heat as the rest of the Desk KW.  I am not a huge fan of these type of switches but someone else can probably better advise you on them.  Electronic T/R switches can be huge TVI generators and the power rating for the switch assumes a very low SWR.  I use a vacuum relay (from Fair Radio) powered by DC from a transformer/rectifier/filter fed by the switched 120V AC from the antenna relay jack.  I can't remember the value now but I chose the filter capacitance to provide a slight delay on break and I have used this setup for 3 years now with no problems and no arcing.  I don't know how many Desk KW's are left but a number of them are missing the desk itself; I imagine a large number were bought without.  Mine was literally in pieces when I bought it, a previous owner had disassembled the unit and had all of the panels including the Ranger newly painted and silk screened.  Reassembly took awhile but the end result was a Desk KW that looked new.

When you get to your Viking 500 make sure to adjust the arc gaps to spec and then if you get any arcing it is probably due to the mechanical delay relay.  The relay controlling the antenna switch contacts in the 500 is supposed to provide a slight mechanically derived delay on break but I don't think that worked out very well.  Mine would flash over the gaps on break and I went in and added a low value electrolytic cap across the relay coil and an isolating diode in series and this provided enough delay to avoid hot switching the antenna.  My 500 still uses the original and much maligned HV connectors without any problem.  I have a strong suspicion that the arcing of these connectors is actually due to a combination of hot switching the antenna and too wide of a gap in the protective devices; part of the manual advises opening the gap somewhat if arcing is a problem and once this is done the arc occurs at the next closest gap which turns out to be between the connector and ground.

Both the Desk KW and Viking 500 get a lot of use in my station and the only failures in the Desk were one of the fans and a crimped power connection that finally gave up after some 50 years.  Before you do any audio mods on the Viking 500 please be aware that for minimum clipping the clipper control must be at full clockwise rotation.  If you turn it fully CCW (to the intuitive minimum position) you have about 30 db of clipping which sounds horrible but fully clockwise there is no clipping.  My Viking 500 is paired with a Pierson KP-81 receiver and is a nice near legal limit rig.

73, Rodger WQ9E

[Detroit47]

I've had the 500 for about 15 years. I found it in the yellow sheets. I wound up trading a guy in Fort Wayne a Pride Dx300 and a Phantom 500 for it. He was a 11 meter operator and it was in peices. The unit came with some spare parts and a new Peter Dahl mod transformer. I don't know which would be better the Pete or the stock one. I put it back together and it works but I want to do it up right. It has a few battle scars but that just gives it a little character. At least there are no extra holes or switches.

73 Johnathan N8QPC

[KL7OF]

Any idea on a replacement for the overload relay??  I also need a mod tranny....  I have serial number 13 complete with desk and chair....Good luck with yours Johnathan....  Steve

[WQ9E]

Steve,

I will go through some info I have tomorrow but you should be able to find a substitute for the overload relay without too much difficulty.  It is a mechanically latching relay that requires a pulse to the reset coil after an overload.  I will go through my box of relays and see what I can find that will physically and electrically fit.

Rodger WQ9E

[W3NP]

As an after thought how would the johnson electronic tr sitch work with it.

I used a Johnson  electronic TR switch when I had my desk KW back in the early 80's. It worked fine except that the signal suckout from the RX was pretty severe and it was very sensitive to the coax jumper cable lengths and the tuning of the final.
I ran mine at full power most of the time (remember - early 80's) without any problems.

As was mentioned the 872's were a source of grief for me and I soon replaced them with 4D32's.

The biggest problem I had with mine, and an area that bears examination BEFORE a problem occurs is the bannana plugs and jackbar for the RF assembly - especially the ones that carry HV. I had some arcing occur there and it was a difficult job to get it straight (carbonized the fiber material between the jacks). Make sure the plugs are tensioned very well (fit tightly into the jacks) and that the entire bar is squeaky clean. It doesn't take much arcing and by the time you discover the source, the damage is done.

I also had a short between the HV choke winding and frame but was able to dis-assemble the choke and repair it. After that happened I mounted the choke above ground.

I thought my serial number was 100600, but if there were not that many made, it may have been 100200. It was one or the other.  I had the desk with mine.

BTW: I had to pay a whole $200 for the complete unit and that after it sat at a hamfest almost all day!!!

I initially used a Viking II with swamping as a driver but switched to a Ranger when I obtained one. I never did use the Ranger audio but instead used an Eico 730 moduator and a suitable transformer.

I sold mine shortly after I got a KW-1 as I didn't have the room for both.

Good luck.

[WV Hoopie]

Aaaahh, those banana plugs. Replacing the RF plug-in back into the jackbar is a job for two. One is needed to lower the unit and another has to guide the beast into place. The banana plug for ground was damaged when I inspected mine, serial #100124, after bringing it home. These items are still available, think I purchased a few from either Newark or Mouser.

While the RF plug-in is out and on the bench, remove all those screws, every last one of them and peek inside every section of the RF unit. Make sure everything is hunky dory! Don't forget to remove the fasteners holding the cooling fans. Then oil the felt pad surrounding the shafts and bearings. Fan motors are now made of unobtainium.

Someone had opened the meter section on mine and used it to learn how to solder. Glad I found the globs and loose connections in there before applying power. Metering would of been interesting.

[Todd, KA1KAQ]

I sold mine shortly after I got a KW-1 as I didn't have the room for both.

Just had to respond to this Dave, because I did the very same thing with my first Viking KW (which I also paid $200 for, complete back in 1986). The KW-1 appeared in '88, and the Viking KW went to 6 Land via Yellow Freight.

Picked up the present one (s/n in 400s) in 1994 from W1MMV's estate, still don't have it online. It was missing a few parts like the wooden frame for the top lid which I recently located in Texas. Still need to track down the relay for atop the mod tranny and leads for the 810s. I've always wondered if ol' Gerry blew the mod iron and was using it for CW only.

I'll echo what Dave, Craig, and others have said: 4B32s are the way to go, drop-in subs for the 872s, easy to find. Good bye arcing and also the lengthy wait for the mercury to vaporize after not using the thing for a while.

Enjoy the beast!