RCA BTA-1M
for
160 Meter AM operation
Well, it finally happened, an RCA broadcast transmitter became available to me. As it turns out it was, for me,
a very good deal. At first I had doubts as most "Big Rigs" require unusual power sources like 3 phase power
or worst multiple power sources and phases.

Well thanks to Ed N3CMI, I didn't have to contend with all that, he had just the unit a BTA-1M, which he no
longer had room for and wanted to get into a good home. I jumped on the deal, and finally got to Philadelphi, PA
to pick the unit up. It looked great, and I fell in "Love" at first sight with the rig. Let me tell you, it is BIG, the manual
states that the unit when all put together weighs 1500 pounds. After loading it and the transformers into my van, I
can truly believe that is correct.

First things first, did I mention I have NO ROOM, let me tell you, this rig takes up space in a big way. So I did the
only thing I could to minimize the need to move it too often, and yet still be able to put it back together, and work
on it and test it, I got yet another Public Storage place for it. Also since it is stored in a Public Storage space, I
figured I would get a portable generator so I would have power for it and also the soldering irons and gun and
test equipment needed to get it back on the air. Lest there be any mistake here is a photo of the Data Plate.



Now we need, to get everything ready to start putting it back together, and get it ready for the big 160 meter
Heavy Metal Rally! Yes folks, I have a plan! So we need a really big dummy load, something that will handle
2.5 kilowatts and not burn out or up. Then we will need a chunk of hard-line coax to carry the RF to the new
Homebrew Antenna Tuner, with the open-wire feeder output to a full size 160 meter dipole. Yep the hard-line
coax is pressurized! We also need a little work bench to hold things while getting it together.

The full Schematic for this particular radio is here!

The transmitter starts out with a single 807 oscillator assembly, with a crystal oven that runs on 115 VAC to maintain
the frequency stability. Then the output goes to the grids of a parallel pair of 807s, which drives a pair of parallel
833A tubes. The output from the pair of 833As goes to a large feedthru insulator, which conducts the RF to the
Homebrew HIGH POWER antenna tuner.

The PDFs for the tube line-up are:
807, 833A, 866A, 8008 and the substitute 3B28.

The audio chain starts with a strap selection of 150 ohm or 600 ohm input, which goes to a pair of 807 input amplifiers.
That output goes to a pair of 807 drivers which provide the audio level needed by the pair of 833A plate modulator tubes.

In the Modulator section, there is a modulation transformer (appx. 150 pounds), and also a modulation choke which
is also (appx. 150 pounds) quite large. The "Low Voltage" rectifiers are a pair of 3B28s changed out from the
original 866a rectifiers. The RF high voltage rectifiers are a pair of 8008 rectifiers. The rig came with a solid state
conversion, that used the tube base and cap and a phenolic form to hold a couple of high voltage strings. These are
made by Wilkinson and are numbered SR-18-2. I also have 8008 tubes and want to make the rig like the original schematic, so I will after testing the rig, put the 8008 tubes in as well.
Follows is a list of things that had to be replaced or changed to get the unit back on the air.

Components that had to be replaced:

Finding an antenna current meter: Well on the E-Bay place I ran across an RF Ammeter out of a "High Powered"
TMC transmitter, that transmitter had to be large as the meter scale goes to "20".

Next comes the reclaiming of the base plate wire entry channels. I had to get some 1/4 counter sunk screws
(6) and nuts and lock washers. The channels keep the cabinet from rocking forward and backward, and
allow the cables to enter the cabinet without having to punch un-sightly holes in it. They were not too bad
I have seen worse in actual (still) on air transmitters, thanks to the lack of technical help to do periodic
maintainance, but that is another story. Anyway, the channels in the before shots had only minor rust on the
bottom part of the channel. There is a plate that is removable so you can put the cables in and route them. Well
I prefer to clean them down to bare metal then put two primer coats of grey on and then Black Gloss in two
coats. This tends to keep the moisture off the metal and helps retard any further rusting.





The Big Rig
with the tubes installed.




Close-up of the driver tube section and the HV
supplies.





I found a RCA BW-11A  Frequency meter on E-bay, seemed like it would go with the transmitter and not need
much modifying as I only intend to use this rig on 160 anyway. I am attempting to drive the xtal socket with my
PTS-40 RF generator for testing, at least until I get the crystals back for the unit. Here are a few e-bay pictures of the
frequency meter. Inside bottom view. Rear view of unit.



  

  
Next on my wish list is the RCA modulation
meter, oh yes and I have to buy or build one of those nifty ON THE
AIR signs so I can get that old time feel. I am working out a foot
switch and relay arrangement to turn the transmitter on (PTT) for
transmit.
The receiver isn't settled yet, I would like an NC-183(R)
or (D), but for the receive
audio chain I am thinking

  
of putting an eico selector and 10 watt audio amplifier (Hi-Fi)
and driving
a large Jensen "Studio" speaker from the

  
receiver audio output.

    

    

    

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