KB7OCY Modulator/Power Supply Restoration

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1950's AM Rig.jpg
1950's AM Rig
1.87 MB
1950's AM Rig-mod section.jpg
AM Rig-mod section
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1950's AM Rig-mod section-rear.jpg
AM Rig-mod-rear
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6 mtr-BB gun.jpg
6 mtr-BB gun.jpg
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73's from KB7OCY.jpg
73's from KB7OCY.jpg
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first layout attempt-no good.jpg
1st layout-not good
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KB7OCY writes:

I received from a fellow ham, Jim W7GNP, an entire dual-rack, 80"-tall transmitter with three transmitters, two power supplies and one humongous modulation transformer.  My buddy Fred and I dragged this beast out from the outside eaves where it had been sitting for who knows how many years, slid it onto our trailer with a winch, and brought it home. 

It was physically in very poor condition: lots of rust, tubes shot out from kids with BB guns, etc.  BUT, all the pieces from an original 1950's home brew build were all intact!  Actually attempting to build a project like this without an entire garage full of "vintage" junk these days might take years of dedicated hunting for parts... but there it was... complete!

main power supply.jpg
main power supply
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main power supply-01.jpg
main pwr supp-01
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main power supply-02.jpg
main pwr supp-02
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mod deck filaments-old-working.jpg
old mod deck fils on
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mod deck-old-front.jpg
old mod deck-front
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mod power supply-old-test.jpg
old pwr supp test
1.01 MB

Here's a little history I have gathered on this rig so far:

Jim told me that the rig was originally built by the Fenwick triplets as a high school project sometime in the fifties.  Are you kidding me?  High schooler's built this awesome rig?  Apparently their  father was an electrical engineer who worked at a motor/transformer rewind shop on East Jefferson and had the giant 2200VA filament transformer rewound into a modulation transformer... I guess they were after the large iron content of the core.  

I began speaking with a few other hams who told me that they have seen it go from one estate to another, some wishing to buy it, but it was just "too heavy" to deal with.  I have also been told that when this rig was on the air, the audio quality rivaled that of commercial stations.  From what I gather, the three transmitters are 50, 144 and 220mhz units.  The main power supply for the transmitters was a pullout from a 2400 volt distribution transformer.  The modulator's power supply is on its own chassis and is rated at 3kv / 500mA, has a pair of 3B28 mercs, a choke and an oil capacitor.

mod power supply-refurb-test.jpg
refurb pwr supp test
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mod xfmr and PS-complete.jpg
mod xfmr & PS
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mod xfmr-back old.jpg
old mod xfmr back
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mod xfmr-front old.jpg
old mod xfmr front
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mod xfmr-front old-01.jpg
old mod xfmr front
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mod xfmr-new-spark gap added.jpg
mod xfmr-spark gap
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With that kind of info (and where are you going to find genuine "Bud Radio" steel chassis), how can you not do SOMETHING cool with this?

I pulled the modulator's rusty, dusty chassis out, cleaned it up a little, and presto!  After a 24 hour warm up on the 3B28's.... 3kv under load; the supply was still good.  So I cleaned it up, leaving all the original components intact and adding just a few mods along with rewiring.

Then I took the covers of the ends of the monster modulation transformer for an inspection.  Having built transformers in my younger years, I noticed what appeared to me as a superb job on the rewind.  A turns ratio and hi-pot check revealed it to be in very good condition, so it too got cleaned up.

modified 6L6 supply-01.jpg
mod 6L6 pwr supp
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modified 6L6 supply-02.jpg
modified 6L6 supply
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new layout-01.jpg
new mod layout
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new layout-02.jpg
new layout-02
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new layout-03.jpg
new layout-03
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new layout-04.jpg
new layout-04
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Now here's where the actual modulation deck came into the picture.

The original chassis had the 304tl's placed on top of the Bud chassis with a "blow dryer" type fan pointed at them to keep them cool.  Pretty lame design, but it must have worked very well.  I checked the 60A / 5V filament transformer, and it too was OK.  Hmmm.  The chassis vas very corroded, tube sockets shot; etc.

It just so happened that as I while inspecting the 6L6 drive circuit and drawing a "mediocre" schematic of the modulator, something struck me as "I've seen this before".  I went to my cabinet and pulled out an old, tattered 1950 ARRL Handbook given to me by a SK's son several years back, and there, on page 292, was the exact schematic diagram of the chassis I was looking at!  Wow!  I was looking at a project I wished I could have afforded to build when I was a young lad!

It was then that I began thinking: "I've got to do something with all this stuff... if anything else, to build a 1950's, era-correct project".  

new layout-05.jpg
new layout-05
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new layout-06.jpg
new layout-06
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new layout-07.jpg
new layout-07
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new layout-08.jpg
new layout-08
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new layout-09.jpg
new layout-09
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new layout-10.jpg
new layout-10
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I began asking fellow hams on the 40 and 75 meter AM "window" their opinions on "how to".  I incorporated their experiences into the "resurrection", if you will, of the modulator deck.  It appears that the most important thing is to keep the 304tl pins cool, followed by the glass.  From what I have gathered so far, Eimac recommends about 15CFM or so per tube, with ample air blowing across the envelopes.  I wanted to keep a short profile on the chassis, so I opted to place the tubes in a chamber.  Air is sucked in from the bottom center using a 100CFM box fan and is redirected through glass chimneys past the tubes.  I understand that this is considered "not conventional" for 304tl's, but my buddy Chris (aka Dr. Spark) has a chimney around his single 304th Tesla coil (20 inch+ sparks) and his use of a chimney allowed him to essentially double the original input power compared to just blowing a breeze across the glass. So I'm hoping this is a good idea.

To help cool off the 304tl pins and reduce the air through the chimneys, I punched a 2 inch hole on either side of the chassis and installed "adjustable" concentric baffles which seem to do an excellent job balancing the air flow as well.

The modulator isn't wired yet, but I'm ready!

new layout-11.jpg
new layout-11
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new layout-12.jpg
new layout-12
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new layout-13.jpg
new layout-13
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old mod chassis-01.jpg
old mod chassis-01
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old mod chassis-02.jpg
old mod chassis-02
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Power Supply.jpg
Power Supply
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There is no room on the chassis for the 6L6 plate or the 304tl bias, so I modified an old Tempo One supply with a remote connector for that.

Sorry about the long-winded explanation. I just thought you might like to know how I got from point A to B with a little Phoenix ham history thrown in.

Power Supply-old.jpg
Power Supply-old
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Power Supply-old-01.jpg
Pwr Supp-old-01
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Power Supply-old-top.jpg
Pwr Supp-old-top
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73's for now

Henry, KB7OCY

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