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Restoring Pioneer SX-790

Discussion in 'Pioneer Audio' started by Knutsul, Jan 25, 2018.

  1. Knutsul

    Knutsul New Member

    Messages:
    13
    Came across this beautifull reciever and decided I have to rescue it from a certain death. Its in a rough cosmetical and technical state. Since there are not many around off them I think its worth to fix it.

    Here a picture of how I found here:
    [​IMG]

    Unfortunatly the case and top of the front plate are damaged:
    [​IMG]
    [​IMG]

    Frontplate is pretty good. Some scratches around the earphone thing and a scratched tuner knob.
    The sound at the moment is horrible both channels are very low and distorted.

    So what needs to be done:
    -Cover needs cosmetical fixes.
    -Top of front plate needs a polish
    -If possible remove the scratches around the headphone entrence.
    -Tuner knob needs a scratch remove
    -Complete technical check

    I know its a lot of work and maybe people wil ask if its worth it. But most of the parts are in not so good shape so it wouldnt be a good parts donor, which means it would go the trash. So thats why I adopted it and wil now try to make a beauty again !
     

     

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  2. parman

    parman Retiree Subscriber

    Messages:
    6,206
    Location:
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    It's called the headphone jack. You have nothing to lose but time. Maybe you'll gain a lot of experience working on it. Good luck!
     
  3. Knutsul

    Knutsul New Member

    Messages:
    13
    Was looking for that word! It is going to be my first real electronic fix project. Do have some good experience with the cosmetic part. I was reading the last week about trouble shooting electronics. Try to understand the way all the parts work.
    Have to say its stil complicated to me but I wil just try and learn while doing. Allot of people came to me with adviece but I want to try doing it on my own and only ask for help when I am stuck or if people see me doing stupid things. Have to practice with the pictures aswel I see.
     
  4. quaddriver

    quaddriver 120 What's per channel Subscriber

    I did an 890 (slightly larger cousin) that is my avatar. You will find it is the same as its "sx-'y'80" counterpart. there is a thread about getting new power packs made from some really smart cookies.

    since I redid my 890, its been my office daily driver for 8-10 hours every week day...

    others might have more in depth history, but what I found was the black dial units were not sold in the USA retail scene as a regular stock unit. they were sold in europe (dual voltage) or canada (single or dual voltage) or at us military BX's as single voltage.

    I had only seen pictures but never owned one, not even when I had my '100 pieces of silver' collection in the early 2000's, so when I saw mine, I had to grab it up ($56.50 was the winning bid - funny how we remember arcane stuff)
     
  5. Knutsul

    Knutsul New Member

    Messages:
    13
    That makes sense since I live in europe and there is a voltage selector on the back. Youre 890 look sweet thats some motivation! My multimeter just arived so i could finaly start.

    Looking to the bottom of the main pcb I can see a couple of things:
    It looks like someone tried to fix this thing or maybe these are old repairs. There are two different powerpacks so atleast one is not original. These are gonna get swapped by the semi DIY powerpack project (no clue how to call it). Since I dont know whether the packs are good or bad I wil check the rest of the circuit board.

    IMG_0798.JPG IMG_0797.JPG

    There are some unfresh solder joints and other things I am not sure about whether to clean it up on the board. Could someone tell what is acceptabel and what not ?
    First piciture I am talking about the 2 rows of 5 pins.
    Secondpicutere about the yellow stuf on the top left
    Third picture about the damaged/burned thing in the middle
    IMG-0800.JPG IMG-0802.JPG IMG-0819.JPG

    Since I had no clue about where to start with the measurments I made up this plan:
    I start at the beginning so thats gonna be the power suply. On the schematic of the pcb I could see there are three secundaire windings on the transformer all three going to a different set of diodes acting as rectifieres. Since the burned solder joint on the picure above is from one of the these diodes I want to measure the dc current's coming out of the rectifiers aswell as the ac current out of the transformer.
     
  6. rcs16

    rcs16 Super Member

    Messages:
    3,892
    Location:
    Baldwin, Ontario, Canada
    I think you should concentrate on check and comparing voltages against the schematic rather than measuring current. You cam measure voltage drops across resistors in order to calculate the current being drawn.
    After you check power supply voltages, you can check those STK-0050 for bias current(V across 0.22 ohm ballast resistors) and o/p DCV offset.
     

     

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  7. quaddriver

    quaddriver 120 What's per channel Subscriber

    $5 says the power pack on the left aint sanyo....
     
  8. Knutsul

    Knutsul New Member

    Messages:
    13
    Resolders the regulators and its now playing on both channels. Stil I want to replace the darlingtons since they are not the same. Does somebody now if the right one is original ? Cause then it could be of use for someone who has 1 original blown.
     
  9. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,791
    Location:
    Glen Burnie Md.
    The one on the right looks like the pair in my 790. I got mine at a thrift store and located the original owner via his Soundex(Dr. License) number on the back. According to him it had never seen a shop. It worked beautifully right out of the box, but underwent an overhaul in 2010-2011. It still has it's original packs, but only gets run approx a week a year, until I build the new pack replacements.

    Knutsol;
    Your's is the SX-790/W4. The standard 790 came with a walnut case. The /W4 came with a Black case.
    78-79 Brochure with Walnut case. http://www.kallhovde.com/pioneer/pioneer78-79b.pdf
    79-80 Brochure with the Black case. http://www.kallhovde.com/pioneer/pio-79-80-b.pdf
     
  10. toxcrusadr

    toxcrusadr AK Subscriber Subscriber

    Messages:
    32,336
    Location:
    Central Missouri
    I've just been looking for info on the SX-750 which is a bit different from the -80 (discrete outputs instead of Darlingtons for one thing). But this thread will have some good stuff you may want to look at - including a lot on cracked solder joints!

    http://audiokarma.org/forums/index....make-it-into-a-doorstop.755909/#post-10275996

    Although if it is playing well then maybe it's fine.

    I happen to have two 780s, one has a blown Darlington. I planned to build some of those replacement boards but haven't got round to it. Only so many hours in the day!
     
  11. Knutsul

    Knutsul New Member

    Messages:
    13
    Thx guys! These old bruchure's are so cool ! If you want you can take over my good darlington to put in your 780. Only problem is I live in the netherlands so shipping might be a pain. But first I have to be 100% sure its oke. The right channel keeps cutting out since 10 minutes. Tommorow I go and start the major clean. But atleast it made some music today!
     

     

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  12. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
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    Location:
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    Check the regulator transistors (3 in a row just below the diodes in the pics) which if you are looking at the 790 from the front would be to the RIGHT of the diodes. They heat up enough to de-solder themselves. You might consider some TO-220 Heatsinks to add to them to cool them down some. These are covered on the SX-780 rebuild lists. One more thing. The original diodes can heat up enough to melt the solder holding them to the board circuits. Especially the rear quad. I used 1n5408's there and 1n4007 for the front quad. Note that I didn't use insulators on the heatsinks for the voltage regulators, so the heatsinks are electrically HOT, and the heatsink wings are trimmed to clear other components.

    780 Relay and Voltage Regulators.JPG

    Voltage Regulator underneath modded.JPG
     
    Last edited: Jan 26, 2018
    emresorkun likes this.
  13. toxcrusadr

    toxcrusadr AK Subscriber Subscriber

    Messages:
    32,336
    Location:
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    Just to clarify, are you talking about the big black heat sinks right in the middle? I see diodes at the top of the pic, so those transistors would be 'below' the diodes rather than to the right.
     
  14. quaddriver

    quaddriver 120 What's per channel Subscriber

    I think he is, his orientation differs from the pic shown, rotate the pic 90* CCW
     
  15. toxcrusadr

    toxcrusadr AK Subscriber Subscriber

    Messages:
    32,336
    Location:
    Central Missouri
    That's what I figured, mobile devices can screw up pic orientation sometimes.
     
  16. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,791
    Location:
    Glen Burnie Md.
    Those pics were as taken. (I edited the text, so the directionally challenged(North is UP) could figure it out):D The Heatsinks are add on's to the replacement TO-220's to keep them cool.
     

     

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  17. merlynski

    merlynski Curmudgeon Electronicist Subscriber

    Messages:
    432
    Location:
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    Larry, I have an SX-780 I am prepping to repair/upgrade. Were you happy with the additional cooling of those heatsinks, or was it just of marginal benefit? :idea: Thanks!
     
  18. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
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    The heatsinks along with the uprated transistors have resulted in NO de-soldering of the pins/pads since I replaced them. The big problem was the original regulators were running at the ragged edge and would de-solder themselves. Replacing them with uprated transistors and adding heatsinks (which PIONEER should have done in the 1st place) prevents this and regulates the voltages better. The new ones still get hot as they are dissipating a lot of heat,(by voltage drop and hi current) but they are well within their SOA. You could drill and tap the center slab heatsink and mount them remotely with(TO-220) Mica insulators and nylon insulators on the screws(The back is the base and electrically hot), which would cool them off more.
     
  19. merlynski

    merlynski Curmudgeon Electronicist Subscriber

    Messages:
    432
    Location:
    High Plains, Eastern Colorado
    Thanks for responding!
    The 'drill and tap the main heatsink with remote mount' was the direction I was heading, but just heatsinks would be easier.:dunno: What uprated transistors did you use to replace/upgrade the originals? I am also going to replace the 4 individual rectifier diodes with a bridge mounted on the main heatsink, and do the hbrown/rcs16 Darlington pack upgrade discrete modules. I will also greatly increase the main PS filter caps and add a soft start relay circuit. Should be a fun project for a 780 with a blown left STK-0050.:idea:
    Not trying to crap on this thread, as everything we are discussing applies to a 790 as well. :naughty:
     
    Last edited: Jan 28, 2018
  20. Knutsul

    Knutsul New Member

    Messages:
    13
    No problem! Only small thing is that I trie to find out stuff myself , but stil apriciate the help! Maybe we could use this threas eventualy ass a big summery. I think allot of people are running into the same things with these reciever. Did some cosmetic work pics will follow when ready and trie to clean the pots, and made a new list of things to do:

    -Meassure or maybe just replace the diode's from the rectifiers stages
    -order hbrown/rcs16 darlington parts at mouser
    -Decide which caps I need to replace
    -Order heatsinks and voltage regulators
    -Solder the power packs

    I do have some difficulties finding parts at mouser website. Is there anyone with some search tips ?
     

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