Sansui 7070 DC offset

Discussion in 'Exclusively Sansui' started by tyella, Sep 2, 2017.

  1. tyella

    tyella AK Subscriber Subscriber

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    I dug a Sansui 7070 out of a pile of electronics at a thrift the other day. I didn't expect it to work at all, but damned if it does work...sort of.

    Before hooking up speakers, I measured the DC offset, and got 14 mv on the left channel and 350 on the right. I adjusted the trim pot and got the voltage down to around 130 or so before bottoming out the pot. DC on the right channel also spiked at nearly a full volt after the relay kicked in. I took about 20 seconds to settle down to the 130 mv. I hooked up some "burner" speakers, and the receiver played sounds. I didn't push the volume too high, but thought I heard some distortion at greater volume.

    Where should I start looking to address the excess voltage in the right channel? A full recap will happen in the next couple of months, but it would be nice to have a taste of the sound first to get me inspired.

    Thanks in advance for any assistance!
     
  2. LBPete

    LBPete Rolling Along Subscriber

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    It's likely that the fuse resistors on the driver board have drifted out of spec. Lots of information available on that. Do a search using the driver board "F" number.

    When you get a chance, please post the serial number for the database. Click on the link in my signature block.

    - Pete
     
  3. tyella

    tyella AK Subscriber Subscriber

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    Thanks, Pete! That's a long thread. I'll check the service manual for those fusible resistors and order caps at the same time to save on shipping.
    I'll check the serial number next time I lay hands on it.
     
  4. Halisson

    Halisson New Member

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    Did you check bias? compare the voltage of the other circuit (L or R) on the driver board and try to identify any changes.
     
  5. tyella

    tyella AK Subscriber Subscriber

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    I haven't checked that yet, but will. Thanks!
     
  6. tyella

    tyella AK Subscriber Subscriber

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    I finally checked the bias.

    Left channel was about 47mA, right was around 70mA. I have them both at spec (30mA +/- 1). However, there was no change in the DC offset. Now I can't get the right channel down below 350mV. Could the trim pot be bad?

    I guess it's time to put together a list of the fusible resistors, capacitors, and maybe trim pots and start the overhaul. Winter is coming, and I'll need a project to keep me off the couch.

    I don't suppose anyone has shopping list for the capacitors handy? Not a big deal to go through the service manual, but it's worth a shot to ask!
     
  7. DougBrewster

    DougBrewster AK Subscriber Subscriber

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    I don't suppose anyone has shopping list for the capacitors handy? Not a big deal to go through the service manual, but it's worth a shot to ask![/QUOTE]

    You may get lucky with a ready made list but if you don't the service manual is probably not to be trusted; you'll need to check what you actually have in there already..
     
  8. tyella

    tyella AK Subscriber Subscriber

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    I suspected that might be the case. Thanks for the warning!
     
  9. LBPete

    LBPete Rolling Along Subscriber

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    If you haven't changed out any of the fuseable resistors, adjusting the amp is futile.

    - Pete
     
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  10. tyella

    tyella AK Subscriber Subscriber

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    Just reporting back. I replaced all of the fusible resistors and DC offset is now 8mv and 9mv! I'll hook up some speakers and give it a run-through tomorrow.
    Thanks, Pete!!!
    One of the resistors was WAAY out of spec. R33 or R34 (can't recall which) was in around 7K ohm, rather than 180 ohm. Strangely, all of the 4.7 ohm and 10 ohm resistors were good. I replaced them anyway.

    Next to tackle the lights. I get no lights whatsoever. One bulb is cut out of circuit, not sure if that effects connectivity, ala christmas bulbs. I'll start with reflowing the solder on the the power leads to the illumination board.
     
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  11. LBPete

    LBPete Rolling Along Subscriber

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    The 4.7 and 10 ohm fuse resistors rarely drift. The 180s are the main culprits followed by the 150s.

    Glad to hear it worked out. Everyone loves a happy ending.

    - Pete
     
  12. tyella

    tyella AK Subscriber Subscriber

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    Another report. After reflowing some solder and checking continuity all over the place, it was ultimately just a fuse.

    Replaced F-04 with another 250v 3 amp fuse and it now glows!
     
  13. slimecity

    slimecity Super Member

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    There ya go....!
     
  14. tyella

    tyella AK Subscriber Subscriber

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    Reporting back again. I replaced the vinyl wrap on the case and it looks pretty good! If I had it to do over again, I'd spend some extra time on surface prep. Live and learn.

    IMG_20171011_231117.jpg


    There are a couple of meter bulbs out and the FM indicator is out too. I'll get to them when I recap a couple of months down the road. Thanks to all for the help!
     
  15. LBPete

    LBPete Rolling Along Subscriber

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    They new vinyl looks good. What did you use?

    - Pete
     
  16. tyella

    tyella AK Subscriber Subscriber

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    DC Fix in Walnut from Amazon. $8 for a roll and I have enough left over for my Pioneer SX750!

    Edit: $9 a roll. Link
     

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