DESK KW MODULATION TRANSFORMER

[W2PFY]

The Johnson KW uses the same tube line-up and the exact same off-the-shelf Chicago modulation transformer without a splatter filter, and those rigs seldom blow transformers.

Hello Don and others, what do you think of the modulation transformer used in the Desk KW?

I have a Desk Kw that hopefully will be on the air before Christmas. Tim, HLR seems to think it's not that great of a transformer. He says they do OK in the low power setting but when switched to high power, performance dwindles.

I plan to use a cathode follower configuration to drive the 810"s and maybe use a hysing reactor with it as well. I don't know of anyone using a cathode follower and if there is someone out there who has, I would like to hear about it.

[WQ9E]

Realize in the low power position the Desk KW can easily be loaded to over 300 watts carrier output and that is how I generally run mine.  In the high power position on 75 meters it runs over 750 watts carrier out so you are over the "new" legal limit and you will definitely find any weak links in your antenna feedline, relay, etc.

One big caution, the rails in the pedestal that support the cage are prone to spreading and if they do the cage will drop between the rails and about the only way to get it out is to do major surgery on the pedestal.   One owner reported having to cut open the pedestal to make repairs; another reported he found the rails were spreading but was able to remove the cage and use wood between the rails and walls to prevent them from spreading.  I decided to redo the roller system in mine while I was rebuilding and used some bearings and angle iron to make a better system.

The furniture moving platform from Harbor Freight is almost the perfect height to line up the cage with the pedestal, you will generally need a 3/4 piece of plywood or MDF to make it the perfect height (assuming the Chinese are fairly consistent in making these the same height).  You can put the cage in the pedestal by yourself (I have done it several times) but having two people makes it easier.  If you do it by yourself, once you have it sliding in use one hand to pull the coax cables out as it slides in so they don't get pinched on the rails or end up where they shouldn't be (like resting on a HV terminal).  They really should have included proper cable guides.

Realize that when these were built, neutral is grounded to the chassis.  You can either go through and lift neutral from the chassis or disregard the current NEC edicts.  The cooling fans are a weak spot; they are plenty big but the motors are starting to fail from old age and they sure make a stink when they do.  These same motors (4 total in the Desk) are used in the 500, T-bolt, and perhaps other Johnson rigs.  When my upper (exhaust) fan failed I replaced both of the pedestal fans with Rotron Patriot fans mounted over the same holes but external to the pedestal.  I wired them in series and run them on 120 volts and they still produce more air flow than the originals and with far less noise.  This also allowed me to add an air filter to the intake fan on the bottom.  Without a filter, the fan in the RF deck that forces air through the tube sockets tends to clog its screened intake up pretty quickly so plan on checking it at least once a year if you use the Desk a lot.

I left the Desk pretty much stock.  I did replace the rather worn plate relay with a pair of solid state relays which are reliable, quiet, and switch on at the zero crossing point.  When I bought the Desk it came with a couple of the big encapsulated 872 replacements so I removed the rectifier filament transformer and used that space and the existing holes to mount the heat sinks for the new pair of relays.  I also added separate inrush current limiting for the HV supply, 810 filaments, and 4-400A filaments.  Finally, I added an external screen current meter since there is no provision in the Desk KW for monitoring screen current.

There are at least 3 slightly different versions of the Desk KW and the variance is in the screen and bias supplies.  For operation on AM or CW I don't think it matters; for SSB the later variations are supposed to be a bit better.  Personally, I would just keep yours however it was set up originally.  The board which contains a lot of the power resistors is asbestos and it is in the air stream but unless you damage it I wouldn't be concerned about it.  There was an auction on ebay quite a while ago for a $1,000 replacement board to, in the seller's words, "avoid murdering your family".  Of course you can find pretty much anything on ebay and once in awhile it is actually something good-the rest of the time it is a worthless replacement board or an expensive SB-610 sporting the Collin's gang colors.

Rodger WQ9E

[k4kyv]

I would definitely use the Heising reactor with it.  That would improve performance and probably offer some protection against crap-out.   Also, I would mount the whole thing on insulation, and let the case float.  No point in grounding the case and placing unnecessary stress on the insulation between windings and core.

IIRC, it has two possible tap connections, one with about a 1.35:1 stepdown ratio, and another with about a 1:65 stepdown ratio.  Both the KW-1 and Johnson KW use the 1:65:1.  With a common modulator/RF power supply, this allows only approximately 100% modulation before the tubes go into flat-topping mode, leaving no audio headroom.  I would use the taps with less stepdown, which would allow some headroom on the positive peaks.  One trade-off is that the greater step-down allows the 810's to run cooler and more efficiently, whereas with the lesser stepdown the tubes will likely glow a dull cherry red when peaking 100% modulation at full strap.

Year before last at Dayton, someone had one of these transformers (unknown condx) at the fleamarket for $100.

[N3DRB The Derb]

ebay can be very good for homebrewing parts, for manufactured gear it mostly does suck. Too many people wanting the same things.

[W2PFY]

ebay can be very good for homebrewing parts, for manufactured gear it mostly does suck. Too many people wanting the same things.

Yo Derb, How does this relate to this topic 

[N3DRB The Derb]

it dont.I just felt like flapping my jaws.

[KL7OF]

I have Desk KW serial # 13...It came to me with a crapped out mod tranny....I know of two other desk KW's that have bad mod transformers as well...It seems to be a common failure.  I am looking for a suitable replacement...I have a couple of pcs of mod iron from BC transmitters but they are physically too large to fit in the space allowed by Johnson....

[Todd, KA1KAQ]

Terry, a good guy to ask about this is Skip, K7YOO. He is quite familiar with the iron as well as the transmitters. My recollection is that it's along the same lines/size as a CVM-4 (can't remember the correct Chicago transformer designation), and having burned up one myself in the recent past, let me say this: be careful. Better still, yank the original and run something with a bit more strap outboard if needed. Like the KW-1, the Viking KW was built for amateur service, not broadcast - despite some uses. As a result, many of the components and designs don't leave much of a comfort margin for the type of AM service we're used to these days.

If you just want to resurrect it and operate it as designed, it should give you good duty. Swap out the 872s for 4B32s, tap the mod tranny for the 12000 P-P terminals (vs 18000), use a bit of outboard airflow as Rodger suggests, and away you go.

[WQ9E]

Great advice from Todd and I would add one more thing to his post; check to make sure that the protective gaps are clean and set properly and make sure you use a good antenna relay.  I use a Kilovac vacuum relay with delay on break supplied by the filter cap in the DC relay supply.

[W2PFY]

Thanks for all the good tips guys, I cut & pasted this information to my Desk KW file.

again, Many thanks'

Terry

[W2PFY]

JOHNSON DESK KW / COLLINS KW1 MODULATION
810 MODULATORS TO PARALLEL 4-250A'S
PRI: 1800/2400 VCT, 18000/11600 OHMS PP
SEC: 1768 VAC @ 283MA CCS (6250 OHMS RFPA)

I was looking a the manual last night and as I recall the cathode current is 470 MA,less the grid currents, the plate should be 385 MA. If I'm correct with these readings, the transformer is running above it design parameter per above information? I have a Peter Dahl mod transformer I might want to put in. If I do use the Peter Dahl it will save the original just in case I would ever want to off this transmitter in somewhat the original condition.   

[WQ9E]

Terry,

Rated plate current is 400 mils for AM Hi and 235 mils for AM lo.  The meter reads final cathode current and Johnson is assuming 20 mils of of control grid current and 75 mils of screen current in either high or low power with resulting cathode meter readings of 495 and 330 mils respectively.

The original transformer seems to do fine running at rated power but I do run mine in the low power position loaded to 400 mils of cathode current with 20 mils of grid drive and 70 mils of screen current.  With my line voltage (stays around 122 or 244 since this is wired for 240) I have 1700 volts on the plates under load with a plate input of a bit over 500 watts and this hits the current legal limit for output.  Running it in the high power position will net you a generally unneeded and unnoticeable 3 db gain at the cost of much greater component stress.

In any case, I would suggest adding a screen meter to your Desk KW just so you will know what it is actually running in terms of plate input.  With a high power tetrode final, a screen meter is a good thing to have and an excellent indicator of proper tuning and loading.

Do make sure the arc gaps are set properly; otherwise you may be one unmuted receiver feedback episode away from a shorted modulation transformer.

Rodger WQ9E

[W2PFY]

The 283MA CCS is the AC level not the DC level

Thanks, I guess I was tired and not reading well at the time I reviewed  your post.

I have no doubt that your post is correct but I wonder where you found this information?

Again, thanks to all that have posted reply's.

[flintstone mop]

Hey
It looks like you have the solution in hand already with your Dahl unit.
But what is the shortcoming of the original transformer? Just underrated for the current or audio or a bad impedance match between modulator and PA?

Fred