AM transmitter/tx audio issue

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

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  1. KB7WG

    KB7WG Ham Member QRZ Page

    Hold on a min. Do you see the nice dc voltage? That's from the audio modulator. It's flat. Nothing from the audio chain is being added.

    The noise is coming from the other end. The rf generator.
  2. KB7WG

    KB7WG Ham Member QRZ Page

    When connected to an antenna, does the receiver work ok?
  3. KC8QVO

    KC8QVO Ham Member QRZ Page

    Receiver works fine.

    As to the DC on the circuit you had me check - I figured that out and did some testing. I injected a signal in to the mic and the signal on the cathode of D8 was as clean as could be, but it was all positive (above ground/0 volts). I dont think thats an issue.

    I have a few more pictures coming as soon as I get through a couple things. I soldered up an RF sampler board for the oscilloscope (so I am not feeding RF straight in to it).
  4. KB7WG

    KB7WG Ham Member QRZ Page

    Alrighty then, I wish you luck.
  5. KC8QVO

    KC8QVO Ham Member QRZ Page

    Here are some more pictures from testing this. The titles should be descriptive.

    Just to clarify - the audio in the scope is from the receiver I am listening to the bad transmitter on, not the receiver from the bad radio.

    Attached Files:

  6. KC8QVO

    KC8QVO Ham Member QRZ Page

    Thank you for your assistance thus far. It is much appreciated.

    This doesn't make any sense yet.

    The signal out of D8 is the modulated control voltage to the driver and final transistors, correct? Since, with no audio injected, the signal out of D8 is DC = the carrier isn't present there.

    Now when I got the picture of the carrier (no audio) on the oscilloscope with the detection circuit (RF side - signal through the coax to the dummy load) the sinewave looks very clean, however the received audio says otherwise. The modulated (with an injected signal) also shows the noise clearly in the RF spectrum.

    I looked at the circuit and tried to find some areas to tap for signals on the RF side prior to modulation. The attached pictures are what I came up with (descriptions in the titles and waveforms there in the pictures).

    The waveforms don't look too healthy. How does the waveform coming out the back of the radio look better? Is it the driver and final stages plus the filtering that is cleaning up the waveform to some extent? Then the next questions I have are: Should the signals here be clean? If so - what could cause them to not be?

    Of note, also, I think - the frequency stability of the radio appears to be correct. The frequency on the oscilloscope with the carrier is correct during transmit and the receiver is working correctly. So if the RF side is messed up - would that not mess up the frequency stability during transmitting as well?

    Attached Files:

  7. KB7WG

    KB7WG Ham Member QRZ Page

    Well, either I mis-understood the original post, or your understanding of AM modulation is different than mine.

    Not knowing your setup, your equipment.......and now I question as to what you are really looking at, I don't think I could help you.

    I was under the impression that when you keyed the mic, and without modulation, your radio was transmitting a bunch of sidebands.

    I was attempting to track that down, but I don't want hours of explanation and debate before you will check the next point.

    I wish you success on your repair.
  8. KC8QVO

    KC8QVO Ham Member QRZ Page

    The mystery continues...

    I pulled the CB out of my truck (uniden 880) to test it along side the cobra 29. It has a 6 pin mic so I made an adapter for the 8 pin to 4 pin adapter I made earlier. I tested it in to the service monitor and set the RF analyzer to demod AM. The uniden sounds fine on the stock mic and when I put the adapters in place and run the Kenwood hand mic - everything sounds perfect. However, when I try to inject an audio signal the 880 does the same thing as the Cobra 29.

    So I went back to the Cobra 29 in to the service monitor with the RF analyzer set to demod AM. On the regular mic it sounds fine. So I put the adapter in place with the Kenwood mic. It sounds fine. When I inject a signal - problem.

    When I made my audio comparisons before they were using an HF radio as the listening device. The Cobra 29 was connected to an antenna (through a remote tuner with a 1:1 match to the antenna) and I was getting the problem on the receive. In to the service monitor I do not have the problem until I inject audio from the service monitor.

    I will agree the problem is on the "RF side", it is appearing that RF is getting in somewhere. I will try and set up out of the shack when I get a chance and see what happens there.

    Weird stuff... Good to work through this, though.
  9. KC8QVO

    KC8QVO Ham Member QRZ Page

    As an aside...

    When I monitored the cathode signal of D8 and injected a signal to the mic connector to watch the modulation it was clean - just all above ground (all positive v). So if I had a clean injection signal and a dirty RF signal - I'm not quite sure how that correlates yet. If RF was getting in through the audio line and the mic adapter I made then I should have seen that on the oscilloscope trace on the cathode of D8, would I not?
  10. KC8QVO

    KC8QVO Ham Member QRZ Page

    I got the radio in question set up in my truck and used my scanner as a recorder. Everything is fine there and same goes for my other Uniden radio - all good.

    So I suppose the final nail in the coffin on this one is what ever was going on last week was RFI. Which I still don't understand it because I can get a clean signal out of another radio under the same test conditions.

    I believe the oscilloscope in that the signal through the radio to modulate the transmitter is "clean". So if that was clean under the test conditions but the emitted signal was not - how is RFI getting in to it? Going back to my last set of oscilloscope images - (R58 an R63 as test points) - those should be the radio frequency side pre-modulation. Those appear dirty to me. If that is showing the RFI, yet the audio line through to modulation is clean, it makes no sense. My test set up works to inject audio in other radios, so maybe it is something to do with the way the receive audio, mic audio and ptt circuitry works with the CB style radios (RX control and TX control - without a mic plugged in linking the RX control circuit there is no receive audio - is how that is designed the source of the issue with an audio source other than a mic element?)

    At the end of the day the radio works - so thats the important thing as far as the diagnostics goes in the thread here.

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