Technics amp broken

Discussion in 'General Electronics Chat' started by jon507, Sep 29, 2014.

  1. jon507

    Thread Starter New Member

    Sep 29, 2014
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    Hi, yesterday, whilst messing around wiring up some new speakers, I shorted the cables against each other. Not a problem normally except that I had left the amp switched on and the wires plugged in at the back of the amp....woops. Now the amp will not get past the safety operation, (won't work other than turn on). Don't really want to replace the amp if I can help it, does anyone know what is likely to have gone and is it fixable at a reasonable cost....by me maybe?
    The amp is a Technics SU-V2X.

    Thanks
    Jon
     
  2. #12

    Expert

    Nov 30, 2010
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    Have you tried looking for fuses? My Onkyo amp has fuses for the speaker terminals.
     
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  3. jon507

    Thread Starter New Member

    Sep 29, 2014
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    Yep, had a brief look under the cover, could only find 2 fuses both of which are fine. Didn't want to start taking anything apart inside without some prior knowledge, so there may be some more hidden under the circuits.
    Thanks for the input.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Just a wild speculation, but perhaps you need to reboot the receiver? My Yamaha has a process by which you can update or reset the firmware. If you have customized settings (radio presets, for instance) you will probably wipe them out. Have you tried unplugging it for a while?
     
  5. jon507

    Thread Starter New Member

    Sep 29, 2014
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    It has been unplugged for a good few hours and again when I inspected the internal fuses, no change. It just stays in safety mode.
    Thanks for the suggestion
     
  6. bwilliams60

    Active Member

    Nov 18, 2012
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  7. #12

    Expert

    Nov 30, 2010
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    Measure pin3 of IC502 for a DC level. That would force it into "safety".
     
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  8. MrChips

    Moderator

    Oct 2, 2009
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    (I attempted to post this hours ago but it never got posted.)

    Luckily, I think you have come to the right place. There are many members here with extensive experience in repairing home hi-fi systems and would be more than happy to lend a hand.

    Firstly, we would need to know the level of your electronics knowledge. Also are you comfortable doing a DIY repair?

    Secondly, do you have access to the Service Manual?
     
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  9. #12

    Expert

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    Service manual is in post #6.
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    Did you register in order to receive the manual?
     
  11. #12

    Expert

    Nov 30, 2010
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    No. You get 2 free downloads per day with no registration.
    The pdf is over 8 meg, so I can't upload to AAC.

    Wait a few minutes, then look under the picture. It takes that site some time to get ready for a download.
     
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  12. MrChips

    Moderator

    Oct 2, 2009
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    Thanks. Got it.
     
    Last edited: Sep 29, 2014
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  13. bwilliams60

    Active Member

    Nov 18, 2012
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    #12, Can you explain why DC on pin 3 would force it into safety for those of us in the learning stage? Thank you in advance for your time. I appreciate it.
     
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  14. #12

    Expert

    Nov 30, 2010
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    Look at the power output in the upper right of the schematic (page 16). Four transistors down is a label that says, "DC detect" Q604, Q605. This is a circuit that detects any appreciable DC level on the output because DC is wrong for a speaker. This goes in IC601 at pin 8, and since IC601 (pin 16) tells IC502 (pin 5) whether it is allowed to run, that would be the "safety" loop.

    IC601 also controls the speaker lock-out relay. (RLY601) and the "safety" LED.

    ...unless I've guessed wrong...:rolleyes:
     
    Last edited: Sep 30, 2014
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  15. jon507

    Thread Starter New Member

    Sep 29, 2014
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    Hi and thanks guys. I have downloaded the manual (thanks bwilliams60) and I am about to start looking through it. I may be a while doing things as I want to make sure I get it all right. I am not an electrical wizard MrChips but I am usually able to come to terms with most things. Thanks, I will report back shortly what i find, #12
     
  16. #12

    Expert

    Nov 30, 2010
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    Right now I'm looking at the idea that you broke the power amplifier chip and it's gone all DC output so the safety circuit shut off the speakers. Go measure it.
     
  17. bwilliams60

    Active Member

    Nov 18, 2012
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    Thank you #12 for the explanation. As I said before, I appreciate it. Always good to learn something new.
     
  18. #12

    Expert

    Nov 30, 2010
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    That's OK. I could not guess who needs the thought process spelled out unless you ask. :)
     
  19. jon507

    Thread Starter New Member

    Sep 29, 2014
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    Hi guys, I'm back, sorry for the delay but I'm lookin' after my 3 young kids, as well as working at the minute, as my partner is away nursing her very sick brother. I need my amp for my sanity!!!! #12 I've tested pin 3 of IC502 and there is a DC level. I'm guessing that's the power amp blown then?
    Thanks
    Jon
     
  20. #12

    Expert

    Nov 30, 2010
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    How much DC level???
    Pin 8 of IC601 wants to see some voltage from the +B5 or +85 volt line. (I can't read it) See if that voltage is being dumped by Q604 or Q605 because the DC on the output line is high enough to turn one of them on.

    If dump = true, then IC502 = bad.
     
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