sx939 intermittent AM

Discussion in 'Pioneer Audio' started by john stumpf, Mar 11, 2018.

  1. john stumpf

    john stumpf AK Subscriber Subscriber

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    so there i was minding my own business basking in the glory of my latest rebuild,listening to the am radio (which i hate) and the set goes dead! i punched fm and all is right with the world,back to am, zip. was my nos ha1138 defective? no all the voltages were reasonable. i probed and fussed all day yesterday to no avail all i knew for sure was if i turned it off for 20 min.'s it would come on and run for a while,tomorrow will be a better day! so this morning i decided to recheck the ic and see if there was much difference in the voltages when it was on as opposed to "faulted out". no apparent difference was noted. i put my finger on the ic to see if it was warm when withdrew my hand i ever so lightly touched f4. atf-020-0 and the set came to life! repeated touching would cause the thing to go off and on. theres no mention of its adjustment in the service manual,but i guess its a ferrite core. i moved the adjustment about 4 degees back and forth and put it back where it was and now i cant get the thing to trip off no matter what i do. i dont believe in magic and all the solder connections are perfect. does anyone have any ideas or comment or know if a NEW f4 can be had any where?
     

     

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  2. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    I'd run it and cringe while listening to the dreaded AM until I felt comfortable leaving it. :biggrin:
    I'm know what advice your looking for. Maybe Mark will chime in.
     
    Last edited: Mar 11, 2018
  3. john stumpf

    john stumpf AK Subscriber Subscriber

    Messages:
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    the plot thickens, after playing all morning on am it died again,this time f4 massage did nothing but touching the b+ pin on the ic brought it back to life im baffled
     
  4. john stumpf

    john stumpf AK Subscriber Subscriber

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    i walked away from it while i was dealing with drilling a new face plate glass. and so today i found a ha1138 data sheet (not easy) measured all the voltages on the ic when in normal am and tripped out. the b+ voltage is 13.25 and is rock steady. while some of the voltages seem to be 1/2 a volt over whats called for in the schematic the rest are reasonable. when it warms up and "trips" pin 13,if output goes to 7.8vdc from its normal 2.6 vdc and pin 15. if input drops to 1.14 vdc from its normal 2,53 vdc. im still haunted by f4's strange behavior. i hate to feed this thing another ic. whatever it is its temperature related and cooling the ic doesnt seem to matter. but something is being effected by heat. i just bought a used 535 tuner board to rob for parts. the fm is completely normal.any and all opinions are welcome.
     
  5. jeffpaletz

    jeffpaletz AK Subscriber

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    You sound very knowledgeable so is it safe to assume you've cleaned the switches and pots with Deoxit?
     
  6. glen65

    glen65 Member

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    How exactly did you come to that conclusion?
     
  7. john stumpf

    john stumpf AK Subscriber Subscriber

    Messages:
    491
    Location:
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    my expired amrip license says so. i do believe i found it last night. i let it warm and trip out. f4,atf--020-0 was warmer than everything else,im not sure WHAT is wrong with it but using alcohol and a q tip to cool the case with the set on "AM" came back to life. i didnt want to repeat the process as the elevated voltage on pin 13 of the ic CANT be any good for it. ive got a bargain basement 535 tuner board coming with a f4 on it am open to anyones ideas as to other possible causes until it gets here,but my mind seems to want believe its f4.
     
  8. petehall347

    petehall347 the brandy coffee man Subscriber

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    bad solder maybe ?
     
  9. glen65

    glen65 Member

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    I wasn't questioning your background,
    I asked because bad solder joints aren't always visible. Even if one looks at a solder connection under magnification,
    doesn't necessarily mean that person will spot something causing an issue.
    Plus, touching one component could be effecting a bad solder joint somewhere else in the general vicinity. Also, there may be more than one causing the issue your having. There could be multiple joints working against you at the same time. There also could be a circuit board trace causing an issue, possibly a separation right were it meets the solder pad.
     
    Last edited: Mar 20, 2018

     

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  10. john stumpf

    john stumpf AK Subscriber Subscriber

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    been there and done that and reflowed f4.i did it as i couldnt think of anything else to do. but f4 has 11.4 vdc applied to one of its windings. the original ic was toast so a n.o.s, ha1138 was installed with the same result except the replacement would play for hours depending on ambient temp. its been in the nineties and with the cover on drastically reduced time needed to trip, sooo, as theres so little in there with regard to am radio and because of the previous bizarre behavior in the beginning lead me to F4 with q tip. i wasnt getting any other helpful advice, and there was nothing to lose as ive no parts on hand. i was rather surprised that it actually did bring it back to life. besides its ALL FUN, isnt it? i do believe that tuner was undisturbed since it was manufactured.
     
  11. john stumpf

    john stumpf AK Subscriber Subscriber

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    well f4 finally arrived yesterday and went in this morning playing marvelously all morning in am,and much better for some reason. it could be that "atmospherics" are better today. i put it it in as i got from a dead 535 board. the service manual makes no mention of it in the alignment proceedures. im tempted to leave it alone, but my curiosity has gotten the better of me. anyone how it's adjusted?
     
  12. rcs16

    rcs16 Super Member

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    I never noticed this before, the sx-950 uses the same chip HA1138 as in the sx-939, sx-939 has adjustments Vr3,4, 950 has fixed values.
    I think you tune F4 for maximum detected o/p, but it probably fine where it is set too.
     
  13. john stumpf

    john stumpf AK Subscriber Subscriber

    Messages:
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    i fiddled with it but in the end i left it where it was. i did note that chip and transformer coil were extensively used in the x3x series. the tuner board was 15$ but the us mail charged 16$ to get it here! at least the damned thing works now and that was the goal. recapped that tuner and used film caps 10mfd and under through out and to my ears it sure made a difference, an fm broadcast had a guitar selection and i could clearly hear the squeak of the artists fingers on the strings! in fact there are NO e caps now in the whole set less than 10 mfd! all stacked film. not sure if it was worth the cost,but it does sound nice with the original output transistors to boot.
     
  14. rcs16

    rcs16 Super Member

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    Splurge, stacked films :) Nice to have some spare parts too. Glad to here you're enjoying your hardwork. Cheers
     
  15. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

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    l checked my service manual also just in case there was perhaps an early and a late version but it makes no mention of F4 adjustment in mine either.

    Great to hear you have the tuner working perfectly now and are able to enjoy your many hours of work. I used many films in the tuner on mine also, sounds good to me.
     
  16. rcs16

    rcs16 Super Member

    Messages:
    3,389
    Location:
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    As part of the Am tune up, you are to also adjust the slug in the bar antenna.
    Pioneers thinking was once they adjusted certain components at the factory, they will never need adjusting again, so why mention them in the tune up procedure :)
    The SX-939 AM tuner is different with those pots that many of the other tuners not having them.
     
  17. john stumpf

    john stumpf AK Subscriber Subscriber

    Messages:
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    Location:
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    the ferrite is adjusted as part of the alignment. i went back and had another go at f4,it WAS out of adjustment as near as i could tell, i adjusted it to the highest output volume and called it a day. thanks for all your help.now im on to the 1934 philco 84b and its a REAL fiasco.
     

     

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