How hard is it to move a Broadcast transmitter to 3.880khz from 1570khz?

Discussion in 'Amplitude Modulation' started by K8CCA, Apr 27, 2021.

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  1. K8CCA

    K8CCA Ham Member QRZ Page

    Looking at the RCA broadcast tuner listed here today got me thinking how cool it would be to have something like that in my shack. I don't know anyone locally that has ever done it and really don't have any idea on what's actually required to move one of these to the Ham bands. I am guessing that the output coils and caps would need to be changed to be resonant 75meters. How big of a job is it to do this? I am pretty good on boat anchors, but never messed with this big stuff before.

    Thanks,
    Eric
    K8CCA


    eta I did find the manual here https://bama.edebris.com/download/rca/bta1mx/bta1.pdf and it looks the listed here has 1kw output! Looking at the pics it seems like it is adjustable to lower output power for day/night ops.
     
  2. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Moving the frequency is fairly easy.

    It's moving the cabinet that will be the problem unless you remove all of the heavy iron before moving.


    Pheel
     
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  3. W7TFO

    W7TFO Premium Subscriber QRZ Page

    QSY can be a challenge, and multibanding is definately a bit of work overall.

    Working the finals up in freq isn't so tough. Hams usually dump the dual output networks for a simple Pi net, as the harmonic suppression requirements are lessened for our use.

    The driver stage(s) are a little touchy, since they were designed not to move around much. A few SS broadband HF amps are on the market and can be used there.

    Most will use a lab-type signal generator to start with, and work up to the necessary power levels.

    Don't forget to use only distilled H2O in the finals, too...LOL

    73DG
     
    PU2OZT likes this.
  4. K5UJ

    K5UJ Ham Member QRZ Page

    A lot of broadcast rig operators come up with a way to drive the final PA with a plastic radio. I've never found any appeal to having a foreign ham rig inside a classic American broadcast rig but it makes sense in one way, especially if the final tubes are tetrodes and only need around 20 w. drive.

    Broadcast rigs are designed with crystal oscillators and are usually RC coupled straight through from the oscillator to the buffer, driver and final PA. That works with a pure crystal oscillator source. But if you hook up an old ham VFO that has lots of harmonics you may have problems with more than one frequency coming out of the rig or FMing the VFO when the modulated PA pulls it off frequency. Ham rigs made to run with these kinds of VFOs have wave trap type tuned circuits between each stage. Broadcast rigs may not have that. A plastic radio or a 50 watt ham rig like a Ranger on CW or an EFJ Navigator driving the PA may fix that problem because they put out a fairly pure RF signal on the desired frequency.
    That rig runs 4-400As in the final PA I think. Yes depending on the commercial use it can be set up for 1KW day 500 w. night, or 250 w. night.

    I think a broadcast rig is safer to work with than an EFJ Ranger. In my opinion the ham rigs can kill you a lot more easily.

    This page here on QRZ details the modifications to a triode rig, a Gates BC1G to get it on 75 m. https://forums.qrz.com/index.php?threads/w1aw-gates-bc-1t-update.599961/page-5
     
    K8PG likes this.
  5. W9JSW

    W9JSW Ham Member QRZ Page

    I thought it was a bit over-priced. Pics show 2 pair of 833's.
     
    N8ZL likes this.
  6. PU2OZT

    PU2OZT Ham Member QRZ Page

    Or else use liquid nitrogen, they give it for free, as every air liquefaction plant pushed oxygen production to the limits, these weeks, in some countries... cannot remember why, however...

    Oliver
     
    N3RYB and (deleted member) like this.
  7. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Most BC rigs can very easily be modified to work 160m. A little more difficult, but can be made to work on 75. Much more difficult to make one work multiple bands, particularly the ones using two triodes in parallel in the final, like the Gates BC-250 and -1 series, since the highly unbalanced circuit doesn't tend to stay neutralised over a wide frequency range. The stock circuit may not even stay neutralised all the way across 160m. One work-around would be to install a control knob with dial, so that the neutralising cap can be manually re-adjusted to frequency, and make up a frequency chart. One more thing to re-adjust when changing frequency.

    One of the easier rigs to convert to multi-band should be the Collins 20V series, because of the modular construction and screen-grid tubes in the final. The entire rf section of the RCA BTA-1 series, for any band including 160m, would better be stripped down and reconstructed from scratch, using HF construction practices. RCA seemed to place the PA components in the most convenient spots mechanically, and used wire leads, however long they had to be to reach, to interconnect them. Maybe not so much a problem in the standard BC band range, but for HF, long leads haphazardly placed can make it a challenge to tame down parasitics and self-oscillation.

    I converted a Gates BC1-T to make it my dedicated transmitter for 160m. If I were ever to make it work multi-band, I would completely tear down and re-do the entire RF section, including conversion of the final to balanced push-pull. I was given an RCA BTA-1R some years ago, but I parted it out since it had water damage and was missing all the tubes and the modulation transformer. Decided that what was left wasn't worth converting.
     
  8. W3SLK

    W3SLK Ham Member QRZ Page

    Rob, K5UJ said:
    The BTA1MX uses a pair of 833's modulated by 833's. The economical -500MX uses a single 833 X 833's. I think the BTA-1MX is frequency agile to 2.5Mhz. My buddy Eric, WB4VVI(SK) used a -500MX on 75M from time to time so it can do it.
     
  9. W7TFO

    W7TFO Premium Subscriber QRZ Page

    The old Raytheon 250 & 1kW Tx were push-pull in the finals, much easier to run up the dial.

    73DG
     
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  10. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    And so does the Gates BC500-T

    And the amount of overkill in some places is downright insane. The RA-1000 uses a 200 mA power supply with 5U4G rectifier, dedicated to supplying the plates of a pair of 6J7s and a pair of 6J5s in the speech amplifier that drive the pair of 845 audio drivers. Probably less than 20 mA total load on that supply.

    OTOH, they skimped on the class-B driver transformer. It has poor frequency response and the audio driver section has an unacceptable level of distortion if the negative feedback circuit isn't activated.
     
    Last edited: Apr 29, 2021

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