RCA BTA-1M AM Transmitter Rescue 8/25/16

Discussion in 'Amplitude Modulation' started by N6YW, Aug 25, 2016.

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  1. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    She's now resting in the garage here at home.
    Preparing to dismantle and part it out. Boxes of tubes puts a smile on my face!
  2. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    No Need to put all of that iron and copper into the dump!
    Nice Save!:p
    N6YW likes this.
  3. AF6LJ

    AF6LJ Ham Member QRZ Page

    Very Nice Thanks You for sharing the pictures I love the one of the strange looking cloud....
    N6YW likes this.
  4. WZ5Q

    WZ5Q Ham Member QRZ Page

    Sorry the condition of the transmitter wasn't as you had hoped, but the Iron and Glass were worth the trip.
    Thanks for the pics, it looks like the scenery was just gorgeous.
    Nothing but swamps filled with Cypress and Tupelo trees down here in Louisiana, lol.
    AF6LJ and N6YW like this.
  5. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    I believe that one's the RCA version of the Gates BC1- line from the -T on, using 807 cathode followers for audio drivers. Not sure whether it was Gates or RCA that came up with the idea first, but it was basically a cost-cutting measure. A pair of 807s is not sufficient to fully drive a pair of 833As, whether in the RF or modulator section. The anaemic RF driver problem is addressed by applying some modulation to the RF driver along with the final; that kicks up the grid drive when it is needed, during positive modulation peaks, resulting in improved modulation linearity and less negative carrier shift. Four 807s and no audio driver transformer is a lot cheaper than an 813 rf driver and a pair of large triode audio drivers along with the requisite transformer.

    The stock audio driver in the Gates line won't fully drive the 833A modulators, plus the Gates circuit board construction tends to become flaky and the resistors burn brown spots, even holes in the phenolic or glass epoxy. I assume RCA used real point-to-point wiring over sheet metal instead of circuit boards. Vacuum tubes on circuit boards was never a good idea in the first place.

    In my Gates BC1-T, I replaced the RF driver PC board and re-built the circuit, with only a few minor modifications, on a sheet of aluminium. It didn't make the 807s any less squirrelly as I had hoped, but the driver works just as well as before with no more burnt circuit board material. I finally got tired of constantly fixing the flaky audio driver, so I pulled the whole thing out and installed something I already had on hand for years, the audio driver section out of a Raytheon R-1000, complete with a pair of 845s transformer coupled to the 833A grids. With the 807 driver, the transmitter would just barely make about 115% positive peak headroom. I have heard others say their BC1-T wouldn't even make 100% modulation, but with the 845 driver and the rest of the parameters unchanged, mine is now capable of up to 140% positive peak headroom. My voice waveform rarely drives it up that high, but I can rest assured that what it does achieve is not distorted, pushed to the verge of flat-topping.
    N6YW likes this.
  6. WA3VJB

    WA3VJB Ham Member QRZ Page

    Billy, a hat tip for saving this old beast, rough-hewn as it is, and in rugged terrain as it was. Among the transmitter rescues I've taken part in, an RCA BTA-1M, the "shutter door" type, as distinct from the 1MX with a solid door like the one you retrieved in Arizona. This one was working when pulled, but had an external modulation transformer from a 5KW transmitter. Definite work-around. The iron was so heavy that the liftgate truck we had backed up to the door of the utility room got STUCK. Drove in fine on arrival, sank in up to the axle trying to depart. A few hours later it was on its way with a crew of four to Pennsylvania.

    And yes, I have a story like yours about a transmitter that was too far gone to be made good. A Collins 300-G, Ser. No. 30, with an internal wiring harness fire deep inside a section of framing in this 1100lb transmitter. I dismantled that poor puppy in my garage and hauled the cabinet to the curb. Told the county's Bulky Item Pickup scheduler that it was an industrial refrigerator. Did not stick around to see the reactions of the municipal workers who picked it up.
    N6YW likes this.
  7. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    Thank you all.
    I enjoyed the trip very much. As of this writing, the usable parts have gone to where they needed to go, and the tubes
    are going to be tested. The carcass will be dismantled and the remaining misc parts will be boxed and stored.
    As the creator of this forum, I find it to be of major importance to undertake these projects and share the info and wealth
    as it slips through my hands. After all, we have a radio society where we all share in this wealth in one form or another.
    It's why I create topic threads like these in order to gain and maintain interest in as many aspects of this wonder we call
    I also appreciate the splendid support and contributions so many of you provide. It makes for a really positive learning
    and sharing environment. Also, I should point out there will be other rescues like this in the near future and I will
    fully mandate that any and all contributions are HUGELY welcomed here.
    Again, many thanks.
    73 de Billy N6YW
    WA3VJB, KD2ACO and AF6LJ like this.
  8. K5UJ

    K5UJ Ham Member QRZ Page

    I completely agree. Thanks for getting the rig out of that shed where it can be dismantled and used. This is a relatively small part of ham radio that needs all the help it can get to continue. In my opinion, buy low sell high parts and rig flippers, delusional eBay tycoons, hamfest bottom feeders and other extreme profiteers should exit and go into furniture antiques and art. Note though that to me, there is nothing wrong with recovering expenses and a modest markup for time and labor. I usually sell things at cost. I don't see vintage radio and AM as my path to riches. We should be trying to help each other; not gouge each other.
    N6YW and KD2ACO like this.
  9. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    The hamfest and ePay bottom feeders may have had at least one positive collateral effect, in that a lot of the stuff that some people think must be made of gold, would have otherwise ended up in a landfill because it would have been considered "worthless". I remember a tale of an old timer who threw his entire collection of QST, dating back to the end of WWI, in the trash because "they're now on CD and all those magazines take up too much space". Unfortunately, when the person found them in the open dumpster, it had rained the night before, and the whole collection was soaked beyond redemption.
    N6YW likes this.
  10. KA4KOE

    KA4KOE Ham Member QRZ Page

    I passed Ray's up simply because it was too big.

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