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Help with fm tuner

Discussion in 'Pioneer Audio' started by globo2006, Mar 20, 2017.

  1. globo2006

    globo2006 New Member

    Messages:
    22
    I bought a pioneer sx-680 and have a problem with the FM tuner.
    The radio stations are heard well but the tuning meter does not move, nor does it turn on the light indicating stereo.
    I changed the FM mute switch but almost nothing is heard
    In AM, the tuning meter works fine.
    When switching to another function, for example phono, or when I turn off the equipment, the tunning meter is centered.
    I have cleaned the function switch with cleaning contacts but the problem continues
    Someone can guide me with a solution
    Thank you very much
     
  2. rcs16

    rcs16 Super Member

    Messages:
    2,728
    Location:
    Baldwin, Ontario, Canada
    Seems to be lots of tuner issues these days on AK. The hardest ones to fix even for the experts.
    It is near impossible to point to a cause with out some troubleshooting and understanding how the circuits work.
    Is the stereo lamp burnt out or is the signal too low to be able to activate it, has to be investigated.
    Did you buy it untested?
    The FM muting switch is used to quiet the audio when you tune between stations.
    if the sig/tune meter work in AM then it has to be either the selector switch or the IF system chip. You could check continuity of the selector switch for the tuning meter and check operation of the AFC circuit, but you need to understand how the ckts work. Wish I had a simple answer.
     
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  3. globo2006

    globo2006 New Member

    Messages:
    22
    First of all thanks for your answer. I try it when I buy it but do not pay attention to this problem.
    I have something else to add because after starting the topic I was trying other things. Also find that in FM the volume potentiometer makes noise as if it were dirty and using it with other sources does not.
    I do not understand much of electronic like to be able to verify what you indicate to me.
    The antenna has and signal received since the FM radio stations listened to them well.
    Thanks again
     
  4. john stumpf

    john stumpf AK Subscriber Subscriber

    Messages:
    291
    Location:
    philippines
    before id blame the tuner itself i'd deoxit the s1 function switch and check the "stereo" indicator bulb.
     
  5. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    873
    Location:
    Victoria, Australia
    rcs16 is correct, they are usually time consuming to fix, l have had a couple of FM issues recently, a TEAC with similar issues to yours ended up just being one of the ceramic cap/resistor packages but it can be anything in the signal chain. I haven't looked at a schematic but l would guess that your SX-680 would have few discrete components in conjuction with IC's as rcs16 alluded to.

    Does the SX-680 have a signal strength meter?
     
  6. rcs16

    rcs16 Super Member

    Messages:
    2,728
    Location:
    Baldwin, Ontario, Canada
    The SX-680 has one meter for tuning, it acts as a center tuning meter for FM and RSSI for AM.
    It gets switched through S1 as said above.
    Then I would check out that S1 switch for continuity for the tuning meter.
    Try cleaning the pot first with fader lube.
     
  7. I LIKE MUSIC

    I LIKE MUSIC Super Member


    These 2 statements are somewhat contradictory. First you state that it receives stations well, then you state that almost nothing is heard.

    Did the almost nothing is heard condition happen after you changed the FM muting switch?

    Some clarification may help us help you determine what is wrong.
     
  8. globo2006

    globo2006 New Member

    Messages:
    22
    [Quote = "John Stumpf, de la publicación: 10397973, miembro de: 137021"]. Antes de Identificación culpa al sintonizador sí estaria DeoxIT el selector de Función S1 y Revisar el foco indicador "estéreo" [/ quote]
    Unfortunately, in my country (Argentina) the deoxit is not marketed. I have cleaned with Contamatic both the switches and the potentiometers.
    Thanks for your answer
     
  9. globo2006

    globo2006 New Member

    Messages:
    22
    When I switch the switch to FM mute is when you do not hear almost anything
     
  10. globo2006

    globo2006 New Member

    Messages:
    22
    I will measure continuity from the switch and verify that the lamp that indicates stereo works. Also check the operation of the FM mute switch. As well as welding.
    I keep track of them later.
    Thank you all
     
  11. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    873
    Location:
    Victoria, Australia
    If you don't receive any stations with FM muting on then that would suggest that your signal strength is not high enough even though you can hear some stations clearly with muting off.

    Thanks for that, l am not familiar with the SX-680.
     
  12. globo2006

    globo2006 New Member

    Messages:
    22
    Thank you Oldsansui441. I‘ll try another antenna
     
  13. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    873
    Location:
    Victoria, Australia
    Your welcome. By all means try another antenna but l don't think this will be your problem, l meant the FM section probably isn't boosting signal strength enough especially since you have no tuning meter deflection, no stereo light and no stations with muting on.
    Normally most receivers/tuners will get near full signal strength on strong local stations with nothing more than a small piece of wire as an antenna.
     
  14. globo2006

    globo2006 New Member

    Messages:
    22
    Thanks again.
    What strikes me is that with the FM switch mute on off, the FM stations are heard well, but as I said in my first post, the signal indicator does not work in that mode, nor does the light on stereo
     
  15. globo2006

    globo2006 New Member

    Messages:
    22
    Reviewing a little of what you said. I wonder if it is possible that the function selector is so dirty internally that it does not make enough contact to send signal to the lamp and the meter?
    I applied a good amount of clean contact and then triggered it several times, but maybe that was not enough.
     
  16. I LIKE MUSIC

    I LIKE MUSIC Super Member

  17. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    873
    Location:
    Victoria, Australia
    Certainly wouldn't hurt, redo any switch to do with tuner function.

    Does it get lots of stations with the muting switch off, even more distant stations? Do you have another tuner to compare station count and reception to?. Just trying to ascertain if in fact it has poor signal strength or whether it is as you have asked, a fault in the tuning meter/stereo light/muting circuits only.
     
  18. I LIKE MUSIC

    I LIKE MUSIC Super Member

    The following things may be more than you want to do.

    First you can clean the tuning capacitor per Dr. Audio's post. This may help with the sensitivity and it is a good first step.


    See the picture below. Next, you can use your DMM to measure the voltage across resistor R125, a 300 Ohm resistor. In the FM mode, this voltage goes directly to the tuning meter through the mode switch. If you measure the voltage here, your are measuring the voltage before the switch.

    SX-680 CENTER TUNING METER TEST CONNECTION.JPG

    If you connect the positive lead of your DMM to the point circled in red and the negative lead to the point circled in blue, when start to tune in a station from below its frequency, your DMM should show a small negative voltage and as you approach having the station tuned in the voltage should go to zero and as you continue to tune up in frequency the voltage should go positive.

    In the next picture I have circled some adjustment points having to do with the sensitivity of the turner and the position of the meter when in the FM mode, but in is not usual for these to be far enough out of adjustment to cause you problem unless someone has changed there adjustment, although sometimes the trimmers TC2 and TC3 can be a little sketchy.

    sx-680 TUNER ADJUSTMENTS.JPG

    You do not want to clean them, but you can use a Sharpie maker and mark their positions by making a mark on what looks the the top of the screw and making a line from there to the body of the tuning capacitor. You can then work them back them back and forth several times, then return them to their original position by lining up the marks. This may or may not help (I have seen it help) but it is easy to do and is part of the process of elimination.

    If you connect you DMM to TP1 and ground as shown, you can measure the received signal strength. It should be around at least 2 to 3 volts DC or more on a moderately strong station. This can be an indication of the sensitivity of your receiver.

    Transformer T2 is an inductor and will likely not benefit from moving its adjustment back and forth so you can ignore it for now.

    I will stop at this point because I do not know if you are comfortable with the above. And if not that is okay, not everyone wants to dive into this level of trouble shooting a tuner and that is quite understandable.

    There are only three adjustments for the FM sensitivity and one for the center tuning meter. As I have mentioned these do not usually go out of adjust far enough to cause your problem unless someone has been in the tuner section. I can post those if you are interested. They use the same test point indicated above, using your DMM with your receiver tuned to a stations at the upper end of the dial.

    Sorry for the lack of pictures, AK has gone sketchy on me, I can not post attachments and I have lost all of the options that are normally at the top of the window. I will try to post the pictures that I referenced at a later time.
     
    Last edited: Mar 22, 2017
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  19. I LIKE MUSIC

    I LIKE MUSIC Super Member

    AK is once again working for me and I added the pictures to the above post and corrected some errors.
     
  20. rcs16

    rcs16 Super Member

    Messages:
    2,728
    Location:
    Baldwin, Ontario, Canada
    Well done "I like music"
    he can also monitor the RSSI drive signal coming out of the FM IF chip(PA3001A-13) at TP1 per the service manual on p14.
    The mute circuit is contained in the PA3001A, s4-b connects pin 12(mute out) to pin 5(mute in)
     
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