Driving the BC-1T With the Ranger Now

Discussion in 'Amplitude Modulation' started by W7KNX, Jan 27, 2020.

ad: L-HROutlet
ad: l-rl
ad: L-MFJ
ad: Left-3
ad: Left-2
ad: Subscribe
  1. W7KNX

    W7KNX Ham Member QRZ Page

    After spending days trying to track down the low level audio noise in the BC-1T audio driver section and rebuilding the toasty board to no avail I finally decided to kill two birds with one stone: I decided to drive it with the Ranger. Not only did I fix my audio problem I don't have to keep switching xtals in the Gates. I can go anywhere in the AM window with the Rangers VFO. Audio recordings I made sound reasonable and I am getting good reports so far. Some day I will revisit the audio board but I wanted to get started on Collins 20V-2 number 2 and in the mean time I have a reasonably good signal on 75m.

    Jeff W7KNX
     
    K0UO and K4CCW like this.
  2. K5UJ

    K5UJ Ham Member QRZ Page

    At least you aren't driving it with a plastic radio.
     
  3. W7KNX

    W7KNX Ham Member QRZ Page

    I don't like to use magic devices for AM either unless I don't have any other options. As time goes on I find I have dwindling interest in my magic radios which are all in studio A. These days I have enough real radios to choose from there is always at least one that is working.
     
  4. K5UJ

    K5UJ Ham Member QRZ Page

    I don't know how you can expect to have a QSO with anyone if you aren't using a modern magic transceiver. All those old tube things are nice to look at but they can't possibly work well enough to be usable.
     
  5. W7KNX

    W7KNX Ham Member QRZ Page

    Too funny. I hope my license does not get revoked for not owning an ICOM 7300.
     
  6. K4KYV

    K4KYV Subscriber QRZ Page

    Do you find the pair of 1614s (factory-selected premium 6L6s) in the Ranger enough to fully drive the 833As?

    I had the same problem with my audio driver board. The audio board with 4ea. 807s wouldn't fully drive the modulator. It had the usual burnt spots on the PC board, but the main problem was its flakiness. I could get it working fairly well, but after no more than 2-3 weeks something else would inevitably go wrong. I ended up swapping out the negative feedback ladder with a similar but much higher quality one from an RCA BTA-1R. I thought that fixed it, but sure enough after a couple of weeks, it wouldn't make 100% modulation without distortion and there was no positive peak headroom beyond 100%, so that was the last straw. I ripped out the cheezy PC board and replaced it with the audio driver section from a Raytheon RA-1000.

    Now, the 833As are driven by a transformer-coupled pair of 845s, which will drive the socks of the 833As. I made the modification about five years ago, and it has never given me a moment of trouble. I completely re-built the power supply module (bottom unit) from the Raytheon, to deliver 1450 volts to the 845 plates, and steal B+ voltage for the low-level speech amplifier stages from the Gates low-voltage supply.

    My 1-T never had the built-in dummy load. I think it came from the factory that way, since there were no impression marks on the mounting holes in the cabinet to indicate previously-mounted hardware. The unoccupied left-hand side of the cabinet had precisely enough space to accommodate the Raytheon driver.

    The large grey cube in the bottom of the cabinet next to the plate transformer is the 50H potted modulation reactor. The power supply filter choke was moved to where the original modulation reactor sat. The original reactor talked back loudly enough to cause acoustical feedback to the microphone which is located about 12 ft away; the new one is perfectly quiet. All rectifiers in the transmitter are now solid state. Removing the 8008 filament transformer liberated enough space for the large reactor to fit.


    RA1000 audio driver2.JPG


    In addition to replacing the audio driver, I modified the BC1-T cabinet so that the rig looks more like a radio transmitter and less like a soda machine. The silly little triangular peep hole on the door was enlarged to a full rectangle, and that stupid grille that protruded out under the door (a.k.a. "ankle-buster") was dismantled and re-assembled so that it is now flat and flush with the cabinet door. The stock BC1-T has to be one of the ugliest broadcast transmitters ever made.

    gatesfrontview3.JPG


    The RF driver board was as toasted with burnt spots as the audio board, although it worked reliably. I re-built the module onto a piece of sheet aluminium, with only minor circuit modifications. It works the same as the old PC board, no better and no worse.

    RF driver.JPG
     
    WZ5Q and W7KNX like this.
  7. W7KNX

    W7KNX Ham Member QRZ Page

     
  8. W7KNX

    W7KNX Ham Member QRZ Page

    I'm not sure how I screwed that reply up by including it with the quote. I'm a dork.
     
  9. KA4KOE

    KA4KOE Ham Member QRZ Page

    We're all dorks and geeks. Nothing of which to be ashamed, lad.
    Fleep
     

Share This Page