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Student learning to troubleshoot has a couple of questions

Discussion in 'Pioneer Audio' started by WoodyJ, Jan 27, 2018.

  1. WoodyJ

    WoodyJ New Member

    Messages:
    21
    Hello all, I am a Computer Science major at the University of Houston, and I am also taking an Electronic Technician certificate course as well. I am really new at this, so please bear with me. I am learning to troubleshoot analog electronics using a DVM. I am using a Cen-tech P98674 5in1 DVM. The reason why I have ended up on this forum is that my dad has stated that in return for him shelling out the bread for college, I will be required to repair his Pioneer SA-6800ii integrated amplifier, which sounds "muddy" and one channel is louder than the other, so by the time I finish the cert course, I better be able to fix it or I'll have a sad dad. To prepare for this great challenge, I have picked up a crappy Pioneer SA-710-2 integrated amplifier for the grand sum of 10$ to work on, so if I screw up at first, it will not be on Dad's beloved. This is one of the "Flouroscan" series, it powers up, the display lights up, but does not output music on either channel. I have been using it to practice using the DVM with the hook probes, and I think I may have found whats wrong with it in the process when I tested each transistor with the DVM for resistance. Two of them, both blue transistors marked NEC A985, show zero on the meter. I am assuming they are what's wrong, I have a couple of questions. First, there is a blue jumper wire next to each transistor that have been cut. Does anyone know why and whether I need to repair them? Also, what would cause both these transistors to fail and what else should I check? I have found what looks like the repair parts on ebay:

    https://www.ebay.com/itm/1pairs-OR-...672846&hash=item25df787633:g:X0AAAOSwmg9Z4dam

    Are those sources a rip off ?

    Thanks for your help!
     

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  2. xlbee

    xlbee PIONEERFREQ Subscriber

    Messages:
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    Welcome to AK!
    The first step in troubleshooting electronics is to locate a schematic and familiarize yourself with the circuitry. Otherwise you are just poking around in the dark. Schematics can usually be found in the service manual. AK has a document database here:
    http://akdatabase.com/AKview/index.php
    The service manual for your unit is there. Download it and spend some time looking at the service manual and the schematics to see what you can learn. I believe the answer to the cut wires is in there.
    You also need to learn how to test transistors. A good start to that is here:
    http://www.audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186
    By doing the above, you will learn a lot and increase your chances of success when a member offers some guidance with your repair.
    Best of luck.
     
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  3. xlbee

    xlbee PIONEERFREQ Subscriber

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    Location:
    Newmarket Ontario Canada
  4. restorer-john

    restorer-john Addicted Member

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    Location:
    Australia
    Welcome to AK!

    You sound like you've got a plan and that's good. The two amplifiers will have completely different issues, but the same basic tests are always a good place to start. Don't underestimate the little SA-710II, it is a lovely amp and packs a lot of punch in a small package.

    Step one, get the service manual for each unit and become familiar with the amps. Pioneer give a lot of circuit description which will be useful for you. There are setup instructions to follow.

    Most amplifiers have some form of protection. The SA-710II has a speaker protection relay which disconnects the amplifier from the speakers if a problem exists. You say it powers up and lights up. Does the relay click in a few seconds after turn on? If not, the next step is to test the DC output of the power-amp stage, prior to the relay and note the voltage. Again refer to the manual.

    We know the power supply is fine, because the meter drive is derived from the same tappings on the power transformer as the main amp, so there's no need to suspect power supply at this stage, but it is always good practice to check your supply rails are in spec.

    sa-712-01.JPG

    If you have a significant voltage present at the output (prior to the relay) of one or other or both amps- you have a main amp issue.
     
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  5. restorer-john

    restorer-john Addicted Member

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    Location:
    Australia
    HiFi engine has the mk2.
     
  6. WoodyJ

    WoodyJ New Member

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    21
    Wow, I was expecting some crusty old engineer telling me to get lost, and instead I got all these answers from what looks like some great people! Thanks! I've had exactly two classes, so at this point I know about five schematic symbols so bear with me.... and yes, at this point I am poking around in the dark but I believe I've got "resistance" figured out. Maybe you guys can tell me if I've got a good workbench started, I've got a good start with tools and equipment that used to belong to my granddad, he was a ham radio buff, I have my granddad's Tektronic oscilloscope model 465B that my instructor says I'm very lucky to have, ( that is a total mystery to me), and I have a Tenma Variable Power Supply that looks like some sort of antique. I bought an Aoyue 968A Rework station that I'm already making decent solder joints with. and my dad bought me this really nice Centek DVM that sounds off alarms if I plug into the wrong jack or if the probe connections are loose. We will be getting heavily into schematics over the next few weeks, I can actually read the diagram about 95% worth or so. I'll download all the schematics and documents you all told me to get and start seeing if I can match up what I'm learning in class to them. I always learn better if I'm actually doing something to put what I'm learning to work. Maybe I can get "analog electronics" figured out this year, digital comes next year, its the one I really need, I've got three dead Xboxes!
     
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  7. dlucy

    dlucy dlucy67 (Doug) Subscriber

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    Your Tektronix 465 is a very well-respected, if somewhat old, oscilloscope. It is analog and mostly manual when compared to today's all-digital all-automatic Rigol DS1054Z, but that's a good thing for getting started. The manual adjustments on your scope will help as you learn how to use it and discover when to use it.

    There is a LOT of info and support for the Tek 465. Grab the manual from this page: http://w140.com/tekwiki/wiki/465

    Hopefully you'll have one or two probes for the scope already. If not, you can buy them fairly cheap on eBay, a little more expensive new on Amazon, or make your own like here: https://cromwell-intl.com/radio/probes.html

    This is more than you'll need, but some good, interesting things about scopes here: https://www.tek.com/document/online/primer/xyzs-scopes/ch1/oscilloscope-basics

    An oscilloscope is a great tool to have available, even if isn't used often. There is a great thread on other tools you'll want to consider here: http://www.audiokarma.org/forums/in...udio-gear-eh-heres-the-tools-you-need.333423/

    One of the cheapest and most-useful tools is a component tester. Plenty of info on that here: http://audiokarma.org/forums/index.php?threads/cheap-component-tester.749729/

    One of the Ak'ers here @ConradH wrote an really nice piece on diagnosing equipment here: http://conradhoffman.com/troubleshoottut.htm
     
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  8. restorer-john

    restorer-john Addicted Member

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    Your Tek is a lovely, classic scope. It will do you for many years and your grandad must have been a cool guy. I expect it will be in your collection long after new high tech DSOs have come and gone- they are practically bulletproof.

    Back to the question, does the relay click in the SA-710II? I suspect it doesn't.
     
  9. xlbee

    xlbee PIONEERFREQ Subscriber

    Messages:
    1,189
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    And now for your first test :)
    Did you read the service manual to find out about the cut wires?
     
  10. redk9258

    redk9258 Super Member

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    Just a comment about those transistors from eBay. Do not buy them. They are most likely counterfeit. Always buy parts from respected suppliers... Digikey, Mouser, BDent, etc. In this case you will most likely need to substitute a modern transistor since the ones linked have been out of production for many years.
     
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  11. avionic

    avionic " Black Knights " Subscriber

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    Never buy your transistors off of ebay.. 99% of the time they are chinese counterfeit junk..
     

     

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  12. avionic

    avionic " Black Knights " Subscriber

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  13. spinroch

    spinroch Unspecified Life Form Subscriber

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    Nice start, WoodyJ!
    Follow logical, sequential troubleshooting steps.
    Read the schematics, check the basics, and have fun using your test equipment and fixing the problems.
    Great scope, by the way!

    image.jpeg
     
  14. John James

    John James "Bob's your uncle" (Stolen) Subscriber

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    Welcome to AK!

    I like your attitude. :)
     
  15. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    Last edited: Jan 28, 2018
  16. WoodyJ

    WoodyJ New Member

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    Yes. According to the manual, you cut those jumpers based on the voltage found between two points, I can barely read the schematic on my laptop, I think I will understand it better when I get a chance to print it out.
     

     

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  17. WoodyJ

    WoodyJ New Member

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    The article by Conrad Hoffman was great. I am reading all of these. Thank you for listing these!
     
  18. WoodyJ

    WoodyJ New Member

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    21
    Yes, it clicks. I can actually see the arm jumping in the relay mechanism.
     
  19. I LIKE MUSIC

    I LIKE MUSIC Super Member

    This has been posted before, but it is very good information for someone just starting out with a oscilloscope. You can't just connect the ground side anywhere like you can with a DMM.




    Good luck on your adventures in trouble shooting.
     
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  20. loweran

    loweran AK Subscriber Subscriber

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    1,009
    Location:
    Austin, Texas
    Welcome! I am a software guy that got into this hobby. I can't say that I am an expert by any stretch of the imagination, but I manage to find my way with the helpful folks here. Learn something with every audio challenge I tackle. With a little knowledge from the folks here, I am sure you will be able to bring that amp back to it's original glory. I have done more than I would have ever expected to do in this hobby because of the helpful resources here. Keep researching and posting and you will be rewarded.

    As others have said, the DBT is a must. Also an audio probe made from some wire an a cap comes in handy for probing as well.
     

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