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Help needed with Sansui 310 FM section

Discussion in 'Exclusively Sansui' started by Thierry, Jun 16, 2007.

  1. Thierry

    Thierry Tape with a passion

    Messages:
    554
    Location:
    Paris, France
    Hi Sansui fans,

    As you may know I am restoring a very old Sansui 310.

    Work can be seen here http://www.dcx2496.fr/sansui310.php

    I am left with only one problem : the stereo indicator doesn't light and the meter doesn't move in FM, although the tuner receives strong signal and produces clean and detailed sound but in MONO

    The stereo indicator bulb is ok (tested it) and the meter is perfectly working in AM. I am quite baffled :scratch2:

    I ordered the 310's service manual from http://www.stereomanuals.com but if a specialist coud show me a track I would appreciate :thmbsp:

    Cheers

    Thierry
     

     

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  2. Thierry

    Thierry Tape with a passion

    Messages:
    554
    Location:
    Paris, France
    Hi all,

    I had now answer so I made some research and as unbelievable as it seems I managed to find a brand new TA7061AP chip :yes:

    Mind this : after 35 years an original Toshiba FM IF amplifier chip was still floating around in a shop :banana:

    I don't know if a new chip will cure the problem but I will replace it as I have no clue about the reason the stereo isn't there. I would appreciate any hint from the FM specialists though :thmbsp:

    Is there any other circuit I should check ? For exemple this red box labelled Sansui on the left of the FM PCB: http://www.dcx2496.fr/Sansui 310/P1000650.jpg

    I remind you that I have an absolutely perfect FM MONO sound

    Thierry
     
  3. Stene

    Stene The Introducer

    Messages:
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    Location:
    Stockholm Sweden
  4. Thierry

    Thierry Tape with a passion

    Messages:
    554
    Location:
    Paris, France
  5. joemango

    joemango Active Member

    Messages:
    157
    Location:
    True North
    The TA7061AP is an IF amplifier and you have mono which means the IF path is likely OK. The flow is RF -> IF -> DET -> MPX(stereo decoder). The problem lies in the DETector and MultiPleX sections.

    The red rectangle on the left could be the 19kHz notch filter. Your pic doesn't show the reference designator for this components so it's hard to tell.

    Sounds like you have no clue and replacing parts in random:nono: I've seen this happen a lot of times and the tuner never gets fixed. Wait for the manual, read the schematic and theory of operation, then troubleshoot. You should have at least a 100MHz scope with a 10x probe and strong stereo broadcast signal to trace the signals.
     
  6. Thierry

    Thierry Tape with a passion

    Messages:
    554
    Location:
    Paris, France

    Thanks, this is the word of the wise man. So I will do (manual has left the US today, should be there next week)
     

     

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  7. Fred Longworth

    Fred Longworth audio fanatic

    Messages:
    3,347
    Location:
    San Diego
    Replace the multiplex IC and all the electrolytic capacitors in the multiplex section. Odds are 2 out of 3 that'll fix the problem.

    Fred Longworth
     
  8. Thierry

    Thierry Tape with a passion

    Messages:
    554
    Location:
    Paris, France
    Thank you very much Fred,

    Once I'll have the schematics in hand I will definitely do so. I can borrow a scope too, that'll help :yes:

    I have noticed something weird : when touching the body of the transistor marked in yellow on this picture with a metal object, the stereo indicator lights up and there's a huge hiss in the speakers : http://www.dcx2496.fr/Sansui 310/trans.jpg

    Thierry
     
  9. joemango

    joemango Active Member

    Messages:
    157
    Location:
    True North
    Problem is, I don't see any multiplex IC in this receiver. It appears to use the older method of bridge Time Division Multiplex decoder using diodes. I now believe the red rectangle on the side is actually the transformer for the bridge and not a 19kHz notch filter.
     
  10. Stene

    Stene The Introducer

    Messages:
    2,200
    Location:
    Stockholm Sweden
    Part of circuit
     
    Last edited: Mar 8, 2009
  11. Thierry

    Thierry Tape with a passion

    Messages:
    554
    Location:
    Paris, France
    Thanks for your support gentlemen, I'll let you know what turns out. The only IC I could spot on the PCB is the IF amplifier. The diagram and the service manual are on their way.

    Thierry
     

     

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  12. Stene

    Stene The Introducer

    Messages:
    2,200
    Location:
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    Another part

    This part of the circuit might be of more help
     
    Last edited: Mar 8, 2009
  13. joemango

    joemango Active Member

    Messages:
    157
    Location:
    True North
    Thanks Stene.

    The first attachment was for the IF amp/limiter and ratio detector.

    While the 2nd is the multiplex circuit that I was talking about, no MPX IC was used.
     
  14. joemango

    joemango Active Member

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    157
    Location:
    True North
    Were you tuned to a stereo broadcast when you did this experiment?
     
  15. Thierry

    Thierry Tape with a passion

    Messages:
    554
    Location:
    Paris, France
    Yes I was.
     
  16. joemango

    joemango Active Member

    Messages:
    157
    Location:
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    This opens up the possibility that the RF section is weak or not getting enough amplification. Mono signal is more forgiving than stereo reception. In mono you can have way less noise for a given RF signal than stereo. How is the sensitivity across the band? Are you getting lots of noise when tuned to a weaker station? You did mention on your earlier post that the tuning meter doesn't move at all.

    Make sure you are connected to a useable antenna like rabbit ears. The connection from the rear antenna terminal all the way to the front end should be solid. That metal can transistor you touched is the MOSFET RF amplifier. Check for anything unusual like charred components or bad solder joints before and around the RF amp.
     

     

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  17. Thierry

    Thierry Tape with a passion

    Messages:
    554
    Location:
    Paris, France
    I confirm that the signal strength meter doesn't move at all in FM, although it works perfectly in AM.

    I am using a 300 ohms FM antenna in the shape of a T and it is connected to the proper terminals on the back pannel.

    I can hear all of the available FM stations accross the whole band (about 40 in my area) most of them are heard loud and clear. On strong stations I can hardly hear any hiss or noise.

    BUT the meter doesn't move at all I can get only mono. I reckon signal force isn't an issue, there's something wrong somewhere else :scratch2:

    Anyway I really appreciate your help :thmbsp:

    Thierry
     
  18. joemango

    joemango Active Member

    Messages:
    157
    Location:
    True North
    You've already tweaked the front end so there is no point of not touching the ratio detector.

    Try tweaking the transformer near D201 (one with the pink sleeve around the tuning slug) while tuned to a strong stereo broadcast. If the detector is mis-tuned there will be audio but not strong enough for the multiplex circuit. This is what is called the S-curve response. Ideally you want to be in the center. This is where the maximum detected signal and symmetry (minimum distortion) is. Just remember to mark the original position if this doesn't work. Hopefully you get stereo this time

    The signal meter operation in FM mode could be something else like you said. Bad contacts maybe.

    Good luck!
     
  19. Thierry

    Thierry Tape with a passion

    Messages:
    554
    Location:
    Paris, France
    Well I have managed to get the stereo indicator work by turning the above mentionned transformer plunger but unfortunately I have stereo ONLY when the receiver is tuned between two adjacent stations ! This means that I can hear both stations together (with considerable hiss in the background)

    In addition this occurs only in FM mid-band, I mean when the index is located around 98MHz. If I make it go left or right from this position I cannot get the stereo light working nor hear any stereo program. This is probably what is called misalignment.

    Mono reception is really impressive though, perfectly silent and without any hint of distorsion.

    I suppose really need proper equipement and schematics to go further I think.
     
  20. MarkMeyer

    MarkMeyer Active Member

    Messages:
    333
    Location:
    Texas
    Yup. I confess that I'm confident enough to tackle the integrated amps I have, but I'm not even going to begin worrying about any of the tuners until I can afford a good scope.

    It's one thing to get a tuner to "work", but it's another to get it working "right" and a scope is a necessity for that.
     

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