Sansui G-8700 no tuning center indicator, no tuner lock led, no stereo

Discussion in 'Tuners' started by jpaudio, May 9, 2017.

  1. jpaudio

    jpaudio New Member

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    Hello to all fellow members. Recently, I got in my hands one nice and shinny G-8700 receiver. Unfortunately, tuner is working only at mono mode without locking feature and tuning center indicator. After a short IF adjustment, I managed to have either right or left indicator arrow lit but not center indicator. Looking at signal comes from tuning detector (IC 02) both lines are equal 5.6V means center indicator might be on with no arrows lit. In addition, locking led is not lit either. I tried several meassurements according to Service manual with no sucess. Last big try, MSM5540 repalcement. I put a brand new but no progress. Note that frequency indication and string pointer much and sound quality (in mono mode) is perfect. But no Stereo, no lock, no center ind.
    Any ideas? Is this common problem of units use the same circuit?
    Thanks in advance for your kind attention and reply.
    Regards,
    John
     
  2. Tom B

    Tom B AK Subscriber Subscriber

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    What is the signal strength looking like? If it is very low even when you tune in a signal, you may have to do some front end alingments to bring the signal up enough to kick in stereo. But be very cautious on any adjustments you try. Always make sure that you keep track of the starting point or position. If you turn a coil slug for example 1 turn, and it has no aparrant effect, you should always bring that back to its original point before you do another adjustment. And as you do any adjustment, you should have a set of headphones on, and view the signal strength, center tune, and other indicators such as stereo light, and tuning lock. Also read up on the operations manual. If you have a weak signal, fm muting usually keeps the sound muted. If you put the muting off, sometime the receiver will disable the Stereo function. Same thing with Stereo/mono function.
    I would think you may have a FM front end issue and FM Discriminator issue. Both issues could be simple adjustments. There is also a VCO adjustment that can potentially bring the Stereo back.
    Last but not least, look for signs of work on the boards. see if someone else replaced parts, or if there is signs that adjustments were attempted. (many adjustable resistors, as well as the capacitor screw adjustments on the tuning cap usually have glyptol or a similar enamel like nail polish that they put on these adjustments to lock them. If you see that the locking enamel isn't matching the screw and the base below the screw, someone has messed with them. It is usually not hard to work through the front end adjustments, even if they have been messed with, but the IF, Discriminator, IF, and MPX adjustments are a bit harder to get right after they have all been messed with
     
  3. jpaudio

    jpaudio New Member

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    Thanks a lot for your reply Tom. All these checks are simple for me and done initially. Signal meter shows 90% so signal strength is pretty enough. No other technician did repair works before. Looking around net for units that have same circuity (Sansui tuners and receivers) there are some that reported same fault but not a solution, I had the feeling that such a problem is common, but I am looking for a "deeper" opinion, it is not an easy repair...
    Regards,
    John
     
  4. Tom B

    Tom B AK Subscriber Subscriber

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    The only problem I have seen with these type of units that I could call "Common Problem" is related to the Selector switch. I worked on a G9700 (very similar), that had no AM or FM, and no digital channel display. I found that the FM boards and the digital display board was not getting the required +12 volts. Looking at the schmatic I saw that +12 feeds through the function switch to the FM board, Tuner, and digital display for AM and FM, across multiple contacts. The switch got dirty, bad connections, and as a result, the contacts for feeding this +12 volts was very intermittant no matter how I tried cleaning the switch. Most units feed the 12 volts to where it is needed all the time, but this G series Sansui switched it on for FM and AM. That switch is like a 7 position switch with 6 or 8 sections of contacts. 42 or 56 contacts total. Its impossible to find. I was lucky to locate unused contacts, and jumper and bridge the 12v through them for AM and FM.
    While I expect more of a problem where FM is out totally, rather than no stereo, and not lock or center tune movement, I would suggest checking the display board, and FM board and make sure the 13v from the RF power supply is appearing everywhere it should (Check -12v as well).
    I have also heard of prescaler problems, but that I believe just affects the digital channel display.
     
  5. jpaudio

    jpaudio New Member

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    Hi. All voltages seem to be in propper values. Since display unit is operating showing correct frequency, signal level, etc. I asume there is no issue about power to this. For an unown reason, lock and tunning center are not done. I would apreciate if an exert could comment or point out where to look in detail. I remind that I replaced MSM5540 IC with no success. Reading arround, this component is the most possible cause of problems but not at my case...
    Regards,
    John
     
  6. Tom B

    Tom B AK Subscriber Subscriber

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    That chip could cause issues with the display, but will not cause a problem with FM stereo. FM Stereo problem is with MPX, IF, muting, or RF
     
  7. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Might be an idea to have this thread moved to "Tuners" - where 'Punker X' may be able to point you in the right direction.
     
  8. jpaudio

    jpaudio New Member

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    I totally agree, maybe moderators can move to right place or I can start a new one, thanks.
    Regards,
    John
     
  9. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Not good to post same question twice - I think if you report your own thread starter post, (using the report link below left) and ask for the thread to be moved - that should do it.
     
  10. Celt

    Celt Super Maude Super Mod Subscriber

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    MOOoooooved...
     
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  11. Punker X

    Punker X AK Subscriber Subscriber

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    I'd start with the alignment. First 7 steps have to do with discriminator/Detector adjustment. Monster receiver would be hard to ship out for an alignment. If adjustment didn't respond correctly I'd be looking at HA-11225, it also is tied into muting functions which often causes issues, leaking or bad switching transistor cause strange problems.
     
  12. jpaudio

    jpaudio New Member

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    Hi Mike, thanks a lot for your response. Can aligment be done without special instruments, I mean signal generator, genescope (!)? What do you mean with term "7 steps"?
    Regards,
    John
     
  13. Punker X

    Punker X AK Subscriber Subscriber

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    FM alignment, 2-3... seven steps there.. but main would be 2 and 3. Along with the VCO on on the MPX alignment. Was looking more at the scat. I have an old analog tuning meter from a tuner with a cap and dropping resistor. Use it in cases like these. I'd hook it up to pin 15 an 17 of the display board and look for meter swings and zero. If you are getting the zero tuned voltage there and the correct 5vDC to ground on 15. I would then look at IC03, TR5 and 6.
     
  14. Punker X

    Punker X AK Subscriber Subscriber

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    Yes you need special test equipment.. Not a genscope but a signal generator, with FM MPX and Oscope and distortion meter and know what you are trying to accomplish with the genscope.
     
  15. jpaudio

    jpaudio New Member

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    Hi. Problem solved, finally. IC03, TR5,6,7 replaced. Re-adjustment done and all indicators are OK. Important: muting level must be precisely adjusted, otherwise will keep tuner off-tuned!
    Thanks all who replied and especially Mike (Punker-X)
     
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  16. Punker X

    Punker X AK Subscriber Subscriber

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    Always good to have a monster receiver back to 100%
     
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