Sansui 771 FM section of tuner Channel Out

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

  1. Zekeman

    Zekeman AK Subscriber Subscriber

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    Good afternoon!

    I have a Sansui 771 receiver which has the left channel out in the FM side of the tuner. Both channels work on the AM band!

    The tuner has been recapped.

    The question I have as I start troubleshooting is; would the issue likely lie after the stereo decoder integrated circuit chip? See that section in the image below.

    Thanks! Zeke

    IMG_5652.PNG
     
    Last edited: Feb 5, 2018

     

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  2. dr*audio

    dr*audio Fish fingers and custard!

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    Start by probing IC03 pins 11 and 12 with a scope. If you have signal there, follow the signal and see where it's getting lost.
     
  3. Zekeman

    Zekeman AK Subscriber Subscriber

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    IThe signal looks relatively even all the way down to the collectors of TR 17 and 18 where they look like this...

    I checked all the components / resistors around these two transistors and I even swapped them out with KSC 1845's with no change...

    Both trannies measure the same voltages
    E: 0.9 C: 5.1 B: 1.5 :idea:

    IMG_5659.JPG
     
    Last edited: Feb 5, 2018
  4. Tom B

    Tom B AK Subscriber Subscriber

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    A question did both channels work before the recap?? If so examine the connections for the (4) 2.2uf caps between the IC and the low pass filter. A bad from factory cap could have been installed, so try just jumping across the 2.2uf cap connections with another, or use one of the originals.
    If the problem existed before the recap, the low pass filter could be bad. You can jump from the input of each channel to its output to see if you can get audio.
    If you have access to a signal tracer, its pretty easy to trouble shoot. Just go from pin 11 and 12 of the IC. If they have the same audio level, go to first set of 2.2uf caps testing both sides. Then transisor first the base then the collector. If still good on both transistors, then go to the second set of 2.2uf caps both sides. If that is good, check the low pass filter. If thats good on both channels, input and output, you may have a wiring problem, or an issue with the stereo mono switch
     
  5. dr*audio

    dr*audio Fish fingers and custard!

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    What's it look like on the bases of the transistors?
     
  6. Zekeman

    Zekeman AK Subscriber Subscriber

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    Guys thanks for the advice I'll take a look tonight post back

    To clarify- I did have both channels before the recap. I have gone back in and tested continuity ( downstream of the IC) and re-soldered all these joints ( downstream of the icy and before the low pass filter ). I also went in and reinstalled the original caps as a test with the same result. I've got detailed photos of before the recap and have confirmed the orientation of all the caps. Additionally I have a salvaged spare tuner from another 771 and I pulled the IC from it and swapped it in - with the same result.

    So I will heed your advice tonight and see what we find .
     

     

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  7. Zekeman

    Zekeman AK Subscriber Subscriber

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    If I recall correctly when I scoped last night they were equal ... it was at the collector where they differ … Again I'll confirm it tonight. Thanks
     
  8. Zekeman

    Zekeman AK Subscriber Subscriber

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    IMG_5668.JPG Here are the bases of TR 17 & 18
     
  9. dr*audio

    dr*audio Fish fingers and custard!

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    Yep, they're identical at the bases. Remove C67 and C68 and see what you get on the collectors. If they are then identical, the filter is bad.
     
  10. Zekeman

    Zekeman AK Subscriber Subscriber

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    Is there any wayto repair it or are the components potted in the case?
     
    Last edited: Feb 7, 2018
  11. dr*audio

    dr*audio Fish fingers and custard!

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    I have seen some that come apart. You can probably pull one from something else and use that.
     

     

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  12. Zekeman

    Zekeman AK Subscriber Subscriber

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    So last night I pulled those two caps and got even signal. Then assuming the low pass filter was bad I de soldered it ( it is in fact potted). This particular filter has five pins on the bottom so I tested resistance between all the pins, which was present across all points. I reinstalled the filter and fired up the receiver and now I have very low volume output on FM with both channels sounding even.… Again at a very muted / abnormally low level, and the previously normal-functioning right channel is now muted... so something changed and now I have a different issue :idea:
     
    Last edited: Feb 8, 2018
  13. Zekeman

    Zekeman AK Subscriber Subscriber

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    Tonight, since I had a salvaged tuner on my shelf in clean condition I decided to do a tuner transplant! (the low-pass filter is different and has 6 or 7 pins vs the other tuner's LPF with 5 pins) The patient came through the surgery but is not doing so well and needs more treatment.

    IMG_5681.JPG

    I have AM and both channels of FM however it will not play any FM stations. I have stereo illumination. Seems like it needs alignment but before I drag it into a shop for alignment is there anything I can do to attempt to get anything out of the FM using my signal generator and scope? ... sort of a quick-and-dirty alignment?
     
    Last edited: Feb 8, 2018
  14. dr*audio

    dr*audio Fish fingers and custard!

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    Do you get FM with the muting defeated?
     
  15. Zekeman

    Zekeman AK Subscriber Subscriber

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    No. Just hiss/faint noise. On some frequencies you can faintly hear tone changes, like it's close to a station, but no clear reception at all.
     
    Last edited: Feb 9, 2018
  16. dr*audio

    dr*audio Fish fingers and custard!

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    I think you got something misconnected. Can you put the old board back in? We were almost done.I would have had you bypass the filter next.
     
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  17. Zekeman

    Zekeman AK Subscriber Subscriber

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    Sure can...
     
  18. dr*audio

    dr*audio Fish fingers and custard!

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    Put 'er back in and bypass the filter. Let me know if you get audio and what signal on collectors.
     
  19. Zekeman

    Zekeman AK Subscriber Subscriber

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    OK back down to the shop to undo the transplant… Stay tuned
     
  20. Zekeman

    Zekeman AK Subscriber Subscriber

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    IMG_5683.JPG Here are the traces off the collectors.

    I bypassed the low pass filter and the volume level came up slightly above the abnormally low level and still remains below normal… At the same time it seems that the audio quality now is a very harsh and high-end sizzley and the low-end is completely gone…
     
    Last edited: Feb 9, 2018

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