RCA BTA-1R1 Transmitter Question

Discussion in 'Amplitude Modulation' started by K5CKS, Dec 13, 2019.

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  1. K5CKS

    K5CKS Premium Subscriber QRZ Page

    I am really happy to find this forum... A good friend of mine Jim, K5JUC asked me to help him figure out a hum problem in an RCA BTA-1R1 AM Transmitter. We pulled the final tubes so we could listen to the 6146 driver stage. The good news was the 6146 output was clean. There are two possible sources for the hum; the modulator output or the P.A. high voltage. I am very hesitant to hang a scope probe on the H.V. because it is sitting at about 3100 volts. Any ideas how to find the hum source??
    Thanks Guys!
    73,
    Rory, K5CKS
     
  2. N2DTS

    N2DTS Ham Member QRZ Page

    Pull the modulator tubes and see if the hum is on the carrier/power supply?
     
  3. K5UJ

    K5UJ Ham Member QRZ Page

    The first thing to do is find out if the rig is really transmitting hum or if you are hearing hum because some transformerless power supply near by is modulating your RF. High power rigs can put RF on the AC line to a transformerless supply. The supply diodes switch the RF at 120 cycles and that signal can re-radiate into your rx antenna and it will sound like you have hum in your audio. If you tx into a dummy load and look at your modulation envelope with an oscilloscope and see a nice flat carrier then that's it. If the cxr is undulating then you really have hum coming from the rig.

    You can put your scope probe on the h.v. to the final if you construct a voltage divider. get some bread board and string around twenty 100K resistors, they can be little 1/2 w. carbons or even 1/4 w. , in series on the board. connect one end to some h.v. wire with an alligator clip and put another clip on the opposite (scope probe) end. If you take prudent precautions, as in keeping the resistors away from surfaces and only touch them when the rig is powered down you should be fine. Connect and set everything up for observation then fire up the rig and key it.

    I would however, check the lower level stages first, beginning with the audio into the rig. The hum may be coming from some source ahead of the transmitter.
     
  4. AC0OB

    AC0OB Subscriber QRZ Page

    Welcome.

    Has this transmitter been modified or unmodified?

    Was this transmitter already modified for Amateur bands and recently brought out of storage?

    When was the last time it was operational and did have any hum at that time?

    Pheel
     
  5. K4KYV

    K4KYV Subscriber QRZ Page

    A good explanation. It can happen with transformer powered rigs as well, since RF can find its way into the rig via the power line and find its way capacitively around the transformer and into the rectifier and other circuitry. Ground loops may exist to exarcibate the problem, and if some of the speech amplifier and other components in the audio chain have their own independent power supply, rf can get in through there, as well.

    In my station the problem can come and go from day to day, with no apparent changes in settings, when I monitor my own signal through the receiver with headphones. Sometimes it helps to ground certain units in the audio chain together with a clip lead; at other times the hum is there and I have to remove the clip lead to get rid of it.
     
  6. W2VW

    W2VW Ham Member QRZ Page

    HP8901A
     
  7. AC0OB

    AC0OB Subscriber QRZ Page

    Filament-to-cathode shorts in any tube in the audio path, such as 1V603-4, can certainly cause a hum.

    Hun in the audio source or ground loops can also be a problem.

    Pheel
     
    Last edited: Dec 17, 2019
  8. K5CKS

    K5CKS Premium Subscriber QRZ Page

    Good plan of attack! We have used a battery powered receiver to monitor the output to eliminate the possibility of powerline induced hum in the receiver. We removed the finals and monitored the 6146 driver and it was quiet. I also want to have a look at the output from the modulators.
    This transmitter is in the Fort Smith History Museum and is running at low power (about 250 watts) into a dummy load at 1 kc. It is operated on weekends transmitting music and old news tapes and advertising spots. There is a local receiver listening to it in the museum. I think your idea for a voltage divider will show us what we want to see. Thanks for the tip!!
    73,
    Rory, K5CKS
     
  9. K5CKS

    K5CKS Premium Subscriber QRZ Page

    This is an unmodified broadcast transmitter running at 1 kc in the broadcast band into a dummy load. The transmitter, located in the Fort Smith History Museum has been operated on weekends when the crowds show up in the museum. The hum seems to have appeared slowly. We have discussed the possibility of leaky filter caps, a pair of 5 uf oil filled. I think what we need to isolate first is where the hum is coming from; the finals or the modulators. Thanks for the reply!
    73,
    Rory, K5CKS
     
  10. K5CKS

    K5CKS Premium Subscriber QRZ Page

    I don't think you can do that on a plate modulated transmitter, the plate current will go through the roof.
    Rory, K5CKS
     

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