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Sansui 771 FM section of tuner Channel Out

Discussion in 'Exclusively Sansui' started by Zekeman, Feb 5, 2018.

  1. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    Leave the filter bypassed and we'll proceed. It would be great if you had an FM signal generator so we could be looking at a sine wave instead of random audio. Down the road you might want to think about getting an FM modulator for use in a car, and a cheap audio oscillator. You power the modulator with a 12V supply, hook up your oscillator to the input and, presto, you have a cheap stereo FM signal generator. Synthesized, even!
    To proceed, what voltage do you measure on the bases of TR19 and TR20? They are tied together so just measure one of them. If you have .6V or above, the problem is in the mute circuit. If you adjust VR03 to one end or the other you should get audio.
    If so, remove the bypass on the filter and see how it sounds. I have a recording gig tomorrow and Sunday so I may not get back to this right away, but this gives you something to do.
    I just looked back to the beginning where you said you had both channels before the recap, but that may be related to this. Check the polarity and solder bridges at C48, C56, C57, C58.
    If all that is ok, and you got sound by adjusting VR03, you can try adjusting VR03 so it mutes on distant very noisy stations. If that makes it mute all the time again, then T06 and T07 need to be adjusted. You can try putting it on a strong station and adjusting T06 and T07 for the lowest DC voltage at the collector of TR13. Then do it again on a weaker station.
     
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  2. Zekeman

    Zekeman Super Member

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    I have a tone generator...
     
  3. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    Consider buying an FM transmitter to plug it into. Like this: http://www.belkin.com/us/p/P-F8Z439/
    Or just buy whatever you can find at Best Buy. I think the one I have is a Belkin. I use it for aligning digital tuners, where the station has to be right on the money or you can't get a good alignment. You will also need a 12V power supply, and you can either solder wires to the 12V plug on the transmitter or buy a 12V socket.
     
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  4. Zekeman

    Zekeman Super Member

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    OK I bought the modulator got it hooked up and am showing the bypassed low pass filter and the measurements on the collectors of tr 17-18.

    image.jpg

    On the collectors of TR 19 and 20 I am getting 0.1 mV.
     
    Last edited: Feb 10, 2018
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  5. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    On the collectors of TR 19 and 20 I am getting 0.1 mV.
    I asked for the DC voltage on the bases. I need to know if it's muting.
     
  6. Zekeman

    Zekeman Super Member

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    They're like 0.1-0.2 mV


    As I think about the FM modulator....I connected the sig gen to only one channel on the modulator...not sure if I need to feed signal into both...I'll check when I get back home later.
     
    Last edited: Feb 10, 2018

     

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  7. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    Yes, you need to feed both.
    .1mV on the bases makes it not in mute. Get the sine wave on both channels and then we'll see where it's being lost.
     
  8. Zekeman

    Zekeman Super Member

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    IMG_5735.JPG IMG_5734.JPG Here it is with both channels from the modulator

    The previous photograph was deleted and is being replaced. Before I had the connections between the modulator and the other devices jumped with alligator clips. Obviously that was iffy at best so I built to RCA union connectors which is a better option and this is the result
     
    Last edited: Feb 11, 2018
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  9. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    Good, now connect the scope to pins 14 and 15 of the PCB.
     
  10. Zekeman

    Zekeman Super Member

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    Ok
    image.jpg
     
  11. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    Well, you have audio coming off the board. Do you hear it? Next step is to remove the bypass from on the filter and re-check pins 14 and 15. what frequency is the generator set to? Looks like 36Hz. 1KHz would be better.
     
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  12. Zekeman

    Zekeman Super Member

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    Here is 1 kHz

    IMG_5737.JPG

    When I remove the low pass filter bypass I can hear the tone but it's at a significantly reduced volume. There is no low-end whatsoever. I don't know if this video will do it justice... I bypassed the low pass filter again for this video . If you're able to listen to it on high quality speakers you may be able to hear what I'm describing

     
    Last edited: Feb 11, 2018
  13. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    There are two possibilities here.
    1. Filter is bad.
    2. Something is loading it down.
    Remove TR19 and TR 20, leave the bypass out, and see if that helps.
     
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  14. Zekeman

    Zekeman Super Member

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    I'll check that tonight. For reference this image shows the trace using the same settings as in the last image but the LPF is no longer bypassed. Note the significant reduction in amplitude.

    IMG_5739.JPG
     
    Last edited: Feb 12, 2018
  15. Zekeman

    Zekeman Super Member

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    OK here’s an image from pins 14-15with TR 19 and 20 pulled from the board and the low pass filter is not bypassed

    52E742E7-3644-4246-9751-537D82A286B1.jpeg

    And this one is with the two transistors still out but I have bypassed the low pass filter…

    1ADEA31B-7119-478D-BA61-85A62E2836EF.jpeg
     
  16. Zekeman

    Zekeman Super Member

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    These images are with TR 19 and 20 reinstalled an are of the input and output sides of the low pass filter - input first output second

    17EDDE1F-9ADD-4D1B-A83F-84483D2B24EF.jpeg


    53B3717C-305C-494D-9263-37A91EDC03DF.jpeg
     

     

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  17. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    Bad filter.
     
  18. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    Take the filter out and take pictures of top and bottom next to a ruler. I'll see what I have.
     
  19. Zekeman

    Zekeman Super Member

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    I’ll post the dimensions tonight but in the meantime here is an image identifying its part number:

    BL12 0910210 381

    Thank you!

    2F777A11-1E3A-4DFB-BA44-F2C46E000C70.jpeg
     
    Last edited: Feb 13, 2018
  20. Zekeman

    Zekeman Super Member

    Messages:
    2,406
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    L X W X H:

    1.5” x 7/8” x 3/4”

    487ED512-74AC-494A-A301-66B98DA969E4.jpeg 5A4A4A8E-4740-4B9A-A163-9EAD557AB2B0.jpeg AB73417C-D256-42C9-A0A8-8E1F9BCE4590.jpeg
     

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