Pioneer SX-1980 Hum on switch-on?

Discussion in 'Pioneer Audio' started by Jon1982, Mar 29, 2017.

  1. Jon1982

    Jon1982 New Member

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    Hi guys,

    Assembled the SX1980 after fixing a couple of issues and now running it on my main system to see what else I can find wrong with it :)

    As title suggests there's a mains hum on switch-on. Its on both channels and independant of volume. It gets louder for a few seconds then begins to fade off, disappearing completely after a minute or so.

    Any thoughts? I'm thinking recapping the power supply board. It looks like someone has already changed a couple. Board looks to be in a good state with no evidence of any frazzling ! :mad:

    Also got an anomaly on the FM, the stations sometimes come in weak and not in the right place (i.e. 106 MHz broadcast appears at 104.8 on the dial) but after a while this suddenly will correct itself... I messed about with the capacitor pots while I had it apart in an effort to improve FM signal.. think I might have stuffed something up? ;)

    Attached a couple of pics complete with new LED lighting (fitted new lamp holders and diode / capacitor to prevent flicker) I thought green was an apt colour for a monster of a receiver :D 001.JPG 002.JPG
     
  2. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

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    I assume you mean the hum is coming out of the speakers.

    pull the pre-out / amp in jumpers and see if it still does it. There IS a protection relay turn on delay, correct?

    THEN if it doesn't - it's time for DMM work. That power regulator board always needs rebulding if it's original.
    You may have some nasty surprises when DMM readings show the power supply is not in as good of a state as it LOOKS.

    I take it from your phrasing, you are in the UK?

    Messing about with the tuner is NOT wise. That tuner is a most finicky beast with very unconventional circuits.
     
  3. inductor

    inductor Super Member

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    somehow green does not grab me as pukka pioneer, but who cares
     
  4. Jon1982

    Jon1982 New Member

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    Thanks a lot for the suggestion of pulling the jumpers out. Surprisingly, this gets rid of the hum. Knackered grounding capacitor somewhere perhaps?

    I only fiddled with the pots on the tuner capacitor and not the PCB. So guessing I should be able to undo things if indeed I have caused it.

    Went for the green since the unit is going to be mainly used with my Arcam separates which are green illumination.
     
  5. inductor

    inductor Super Member

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    try the preamp with another power amp, suspect the hum is coming from the preamp though
     
  6. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

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    :yikes::yikes: :eek::eek: :(:( :no::no:

    That means you had the shield off? Or stuck a screwdriver through holes?

    Those are hyper sensitive - both to adjustment, TYPE of tool, and susceptible to damage.

    And MORE irreplaceable than a PA1002a chip. Pioneer BOUGHT them, and only for the 1980.


    The rest of your problem is probably the power supply feeding the preamp circuits.
    Since we knew the power supply needed rebuilding - just another confirmation...
     
  7. Awesomeaudio

    Awesomeaudio AK Subscriber Subscriber

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    I knew this was coming. After all the threads I've read about messing around with tuners, It's one place I dare not venture without the appropriate knowledge and tools.
     
  8. Jon1982

    Jon1982 New Member

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    Thanks again for the comments, constructive or otherwise ! :bigok:

    I am planning on recapping the power supply board, also the flat amp and the power amps.

    Is there anything else I should pay attention to? The innards of the receiver look in good fettle compared to some pictures I've seen on here. I've read about a dodgy 35V zener?

    As for the tuner, while I'm far from an electronics mastermind, I'm not a butcher. I'm confident that I won't have physically damaged anything. I'm wondering if the problem might be connected with being unable to set up the APC as I mentioned on another thread. Or perhaps having turned the pots after years of being stationary I might have left one on a 'dirty' spot. Might try a squirt of servisol? (its a switch cleaner here in the UK)
     
  9. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

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    recapping has become an imprecise term here. To be clear, the power supply needs all new silicon (diodes, transistors) as well as electrolytic caps.

    I didn't connect it in my mind with the earlier APC thread, been quite distracted with my imminent crunch time here ( Tomorrow!!! :yikes: )
     
  10. Jon1982

    Jon1982 New Member

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    37
    Thought I would share this with you guys.

    Seems my SX1980 power supply is not as other SX1980s!

    It would appear that 3no. transistors have been relocated to separate heatsinks bolted to the chassis and the onboard heatsink has been removed. Presumably for improved heat disappation.

    Someone's also changed a couple of caps by the looks of things. You might have thought they'd have changed the lot while they had gone to the trouble of taking the board out. Oh well... :oops:

    I've measured the power supply voltages and surprisingly, all are close to the quoted figures. (+82V -82V, +35.6/-35.6V) The furthest out being the 13.5V to the tuner which is only reading 12.75V on my DMM. Wondering if this may be a cause of the weird tuner issue I'm having.

    Thinking I will proceed at least with a recap and see if that cures it. Reluctant to start swapping out semis at this stage
     

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  11. Jon1982

    Jon1982 New Member

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    Think I need to replace the filter caps. My cap meter measures only 11 or 12 mF across the terminals. Was expecting to see circa 44 mF there?

    Would it be advantageous or at least not detrimental to use 33 mF replacements? Thinking of getting a close match to original can size would make life easier.

    EDIT might have answered my own question here. Looking at the diagram they're arranged in series not parallel so assume the reading I'm getting is normal.
     
    Last edited: Apr 18, 2017
  12. zebulon1

    zebulon1 Cleaned up the shop and half finished sets. Subscriber

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    It very well could be but the regulator for the 13.5v supply - for the tuner, is at the top of the image you posted. All the caps I see are original. Well the ones for the 13.5v supply. The difference in voltage you see is minor. You are on the right track.
    The mod, relocating the hot regulators was thought out and looks to be a little messy but functional so-far (Is the TO-220 the proper one for the job? Q203 :dunno:). There are better ways to achieve the same result.
    Regardless, the board needs to be rebuilt. No if's and or but!
    Rebuild the PS and go from there.
     
    Last edited: Apr 19, 2017
  13. Jon1982

    Jon1982 New Member

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    Gentlemen, I come again to humbly sip from your eternal fountain of knowledge ;)

    Work on the power supply board was going well.. replaced all the electrolytics, finding quite a few duffers along the way, also replaced the dodgy zeners.

    Reassembled and powered back up, all seemed well for a second and then the source selector lamp extinguished. Checked the fuses on the power supply board and a couple have popped. D'oh! At least there was no pop, smoke or smell!

    I'm wondering if I might have lost concentration at some point and put an electrolytic in back to front, is that probable? I had to use a slightly higher capacitance rating for the large 35V electro as I couldnt find an exact replacement in axial form but I'd be surprised if that were the cause.

    EDIT EDIT EDIT..... found a clear picture on t'google and it seems that's EXACTLY what I've done!

    Anyone got a dunce's hat?
     
    Last edited: May 3, 2017
  14. pustelniakr

    pustelniakr Silver Miner at Large Super Mod Subscriber

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    I know that it is mostly too late for this comment, but I only just saw this thread. In any case, I strongly recommend repair of existing problems, BEFORE wholesale replacement of any kind of parts. You need to get a gem like this working fully, before shooting for longevity, which is what wholesale parts replacement is for. You need to discover exactly what is wrong, and fix it. If you have several issues, fix them, one at a time. In this way, you will be able to determine if additional issues were cause by you, or were always there, ready to manifest. You will also be able to have confidence that you solved the problems, because you will know what they were.

    Yes, unless you have developed very obsessive procedures, as I have, it is quite easy to get a part in backwards, or damage a trace, or get a solder splash somewhere. If any of that happens, if you have been at it in any kind of wholesale fashion, it can be quite difficult to find out where the problem is.

    I hope you will be able to get back to where you were, and then, up to full steam, without too much trouble. If Mark is willing to guide you, I would stop now, and wait for his instructions from this point, ignoring all others. If you carry one as you have, you could wind up with quite an expensive mess. There are others that can competently guide, but you need to select a single voice and follow that, otherwise, you will be chasing your tail, trying to do what several voices are saying. Mark has already been in here. That is why I suggest him for remote troubleshooting guidance. Others will not wish to step on his toes, unless he demurs.

    Good luck,
    Rich P
     
    Last edited: May 3, 2017
  15. Jon1982

    Jon1982 New Member

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  16. pustelniakr

    pustelniakr Silver Miner at Large Super Mod Subscriber

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    For the big caps, remember that the ground is tied to the positive terminal of the caps, for the negative supply voltage(s). That is a common error in this area.

    Enjoy,
    Rich P
     
  17. Jon1982

    Jon1982 New Member

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    Its getting quite late here but I hate to go to bed on a problem, went into 'quick sticks' mode and flipped around the one capacitor I had gotten wrong, reassemble, replace fuses, and now all is well with the PSU, it would seem :)

    Having found so many dodgy caps I feel it would be remiss of me not to now look at the other boards. I'm wondering if the weird tuner issue I have may be down to a capacitor too.

    Thanks for the well wishes Rich. There's people on here far more knowledgeable than me, but I am tenacious and pretty handy with the soldering iron :biggrin:
     
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  18. pustelniakr

    pustelniakr Silver Miner at Large Super Mod Subscriber

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    Unless you are a competent audio tech, I would slow way down, and wait for guidance. I can appreciate zeal and tenacity, but your 1980 will not. You do not seem to have let the magic smoke out yet, but it is currently looking for a way to get out. I would power that puppy down, and keep it that way, until you know what the next steps should be, and they will probably be some careful measurements.

    I will climb down from the crow's nest now.

    Enjoy,
    Rich P
     
  19. Jon1982

    Jon1982 New Member

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    Being British I'm perhaps a little modest and am a bit better than I let on. I managed to bring a marantz pm-94 back from the dead a couple of years back, that was a pretty complicated beastie and it had multiple issues. I'd have thought if the amp was going to pop it would have done so by now! I drove it pretty hard before I had it apart! Checked the power supply voltages and they're still nice and stable. It was gratifying to find that most of the bad caps I found were related to the pre-amp supply, which was the source of the hum. I'm just going to be patient and methodical and perhaps delegate the tuner issue to someone who understands the circuit better than I, if I can't easily find the problem.

    I really appreciate you thinking about my precious amp! But don't worry, she's not in quite as bad a pair of hands as you think :bigok:

    I did spot that the amp was set to the wrong supply voltage on the back, something that had eluded the previous owner! She must be fairly robust to have withstood that :eek:
     
  20. BOUXY

    BOUXY Super Member

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    I like that Green!My more modest Receiver I did the Green thing and it looks ever so good:)
     

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