The AM Forum

Very new to the group and new to AM...

Is it true to say that the modulation transformer in the desk kilowatt is a weak link?
Is the reactor addition a recommended modification for the desk?
Has anyone successfully completed this? Results?

Warm Regards,
Len N2LEN

Haven't heard of a lot of them shorting in the Viking Kilowatt, Len. It is the same transformer used in the KW-1, and in that service it has indeed earned a reputation for being a weak link. Partly due to its sizing perhaps, but likely related more to the splatter filter/choke system used in the KW-1. Terry, W2PFY was asking a similar question recently so you might want to look a ways down the list for his thread.

As someone who has punched a hole through the insulation of one in the latter, I'll say the less you hammer it, the better. Average use will likely be okay, but if you start playing with the values of components and driving the percentage of modulation up, that old paper will eventually object.

Not that it wouldn't be a good idea, the better question might be - where would you put a reactor in the Viking KW?  ;)

You would probably have to put it outboard in another enclosure, connected to the transmitter through HV cable.  I doubt if you could find space for a reactor in a KW-1, either, unless removing the splatter choke left enough room to shoe-horn one in.

Another reason why I have always built my own.

There isn't much room on the chassis of the Viking Kilowatt but there is plenty of space above the chassis mounted components...A reactor can be mounted (hung upside down) on two angle supports that span the top rails of the chassis..I have viking kilowatt S/N 13 and it came to me with a shorted primary on the mod tranny...  good luck   

A mod xfmr and reactor combination from one of the later versions of the Gates BC-1 series would probably fit.  The mod xfmr is considerably smaller than the  stock KW-1/Johnson xfmr, and the matching mod reactor is about the size and shape of a larger TV power transformer from the early 50's, and would probably easily fit inside.

Hello Don, are you sure there was a splatter choke in the desk KW?  I don't recall seeing it in the schematic.  

No splatter choke in the Johnson Kilowatt.  There is one in the Collins KW-1.  They both use the same mod xfmr. The splatter choke is the likely cause of so many KW-1 mod xfmr crap-outs.

Len,

I don't believe the mod tranny is a weak link in the Johnson Desk KW. This kilowatt amp/mod plays well with a Johnson Ranger and sounds gud on the receive end of the QSO.

The first thing to do; get a scope connected to that station and at least watch the  waveform pattern at all times. By not overmodulating the rig, the mod tranny will last longer.

Don't piss beat the desk; it will run a kw input to the rf final but why bother. Look at the numbers, about 402 desks were built, over 11,000 rangers, I don't remember how many transmitters in the 100w carrier range by griefkit, junkston, and others. My money says at a kw input, you are transmitting to stations that can hear you, but you will not hear them.

At low power, factory recomended settings; 305 watts input, I get about 220 watts of carrier and there is no problem being heard.

Heavy Metal Rally is a different matter, I only run the kw into something labled as a dummy load. ;D

73's
wd8kdg
Craig

In your county are you not allowed the full gallon?  ;D

"Heavy Metal Rally is a different matter, I only run the kw into something labled as a dummy load. Grin"

Does your dummy load sit at the top of a tower and have lots of aluminium elements sticking out of it?

Some would interpret that as a Dipole :)


--Shane

In my country & full gallon:

Well there are some strange agents here in one of the suburbs of "The people's republic of Eugene" Orygun. Lots of hippies stuck in the 1960's. So don't the regs of the 60's apply? We have our own rules in this country anyways. ;D

Craig,

Full gallon is anything below shooting fuel into the afterburner

If it worked for me in the 60's, it'll work for me now.

Timely discussion, I was talking with Robert W0VMC this past weekend on 40, he said he's redoing a KW-1 for one of the guys back east and highly recommends adding a reactor, considering the mod iron issues they have. Helps the fidelity as well as protecting the iron, he says. Good stuff.

So how would you go about sizing a reactor for an existing set up like these two transmitters? Building from the ground up is preferred, sure, but if you've got a rig that wasn't designed with a reactor in mind? Steve makes a great point about the big Johnson, plenty of room between the RF unit and the back deck to hang something in the middle. The splatter filter is mounted vertically inside the back of the KW-1, directly above the plate iron. It would have to be a fairly small unit to stay put there.

Please tell us about it :D :D


What are we talking about here?

There must be some confusion between the Big Johnson and hanging in the middle? Big Johnson's do not have a splatter choke and  have never have been know to hang.  Perhaps you thinking about the KW-1?

I dont remember what the inductance of the splatter choke is, but...............
I wonder if you could reconnect it as a mod reactor? ? ? ???  ???

Splatter chokes are usually one or two MFD. Too small to be used as Modulation reactors.

They are, however, designed for audio use with low capacitance between windings so I've heard of people putting them in series with a couple power supply filter chokes used as mod reactors to keep the series capacitance down. Might help with high audio frequencies.

Always wanted a Big Johnson QSL card ;D

No confusion here, Terry. It was covered clearly in my first post, and again by Don when you asked about it the first time. Steve mentioned hanging a reactor in the Viking KW in his post.  ;)


The caps are those blue tubular Plasticon types, .006 mf at 10,000 VDC and the reactor is .02 - 1.5 hy @ 10,000 VDC

You can see the actual layout here, 3rd page down:

http://www.collinsradio.org/archives/manuals/KW-1-(01-52)Sec6.pdf

L-503 is near bottom right, and C-503 & 504 are just above it.

Looks at least like a reactor of the same physical size could be hung in its place, and there's plenty of room for a big cap or two to sit right on the bottom deck. The plot thickens....

You must mean Henries instead of mfd.


That would work.  At higher frequencies where a power supply choke would be likely to roll off due to internal capacitance and core design, you would need less than 1 henry of inductance in the modulation reactor.  The splatter choke would still function as a modulation reactor at frequencies where the power supply choke acts like a dead short, but the power supply choke would serve  well at the lower frequencies.  That's a good use for those variable tapped splatter chokes sold back in the 50's by some of the transformer companies.  You sometimes see them on ePay and at hamfests.

Using one in a pi-section filter between modulation transformer and final is a good way to end up with a blown transformer. I speak from experience, and you can ask many of the owners of KW-1's.  I have been told that M-derived filters are less hard on mod xfmrs, but they act more like a sharp notch filter at cutoff, less effective out 5-10 kHz beyond the cutoff frequency.

"You must mean Henries instead of mfd."

Yup -- sorry for the confusion.

Thank you for all the input....

Would the same apply if upgrading to a Dahl replacement mod transformer....

Regards,
Len

Beware!

The initial reports on MagCap / PWDahl Co transformers was mixed.

Out of 3 people I know who have a transformer, 1 is happy, but his seemingly MAY be an old original PWD. 

The other two, one has large amounts of voltage sag, and the third guy is seemingly happy.  It's his first foray into HV and amps, so he has little to compare it to.

The one with Vsag has been replaced with a (used) PW Dahl, original, and the sag is about gone.

Transformers cost real money, which is something most people can't afford to toss around, purchasing an xformer, paying shipping to the house, finding out it's not going to work, paying shipping back, etc....  I'd do my research. 


--Shane