sx 828 tuner is out of whack

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

  1. john stumpf

    john stumpf AK Subscriber Subscriber

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    im trying to do the fm tracking adjustment on my rebuilt 828. no matter what i do i cant seem to adjust the local oscilator L4,below 94 mhz im starting to back the slug out of the coil. the var cap is clean as a whistle and the shaft wipers were deoxited and lubed. im using a panasonic vp-8177 signal generator its ancient but it works. i verified it at 90 and 106mhz on a working pioneer tuner (sx-1500td) i know newbies arent supposed to fool around with tuners but this one was so whacked i figured i didnt have much to lose and do have a spare 828 front end. ive touched NO other adjustments. ive got 3 DIFFERENT pioneer 828 tracking proceedures and im following the proceedure from what i believe to be the latest version that has step 6 "adjust generator and tuning dial to 90 mhz" ..... ive verified that the air cap opens and closes completely and that the pointer covers the entire scale spectrum. i really want to learn how to do this/
     
  2. kevzep

    kevzep Its all about the Music Subscriber

    Yes well, tuners are not exactly the easiest thing to be fiddling with. But since you are in there......

    Are you using a Balun or something to load the Antenna input properly, the tuner wants to see 300Ω and the output of most RF gens is 50Ω or 75Ω...So that needs to be addressed.

    How much level are you feeding the tuner?

    Now you are starting at the low end so you are adjusting the coil, L4.
    If it is just the tracking alignment and not something else, you will have to get it as close as possible with the coil at the low end, then go to the top end and adjust CT4 to get it close, then go back down the other end and readjust the coil, then go back to the top and readjust the CT4.
    But I would not wind the slug almost right out of the coil, adjust it until its not quite at the top, so it still has some adjustment.
    The variable cap (CT4) will severely affect the alignment at the low end even though it is for aligning the top end, its a VERY sensitive adjustment and you can get the alignment miles off by just thinking about moving it, and if it it miles off at the top, it will pull the low end of the scale way up.
    what happens at the top, also affects the bottom.

    I had a JVC that was like this, took over an hour to get it tracking right!!

    Have another go and see how you get on, use a non inductive plastic type screwdriver for the CT4, they are VERY VERY sensitive to anything metallic and will throw the whole thing off.....
     
  3. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    Kev, the antenna has no effect on the LO. Just sayin'. Try to adjust the high end first. If the cap is way out of adjustment it will affect the low end adjustment. Also verify that when you tune to the extreme ends of the dial the pointer is spaced equally compared to the dial numbers. Maybe some idiot moved the pointer.
     
  4. john stumpf

    john stumpf AK Subscriber Subscriber

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    well the service nanual didnt address the balun issue but i did read it elsewhere. the 828 has two antenna inputs in the back a 75 ohm for a coax and a 300 0hm for a dipole i used the 75 ohm straight from the generator out put. i couldnt understand stacking baluns upon baluns as the set has its own internal balun which i must admit i dont understand that well. i started at 20db as recomended and as i dont know how tired the generator is i couldnt get a tone below 40db so i left it there. ill try your method tomorrow feel free to hurt my feelings its part of the learning curve
     
  5. john stumpf

    john stumpf AK Subscriber Subscriber

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    well the idiot would have been me as i had to restring the thing as i got it broken it was done to the service manual. i thought about moving the pointer but it was apparent that would have shifted it too far. at 87.4 mhz the cap is fully closed and the dial scale stop agrees with the mechanical stop of the cap
     
  6. john stumpf

    john stumpf AK Subscriber Subscriber

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    i have all the correct tools. but my screwdrivers have tiny steel blades i assume thats ok?
     
  7. kevzep

    kevzep Its all about the Music Subscriber

    Where did I say that??Or should I not bother trying to help? I wouldn't be advising if I didn't understand how a tuner works....."Just saying"
    I was first making sure the test equipment is connected correctly?
    As I happens, we more or less recommended the same approach.
     
  8. kevzep

    kevzep Its all about the Music Subscriber

    You might have to "over compensate", you'll see as you get the metal blade near CT4, it might start affecting the adjustment as you move nearer to it.

    What DB scale is that you're using, the one Pioneer is referring to is with respect to µV. 0db=1µV, So they ask for 20db for this adjustment which is 10µV. Your generator may have the correct scale,I'm just just checking. Yes Warren I know, this has no effect on the Local Osc, just saying.

    As the Doctor said, the dial pointer may be in the wrong place. Check that before you make any wild adjustments...
     
  9. john stumpf

    john stumpf AK Subscriber Subscriber

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    we are making progress! i got a tone on 90 mhz and 106 mhz! and the stations are where they are supposed to be! would like some feedback on the balun issue and the tools before i go further and YAWN its late! i will catch up tomorrow
     
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  10. john stumpf

    john stumpf AK Subscriber Subscriber

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    yes its a national vp-8177a and it does say 1uv=0db dial pointer agrees with the service manual whic says turn it down low and install the pointer on the low end of scale so its mechanically all the way to the left and the pointer is on the FIRST small lime on the scale
     
    Last edited: Oct 17, 2017
  11. rcs16

    rcs16 Super Member

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    Hi John,
    a "BIG" if you have a freq counter and a fet probe(best to use for loading of the LO), you should be able to measure the LO freq range and adjustments. You should see the freq change as you insert your adj tool if it has an effect on it. Could use a 100K R in series with the probe to minimize cap loading of the LO ckt, since it is not buffered
    I do not follow what you guys are talking about adjusting the balun, L4(T22-025)? there is no adjustment on the balun, are you referring to the 1st tuned RF stage consisting of L1,Cv1,Ct1?
    Your generator should be connected directly to the 75 ohm input jack. I am assuming national vp-8177a has one.
    I would not worry about the dial pointer, it could be out of wack, what is important is what freq gets tuned when the var cap is at min/max positions. The LO freq should cover the whole band ~87-109MHz, your LO adjustments are for this. Assumption is that the LO is hi side injection so add the IF (10.7Mhz) to ~87-109MHz = ~97.7M-119.7MHz
    To note the BW of the RF front end is usually in the order of a few MHz, so the adjustments usually have little effect once they are close to center.
    Sometimes it is advantageous to disconnect the AGC when doing adjustments.

    Good luck
    Rick
     
  12. kevzep

    kevzep Its all about the Music Subscriber

    Referring to the procedure in the SM for this tuner's FM tracking.
    They want you to set the RF gen with a 90mhz carrier with a 400hz tone at 100% modulation (which I always assume is about 75khz.)
    Then they want to to set the tuner dial at 90mhz.
    The they say Adjust L4 core first, then adjust L1, L2, L3 for maximum reading on VTVM and to make sure the tuning meter is centered.
    This is what Pioneer state in the manual, and its pretty much the same for all the analogue tuners I have worked on...

    I do like what you are suggesting though Rick, and appreciate you taking the time to post. Sometimes the procedures in the SM's are a bit out of sorts, especially Sansui and their love of those damn Genescopes...
     
  13. john stumpf

    john stumpf AK Subscriber Subscriber

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    well folks i repeated the procedure a half dozen times. i know now WHY you must do it more than once but really there was no significant change after the third time. the operation was a success! it tunes on station and the stereo on lite seems to function. adjusting t1 had no effect so i marked it and left it where it was originally. step 7 leads one to believe that the tuning needle is centered during this process. this does not seem to be the case. is there any way to remedy that? im not sure my generator is capable of doing the rest of the alignment and maybe i should quit while im ahead in the game as im fumbling around in new territory! i also did the am tracking but my god its noisy an awful lot of interference during the day check it out again tonight
     
  14. kevzep

    kevzep Its all about the Music Subscriber

    Yeah T1 is the IF transformer, you need to just leave that one, you're getting into tuner voodoo world there.

    If your tuning meter is off its most likely the discriminator is as you say "out of whack".

    To deal with these two parts you need a sweep gen with markers so you can find the centre. Just leave them for now.
     
  15. rcs16

    rcs16 Super Member

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    I just read the sm procedure, they talk about T1 and T2 bottom core, but nothing about the T2 top core, I think there is missing info.
    I think that there is more than one way to adj the discrim,
    the sweep method with the marker is one way to center the discrim. and adjust for best eye linearity on a scope. This is a method if you do not have a a low THD RF gen and a THD meter.

    you can zero the discriminator with a shorted RF or IF i/p , measure the DCV at R29/C28, adjust the bottom core for closest to 0DVC.
    The T2 top core, is adjusted for lowest THD, using high RF level, config as mono 75KHz deviation, 1KHz or 400Hz modualtion
    I think that you adjust T1 for max detected AC, at the o/p of the discrim with modulated carrier. You can also check T1 adjustment and monitor for THD to fine tune if it makes any diff.
    The discrim has to be DCV neutral or zeroed, for the deviation mute ckt to work properly, Q7,8,9. it is designed to apply mute as the carrier is deviated from the 10.7MHz center point.
    if can see this DC shifting action as the 10.7Mhz IF is deviated off of the center.

    I have a QX-9900, It is the same IF ckts, one day, I'll go over it and test my adj procedure.
     
  16. john stumpf

    john stumpf AK Subscriber Subscriber

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    i dont know if i could manage that without adult supervision. i know significantly less than you give me credit for. can you or anyone else recommend a sweep generator suitable for a home hobbyist? if you could suggest 3-4 models and makes that would have been used commercially i would appreciate it. theres a lot of surplus sold in manila for a song. my vp-8177a cost me 60$! i wasnt satisfied with the front end. i noticed that i couldnt even touch the plastic fm string pulley without throwing the station off. in fact any contact with font end threw it off. as i said the it was deoxit treated twice and lubed. and a non residue solvent used to clean the the cap fins. i R and R'ed the front end as i had another and repeated the tracking adjustment and success. the adjustments on the new frt end were very touchy and the signal generator signal could be heard at 20db and less! its going to take a while to find a signal generator, sure would like to know the best potential candidates for acquisition. i have a rigol digital o scope. but no distortion meter. if i botch the job i do have another tuner card. but im tired of sitting in the darkness!
     
    Last edited: Oct 19, 2017
  17. kevzep

    kevzep Its all about the Music Subscriber

    Thats a good method Rick, I don't see why it wouldn't work, I might try that, its definitely worth testing it out.

    I have to buy a Spectrum analyzer for some of the other work I do, so I am going to use that to look at IF stages.

    Then I have a Wavetek sweep gen with markers I was going to use for the discriminators, I am a visual guy, lik to be able see a visual representation.

    I actually like the procedures Pioneer use, no need for ridiculous gear....Sansui love the genescope method.
    Pioneer even go for the Lissajous method for setting the Multiplex which is goo.

     
  18. rcs16

    rcs16 Super Member

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    The IF sweeper method to adj detector linearity has its flaws. Look at later model procedures and they use this method.
    Most ratio detectors and quadrature detectors use the same method, DC zero with no IF i/p (only noise) and then adjust for linearity with full mono 75KHz modulation at a reasonable level to get the recovered audio at its S/N limits, say >100uV-1mV.
    The best method is with a THD or spectrum analyzer, as your eye can not see this fine differences in linearity, eye/sweeper is a coarse adjustment method.
    Methods using IF sweepers/markers wereused for aligning tuned IF stages (L/C) since it was easy to see the filter responses and symmetry.
    Sweeping a RF front end is an excellent way to adjust the RF tuned stages since they are usually L/C ckts. Not too many people have RF spectrum analyzers and tracking generators, I for one,
     
  19. john stumpf

    john stumpf AK Subscriber Subscriber

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    is there anything at all i can do to at least improve IF alignment?
     
  20. rcs16

    rcs16 Super Member

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    Try what I said to adjust for DCV 0V on the detector with no IF i/p.
     

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