SONY ST-J75 Muting Adjustment

Discussion in 'Tuners' started by spark1, Mar 10, 2018.

  1. spark1

    spark1 Well-Known Member

    Messages:
    863
    Location:
    The Great Midwest
    I have the ST-J75 tuner, and it has a tendency to momentarily mute the signal when it contains heavy bass. This behavior seems to subside quite a bit after the unit is warmed up (15 minutes or so), but still occasionally does it.

    I plan on giving this tuner, and a matching Sony integrated amp, to a friend who loves music but cannot afford a decent stereo system. She lives about 45 minutes from a major metro, and currently uses an old boom box. She also likes listening to public radio stations. I'm including a pair of Boston A60 speakers and a pretty good Rotel CD player...I think she will really appreciate a nicer system.

    Anyway, I am recapping both the tuner and the little amp before I give them to her...mostly to reduce the chances of future problems. I have no plans to adjust the tuner (don't have the tools or the skills!). However, I would like to try to back off the sensitivity of the muting via IFT101. While I can certainly experiment, I would prefer to minimize the amount of of adjustment I do. So, was wondering if anyone can confirm which way to turn IFT101 to reduce sensitivity to low-frequency signals?

    Thank you!
     

     

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  2. Punker X

    Punker X AK Subscriber Subscriber

    Messages:
    3,926
    Location:
    Panic in Detroit
    My experience is with this it is not the muting adjustment, but the discriminator adjustment. If not zeroed correctly the heavy bass makes the tuner seem like it is not zero tuned and it mutes. Easy adjustment with a voltmeter. Wouldn't use a metal tool to adjust this.
     
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  3. dr*audio

    dr*audio Fish fingers and custard!

    Messages:
    18,484
    Location:
    Cleveland, Ohio
    You don't need a dmm to do this. Adjust IFT101 for the midpoint at which you get a stable stereo signal on a known good channel.
     
  4. spark1

    spark1 Well-Known Member

    Messages:
    863
    Location:
    The Great Midwest
    Thanks for the guidance...I appreciate it.

    However, I don't know what you mean by adjusting it to the midpoint of a stable signal? Are you saying just keep adjusting until it quits muting?

    Thanks again!
     
  5. spark1

    spark1 Well-Known Member

    Messages:
    863
    Location:
    The Great Midwest
    After looking at the service manual (I don't know why I didn't do this before...my bad), page 18 does show a 2-step adjustment of the discriminator circuit. The second step requires only a multimeter, but the first step requires and signal generator and distortion meter (I have neither).

    Will doing just the second step solve my problem better than adjusting it by ear (which I'm assuming is what is meant by adjusting it to the midpoint of a stable signal)?

    Sorry to be so ignorant...I've always stayed away from the tuner sections in any gear I've worked on!
     
  6. dr*audio

    dr*audio Fish fingers and custard!

    Messages:
    18,484
    Location:
    Cleveland, Ohio
    This is not the discriminator adjustment. This tuner uses a second IF filter, IFT101 to decide where to lock the signal in. Using a non metallic tool, Adjust IFT101 by turning it back and forth and noting where the stereo lamp goes out or when it mutes (stereo lamp is a better guide.) Turn it one direction, and the lamp goes out. Turn it back and the lamp goes on. Continue turning it in the same direction and the lamp goes out. Midway between the two extremes where the lamp goes out is where you want it. Then fine tune it if necessary listening for it to mute on bass notes. A Rap or R&B station is good for this. Tweak it so it stops muting.
     

     

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  7. spark1

    spark1 Well-Known Member

    Messages:
    863
    Location:
    The Great Midwest
    Doctor, thank you VERY much for such clear instructions. I think I can handle this! I appreciate your help with this, and appreciate the many other things I've learned reading your posts.
     
  8. spark1

    spark1 Well-Known Member

    Messages:
    863
    Location:
    The Great Midwest
    Well, after buying a set of plastic adjustment tools and upon opening up the tuner and locating 1FT101, I think I see the problem! Apparently someone before me tried to adjust it and damaged it. The red "ring" at the top is missing, the carbon(?) material seems cracked and it will not turn in either direction...although I did not try too hard to force it.

    Is there anything that can be done, or is this nice little tuner destined for the recycler? Thanks!
     
  9. dr*audio

    dr*audio Fish fingers and custard!

    Messages:
    18,484
    Location:
    Cleveland, Ohio
    It's called a ferrite core and someone here wrote about how to extract a broken ferrite. You can search here for how to extract a broken ferrite, but I did a Google search and found a method which I think is the same one. You drill it out with a drill bit, carefully by hand. Use a bit holder, Harbour Freight sells them. Then you buy a replacement or steal one from a junk unit.
     
  10. spark1

    spark1 Well-Known Member

    Messages:
    863
    Location:
    The Great Midwest
    Thanks. Sounds like it might be a bit more than I can handle, having had no experience with this type of component. It looks like there are a wide variety of "slugs" out there, with differing values, threading, sizes, etc. Too bad, because this tuner is in nice shape otherwise, and does an amazing job of picking up stations....much better than any other tuners I own (either in receivers or separate).

    To be clear, the one in question is in a two-core assembly...what i would assume is a metal housing, containing 1FT101 and 1FT102. The whole device is labeled IFT102 on a blow-up in the service manual, but from what I can tell 1FT102 is the one with white ceramic/paint on the slug(?), while 1FT101 is the other (supposed to be red).

    The service manual doesn't mention anything about low-frequency cut-out. It does show an adjustment procedure in which you adjust for 0 voltage, measured at a nearby resistor, with the unit "de-tuned" (which I assume means not on a station).

    All somewhat academic now for my situation, but might be helpful to someone else.
     
  11. kyrie59

    kyrie59 New Member

    Messages:
    45
    Oh please, don't scrap it, these vintage pieces are audio history that can't be replaced!! When young I bought the ST-J55, and loved it... but lost it in a divorce. Got out of hifi for a long time, but got back in about 10 yrs ago, and bought this tuner from eBay. Got lucky in buying it from a repair tech that had just refurbished it, sounds great. These Sony tuners were some of the best in S/N ratio, and sound great today with all the trash floating around in the air.
    Good luck!!
     

     

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