AM transmitter/tx audio issue

Discussion in 'Radio Circuits, Repair & Performance' started by KC8QVO, Nov 24, 2019.

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  1. KC8QVO

    KC8QVO Ham Member QRZ Page

    I suppose I will probably get proverbially shot at for this one but here it goes...

    I am trying to troubleshoot my old Cobra AM CB (29wxnwst). I made a mic adapter for an 8 pin kenwood round (I have adopted that "standard" across the board with everything from a KX3 to IC-718 etc). I used the MC43S hand mic that came with my TS-2000 as a test. I used a 12v DPDT relay to do the T/R switching with the 12v feeding both the mic and an external PTT (so I can use my 1/8" mono input adapter, no PTT on it, to inject a signal for testing/comparing... ironic, eh?). Now I get a real dirty carrier and when I inject a 1000hz tone at 2mv the signal is horrendous. I can hear audio in the background under the noise, but it is absolutely not clean. Last time I had the radio in-use it worked fine (early-mid June), but I did not test it before I tried the adapter. So it is conceivable something could have gone bad in that unused time, but not likely.

    Of note - I pulled the 1st audio amp in the mic circuit before the signal goes to the modulation circuit - I get the same trash on the carrier when listening in another radio (of course theres no tx audio at all under the noise as with the amp out it can't get to the modulation circuit).

    I haven't found a way to pull the audio, further down stream from the amp, from the modulation of the transmitter drive and final transistors (one of each in series, with the same feed from the modulation circuit as best I understand the schematic).

    I am pretty certain the transistors (driver and final) are good as I am getting about ~3 watts carrier and 12-ish watts peak (prior to pulling the amp). What I'm thinking is trying to figure out how to pull everything in the modulation side of the equation except for the carrier - if that is possible. If anyone has pointers on what that may look like on an AM transmitter (where in the circuitry) that would help. The service manual states the tx audio goes through the transistor I pulled then to another audio amp (a ta7222ap). However, when I pull the output (pin 9) of this it mutes the speaker/receive audio and when I pull the input (pin 4) it does nothing to the transmitted audio trash.

    I checked over caps in the circuit. There is one ceramic that looks bulged out, but I looked at it real close and that appears normal - there isn't any char, discoloration, or cracks in the coating.

    What else would cause the audio to distort, and really just the trash noise on just the carrier?

    See spectrum pics for reference.

    The spike on the dirty carrier spectrum on the right only happens on keying.

    What I don't get is the carrier spectrum on the one that should be depicting the dirty signal does not appear weird, although the demodulated audio is.

    The clean images were from another radio that works fine.

    I haven't figured out how to get 2 tones. I can set up a 2 tone test for the RF generator, but I used the function generator out of the audio port on the service monitor and that only lets me set one frequency. I can look at it again when I get some time and see if I can set the 2 frequencies in the RF gen to route to the audio out port instead of modulation on the RF signal....
     

    Attached Files:

  2. KB7WG

    KB7WG Ham Member QRZ Page

    Upload your schematic for us. And show us a schematic of your modification. Have you the stock cb mic?

    Have you a scope?
     
  3. KC8QVO

    KC8QVO Ham Member QRZ Page

    I have 2 mics that work. I retired the stock one and went to an Astatic 6L6. I used that fine for a while.

    As to a scope - have 2. One in the service monitor and a stand alone 2 channel 100mhz digital scope.

    I think I am seeing where you're going with it - probe the modulation to the transmitter driver and finals?

    The schematic I am working with is here:
    http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/cobra_29ltdwx-st_29ltd_sch.pdf

    The service manual is here:
    http://www.cbtricks.com/radios/cobr..._st/graphics/29ltd-st_29wx-st_sm_pg1_pg11.pdf

    TR17 is what I pulled. TR4 is the op amp I pulled the output and input on (left in place, just pulled the legs).

    Attached is my adapter schematic. Not shown is the part that adapts to a 1/8" mono - pin 1 is audio and 7 is audio ground.

    On the 4 pin -
    1 = Audio ground
    2 = Audio signal
    3 = TX control
    4 = RX control

    8 pin - kenwood pinning
    1 = Mic audio signal
    2 = PTT Signal
    7 = Mic Audio ground
    8 = PTT Ground

    When I inject the signal from the signal generator I feed audio in to pins 1/7 on the 8 pin and use the PTT button on the adapter.
     

    Attached Files:

  4. KC8QVO

    KC8QVO Ham Member QRZ Page

    Well I learned something new today. The output in the RF generator can be set to AF. I've done some calibrations in the past feeding receivers from the RF generator using 2 tones, but I never tried to drive an external transmitter with the 2 tone audio. Now I know how to do it.

    I bumped the audio level to 3mv and used 800hz and 2000hz signals, arbitrary values and not from any "standard", just ones I picked to try it.
     

    Attached Files:

  5. KB7WG

    KB7WG Ham Member QRZ Page

    It's difficult to put all this together. The only thing I have to go on is the print. IF you have changed that print, we are wasting our time.

    When you said TR4, did you mean IC4?

    Restore radio. Connect a dummy load. Do not inject signal. Key radio. What does signal look like a the cathode of D8? D8 is connected to T1.
     
  6. KC8QVO

    KC8QVO Ham Member QRZ Page

    TR17 is back in - that is how I was able to produce the dirty spectral image earlier today (TR17 out would have blocked the audio aside from the carrier).

    As far as a dummy load - I'm using the service monitor is the dummy load. Would there be any merit to connecting a dummy load to the radio? Unless I feed straight in to the service monitor I wont get the spectrum. I don't think the "load" (antenna, service monitor, or dummy load) would change the internal scope traces though.

    Attached are pictures of the cathode trace on my oscilloscope keyed and unkeyed. It is DC - 12v unkeyed and 10v keyed. There is no signal injection, just dead audio (no source connected, either) and keying the transmitter.
     

    Attached Files:

  7. KB7WG

    KB7WG Ham Member QRZ Page

    Load and monitor is ok. What does spectrum look like now?
     
  8. KC8QVO

    KC8QVO Ham Member QRZ Page

    Still a mess.

    Again, the odd spike on the side of the unmodulated signal spectrum envelope is only on keying/unkeying.
     

    Attached Files:

  9. KC8QVO

    KC8QVO Ham Member QRZ Page

    Also - the terminology earlier between "TR4" and "IC4" - you are correct. I meant to type "IC4".
     
  10. KC8QVO

    KC8QVO Ham Member QRZ Page

    One test that I am wanting to do is remove the audio stream from the mic chain to the injection point in the AM modulation so as to isolate the dead carrier with no other components in the audio chain up stream from there. I can't figure out where I can break that.

    The dead carrier sounds dirty in another receiver so by cutting out all the components leading to where the mic audio goes in to modulate the carrier it should, in theory, take out their influence on the carrier. If there is a bad component in the audio stream the carrier should then clean up. If there is a bad component in the rest of the transmitter, or in the creation of the carrier, then there would still be an issue.
     

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