More AM retuning work

Working on another old AM station, this one is a simple Class C one tower on 1230 KHz.

Broadcast Electronics AM Output Tuning Network
Broadcast Electronics AM Output Tuning Network

The main problem today was this BE AM output network unit between the BE AM1A and the ATU.  This site has had some dirt difficulties over the years and the internal parts of this tuning unit arc at full power.  I attempted to drive the ATU directly with the transmitter, which was a no-go.

Gates Radio 1 KW AM ATU, circa 1947
Gates Radio 1 KW AM ATU, circa 1947

I took a look at the ATU, which is a pretty standard Gates 1 KW ATU from the late forties or early fifties.  I have seen perhaps dozens of these things.

My first thought was that over the years, likely due to changes in the ground system, the base impedance has shifted away from its licensed values.  However, a quick measurement of the base impedance shows it to be exactly at the licensed value, 17.3 ohms.  The tower is 67 degrees tall so the impedance value is right in the theoretical norm.

I  measured the input to the ATU, which showed 38 ohms with about 7 ohms of capacitive reactance.  I can only surmise that it has always been this way.  The transmitter in use before the BE AM1A was a Harris/Gates Radio BC-1G.  That model transmitter will drive anything including an open transmission line.

Retuned ATU input; 49 ohms resistive, 0 ohms reactance
Retuned ATU input; 49 ohms resistive, 0 ohms reactance

Having the bridge on hand, I decided to retune the ATU for a better match. I put the bridge on the input terminals of the ATU and set it to 50 j0.  Using the remote control, I turned the transmitter off and on while making small adjustments to the output strap on the coil until the resistance was 49 ohms with zero reactance.  I would have gotten it to 50 ohms, but the strap on the output side of the coil would not stretch far enough to reach the proper spot on the coil.

Now the transmitter will run into the ATU directly at full power with about three watts reflected.   The BE AM output matching network unit has been removed for cleaning and repairs.  I will reinstall it once those repairs are completed.

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8 thoughts on “More AM retuning work”

  1. when you have to make adjustments that push the limits of the hardware, just to get a reasonable test result… is that more deviation over time, more accurate modern testing, or just a less than desirable install and a get it running-damn the torpedoes mindset of the original installer?

  2. That is the cleanest Gates ATU of that age I’ve ever seen…of course being not exposed to the elements helps. My first AM job had one that was rotting out after 30 years of neglect, and was full of mouse everything, but somehow still presenting a good match to the MW-1 feeding it.

  3. Christopher, I think perhaps it was set up this way on purpose, but I don’t really know why it would be done that way. One thing to note, this site used to use open wire transmission line. That might have something to do with it.

    Gregg, the MW-1 was a pretty finicky as I remember it.

    Scott, that is a great question. I’ll get one and update the post.

  4. Would this perhaps have been the old WHUC Hudson? I was imprisoned…. err…. employed there 1980-84.

  5. Bill, it is indeed WHUC. I did not know that you worked there. It is playing country music these days, which is a good fit.

  6. Got my first exposure to Gates TX gear there through a great contractor named Henry Ellenbogen. The studio (formerly on Rt. 66) had a fire that destroyed most of the studio gear in the mid 1970s. Bill Bingham and company at Northeast Broadcast rebuilt it. Truthfully, it was an OK place to get one’s feet wet in broadcasting.

  7. You have a typo in

    the base impedance has shifted away from it’s licensed values.

    its, not it’s

    Admin note: fixed it.

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