sx 828 tuner is out of whack

Discussion in 'Pioneer Audio' started by john stumpf, Oct 17, 2017.

  1. kevzep

    kevzep Its all about the Music Subscriber

    Thanks Rick, really appreciate your responses here, I'm learning a bit from this...I'm going to try this method for the discriminator, I appreciate the angle you are coming from with regards to the visual...
    I guess the manufacturers assumed in the service shops, the tuners would be pretty close to start with, so quick check and if anything was "out of whack" it would be fairly obvious..

    I need a Spec analyzer as I work for an audio hire company, and we have a large amount of RF microphones and wireless personal monitoring, its good to use the spec to compile a large system with them all working together, in the same rack, we can have upwards of 16 RF mics and around 10-12 personal monitoring systems, if you are not careful with assigning you frequencies, all hell can break loose as you can imagine....
    I digress though....

    I'll report back, as I have a tuner here I need to look at in a Sansui QRX7500A, I think its out of whack, so I will try this method.

    I hope you don't mind me jumping in here John, we can all learn a bit more this way?

    I agree with what Rick has said, try that method, you'll get your tuning meter lined up which will in turn bring you a better alignment, when the discriminator is off, FM stereo can be quite distorted etc etc...
     
  2. rcs16

    rcs16 Super Member

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    Glad to hear kevzep that you are following along. I was referring to looking at the FM detector o/p with a SA or THD meter to adj it for linearity. Of course it is nice to have a SA, for RF purposes too, one day for me hopefully.

    I also thought since he has a o'scope, assuming it can get up to 100MHz, he can monitor anywhere in the RF chain to check for level as he adjusts and can also change i/p freq so look at the RF FE filter responses, to see where they are currently centered at, can even determine response curves/BW. With changing the RF freq up or down he can see where the filters are centered at.

    Another resource is fmtuner forum which I belong to, this is where the experts reside for any tuner ?'s
     
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  3. john stumpf

    john stumpf AK Subscriber Subscriber

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    and at the risk of appearing to be a complete ignoramus could you explain in detail how i should go about doing that?
     
  4. rcs16

    rcs16 Super Member

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    Try shorting the IF i/p,using a short piece of wire or jumper clips, pin 22 to ground 23 on AWE-016, measure with a DMM in DCV mode, the DCV at terminal 9 to ground. Using a hex non-metallic (nylon) tuning tool to adj T2 bottom core to see how close you can get to 0DCV. You can also watch the center tuning meter, as a coarse adjustment.
     
  5. kevzep

    kevzep Its all about the Music Subscriber

    I have used the Tuning meter as a coarse adjustment before too, its going to get you pretty close. I had one tuner someone must have been fiddling with as the discriminator was miles off....

    I actually am a member of the FM tuner group, but I can't remember my log in, I'll have to sort that out, its crossed my mind a few times to get back in that group...

    John, you'll have this tuner running in no time.....you're getting close!!
     
  6. rcs16

    rcs16 Super Member

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    I just did a quick test on the qx-9900. I noticed that when I tune to some fav weaker stations the detector is not at 0DCV, to get a clear signal. If I tune to a strong station I can get it to zero with clean audio.
    I shorted the IF,pins 22,23, the signal still feeds through but more noise. I used some long leads. If I remove the RF ant i/p as well, I get noise and the detector is at 250mV.
    The detector moves about +/-1.6VDC.
    IF I have the RF open I get some noise, if I salso short the IF I get less noise and detector is at 250mV
    My muting does not work very well. I'd have to muck with it to see what the reason is. I think that the detector needs to be zeroed.

    See if John gets the same results
     
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  7. john stumpf

    john stumpf AK Subscriber Subscriber

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    i did as you said. it started out at .006vdc. i had to lift the board as the slug was quite stiff. the core needed to be turned approximately 1 turn counter clock wise as viewed from the top to achieve 0 vdc. the needle will center and "0' can be achieved on all stations with a clear signal. BUT the highest voltage seen on pin 9 is. 008 vdc. selectivity is good but sensitivity could be better
     
    Last edited: Oct 23, 2017
  8. kevzep

    kevzep Its all about the Music Subscriber

    Nice work, looks like you are making some good progress there...
     
  9. john stumpf

    john stumpf AK Subscriber Subscriber

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    is this all that can be accomplished?
     
  10. rcs16

    rcs16 Super Member

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    The detector should swing about a good volt either way of center tuning. try again and slightly de-tune the var cap in either direction on a strong signal. The ckt has to swing this much for the deviation mute detector ckt to work properly, you have to turn on D8,9, Q7,8 for it to work as intended.
    You need to go back and try to adj the FE to see if you can fine tune it some more for better sensitivity. If your scope can read 100MHz range, you could inject a fairly large signal into the FM ant i/p (~1mV) and probe the o/p of the second RF stage at Q2 drain, watch the response of the tuned ckts, as you slightly change the i/p freq.
    You can also scope the IF section say at the 1st IF filter CF1 o/p and adjust the RF FE,LO to get best IF signal peaking.

    Good luck
    Rick
     
  11. john stumpf

    john stumpf AK Subscriber Subscriber

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    to be continued....
     
  12. john stumpf

    john stumpf AK Subscriber Subscriber

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    its been a while but i finally am able to give you an update.no matter what i did i couldnt improve sensitivity on the sx-828, while it did play well on channel's it would tune in on. i even recapped it as i had the caps to no avail. with much anguish i elected to replace the board with a spare i got with the set after a tracking alignment was an INSTANT WINNER. i can understand now why few respond to these threads written by the un trained. its damned difficult to explain how to proceed.i'd never used a signal generator in my life but that was the challenge. i didnt even know if it still worked! i even had to listen to a 400 hz test tone on you tube several times to burn it into my ears. in the end that 60 $ generator was the best 60$ ive spent in a long time! believing in my heart i wasnt dangerous anymore i moved on to an sx880,890 and a 1010 and finally an 850. i found that all were out am and fm to a significant degree. i discovered that after a air cap cleaning and lube, the next thing to do was CENTER THE POINTER on its entire SCALE. that is to say, make sure its travel was equidistant on its travel on the scale from center to both ends. only then was it time to run a alignment. i found that for me at least that following the service manual worked. but was a bit disappointed that while the tape rec. out was a mv ac test point to the highest un-distorted value it failed to say what that value should be and they were all a bit different. adjustments are subtle most of the time but some times not. the real surprise was the "clunker" sx850 i bought for 49$ on ebay several years ago. i had rebuilt it and it played well but was off station. after centering the tuning needle on the scale it was virtually spot on with hardly any drift and only a minor tuning needle adjustment to center it.i found that the newer tuners are easier to align as they dont require a sweep generator which i cant find here at all at any price. neither can i do mpx adjustments without a stereo signal generator,i can only speculate what the results of all this would be with a PRO doing it with all the right gear. but all now are WAY better off with working stereo lights and centered tuning needles and reasonable separation.im not scared of tuner's anymore even though troubleshooting them still eludes me! p.s. there are several errors in the sx 880/890 schematic the tuning and signal meter's are switched and c418 is installed backwards in the set as the silk screen is opposite of the schematic and circuit board.
     
    Last edited: Nov 13, 2017
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