SX-1050 Transformer Hum

Discussion in 'Pioneer Audio' started by cool cabbage, Jan 31, 2019.

  1. cool cabbage

    cool cabbage New Member

    Messages:
    3
    I have recently made a series of mistakes that have brought me here for some assistance. I have a limited knowledge of electronics, but am very experienced with computers, and I learned in the same way that I am attempting to learn about electronics. I am prepared to hear all about how much of an idiot I am, and everything I did wrong. Excellent.

    With that out of the way, I'd like to explain my situation.

    I have an SX-1050 that my father purchased new around 1976. I used it and loved it in my teenage years, moved away from the area for several years, and now we've been reunited. It never seemed to work quite right; it would click off for a few seconds, and then back on. It seemed to get worse as it warmed up. The filter switches needed to be fiddled with a little bit every so often to make everything sound right. My father said that it had always been like that, even when it was new. I used some electronics cleaner on all of the switches and knobs, removed the preamp jumpers (which he said were also always a problem) in order to run it through an equalizer, and blew out as much dust as I could with an air compressor.

    Everything worked and sounded great.

    I looked for any bad or leaking capacitors inside the unit, and located two on the Power Supply A board. C1 and C9 in the service manual, 220 micro farad 80v capacitors, which had leaked all over the board. I ordered 2 Nichicon UFG1K221MHM capacitors from mouser as replacements. I removed the shield from the back, removed the old capacitors, and installed the new ones.

    During the removal, the copper pads to the traces lifted off the PCB. Both pads on one of the connections, and only one of the other. I exposed some of the trace, pre-tinned the trace, pre-tinned the capacitor leads, and made the connections.

    The time came to test the unit, and a got fizzles, sparks, and smoke. As it turns out, the coating on the traces was much more delicate that I had expected it to be, and I was much more careless with the soldering iron tip that I had intended to be. I never disconnected all of the wrapped posts on the underside of the unit, so I was kind of working with it dangling from those wires. It was a precarious situation, and a mistake I will be sure not to make again.

    I have a younger brother that is a little more experienced in these matters than I am, and he suggested covering the back of the board with liquid electrical tape. We did that, and what I assume to be short circuiting seemed to stop. No more sparks, no more sizzling and fizzling, no more smoke. Great, I thought. Everything should be fine.

    I also replaced a blown 1A fuse, numbered "13" on the Power Supply B board at this time.

    Now, the transformer hums, and it can be heard very loudly through the speaker. No switches or knobs have an effect on this hum.

    I am looking for some advice on this repair, getting rid of the hum, and getting this back to working condition. Replacing the Power Supply A board seems like an affordable and easy solution, but if it would be possible to repair this unit without spending the money, I would prefer to do that.

    Thanks for taking the time to read my life story.
     
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  2. zebulon1

    zebulon1 Building a new bench. Finally! Subscriber

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    You don’t happen to live in Las Vegas?
    Sorry for your predicament.
    What started out a simple service turned into a mess. Live and learn. Sweet deal living with the set all those early years and dealing with the spotty reliability.
    Not near my data but wanted to mention getting the proper equipment to repair this type of vintage audio. Poor tooling and skills add to your unsuccessful repair. All can be learned and got for the cost of a professional.
    I’ll add more information once home tonight.
    If you are planning to continue the repairs build a DBT. Dim Bulb Tester.
     
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  3. cool cabbage

    cool cabbage New Member

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    Not in Las Vegas. Recently lived in Sedona though, so if I had tried this a year or two ago, you'd probably be the perfect resource.
    Thanks for the kind reply. I've acquired many skills through trial and error during my lifetime, and some forums can be quite vicious if you don't know everything that the most highly skilled person on the forum knows.

    I definitely have inadequate equipment, but as you mentioned, it should've been a simple service. The success I had with cleaning and improving the unit's reliability increased my confidence and allowed me to decide to move forward.

    The Dim Bulb Tester definitely seems as if it is something that I need to start on before continuing on this project, so thanks for that.
    Looking forward to hearing more from you, and again, I deeply appreciate your kindness and understanding.

    As I hope for this unit to be a sort of family heirloom, I expect to repair it, hopefully by myself, but if I have done too much damage already, then it may end up in the hands of a professional.
     
  4. Watthour

    Watthour Electron Rancher - JS3600

    Chances are that any replacement power supply board would need some attention, even if it is working. It's a bit of a weak link in the series, and is fairly easy to update and upgrade. So long as the board itself isn't completely destroyed, it could be a good candidate for rebuild.

    The hum you are hearing could be unfiltered power getting through the supply. Along with the smoke and sizzle you experienced, and the replaced caps, there may be a damaged diode or two, or a regulator failed, or additional caps failed. Get a copy of the service manual with a clean copy of the schematic. Then test the diodes in the power supply, and check the remaining caps for shorts and complete open-circuits.
     
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  5. merlynski

    merlynski Curmudgeon Electronicist Subscriber

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    High Plains, Eastern Colorado
  6. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    Dude, you already wrecked one board. This is NOT the receiver to learn on. Take it to a pro before you make it even worse. Ten-to-one the caps you replaced weren't the problem anyway.
     
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  7. zebulon1

    zebulon1 Building a new bench. Finally! Subscriber

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    This is a perfect read for tonight:
    http://audiokarma.org/forums/index....udio-gear-eh-heres-the-tools-you-need.333423/

    Many members started where you are and had successfully repaired their audio equipment. This forum has tons of info on what's needed to repair these vintage sets, you have to spend time researching it. Use Google or some other preferred engine. The secret is using Audiokarma as a final search word. It will give all the hits you'll need.

    Both you and your brother should have enough combined brains to make this a successful outcome. :D
    Just - Don't get ahead of yourselves.
     
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  8. merlynski

    merlynski Curmudgeon Electronicist Subscriber

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    778
    Location:
    High Plains, Eastern Colorado
    Last edited: Feb 1, 2019
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  9. WE6C

    WE6C Well-Known Member

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    Location:
    Colfax, Northern California
    Pull one leg of each part on that board and test. Replace any out of spec. You'll get it.
    Bob
     
  10. cool cabbage

    cool cabbage New Member

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    3
    Replaced the board and everything works fine. Thanks for the help, those who helped, and for the sarcasm, as was expected.
     
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  11. zebulon1

    zebulon1 Building a new bench. Finally! Subscriber

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    Location:
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    The sarcasm was all in good fun and you took it well.
    Welcome to AK and I hope you visit again with another Pioneer project..
    I'll be glad to help.
     
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