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AU-7900 Repair - blows 1A fuse

Discussion in 'Exclusively Sansui' started by EastPoint, Apr 24, 2018.

  1. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

    Messages:
    4,549
    Location:
    Milwaukee, WI
    Hi folks. I just picked up a non-working Sansui AU-7900. It has no output on either channel, either through the whole amp or directly through the main in jacks. I measured about .5 VDC on one channel and like .005v on the other. It did come out of protection fine. I opened it up and saw nothing obviously bad, but the 1A fuse underneath was blown, though not badly smoked. When I replaced it and powered it up on a dim-bulb tester, it lit the bulb and would not come out of protection.

    All four of the 220 ohm fuse resistors on the power supply board measure way over 220 ohms in-circuit--I'm getting like 2.2 meg ohms on two of them, 330k, and open line. I read through this thread here. After I replace the fuse resistors and replace the little glass bead diodes with a pair of 4148N in series, is there anything else obvious that I should do first? I don't have a scope but I have a multimeter. I am planning to recap/re-transistor the boards but obviously I wanted to get it working before I did that.
     

     

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  2. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

    Messages:
    4,549
    Location:
    Milwaukee, WI
    I replaced the four 220-ohm fuse resistors, all of which were bad. No change in amp status. I tested the six diodes d603-608 and three of them seem bad, so I'll get those ordered and replace them as well. I will order all the caps for the PS while I'm at it.
     
  3. Willy6

    Willy6 AK Subscriber Subscriber

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    1,334
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    First thing I always check is the op transistors if they all test good start working your way inward from there checking regulators and differential transistors next go board by board untill you find the short circuit
     
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  4. slimecity

    slimecity Super Member

    Messages:
    2,891
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    New Zealand
    Second what Willy said. A dead short is usually something simple like blown outputs or more rarely, blown rectifiers.
     
  5. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

    Messages:
    4,549
    Location:
    Milwaukee, WI
    I gotcha. I'm gonna get those power supply diodes done and then check the outputs, and then go from there.
     
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  6. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

    Messages:
    4,549
    Location:
    Milwaukee, WI
    Hi folks. I replaced all 6 of the diodes in a row, then replaced all the little glass bead diodes with a pair of 4148 diodes in series. The amp now comes out of protection and plays. However, there is a bit of AC hum on both channels. It does not vary with volume, and it is there when I go in directly through the Main Ins. I replaced every single electrolytic capacitor on the power supply, and the hum is still present. Thoughts?

    Hum fast facts:

    Both channels, does not vary with volume.
    Present in headphones or through speakers
    Present when rear switch is on "connected" and I am running through regular input
    Present when rear switch is on "separated" and I am running directly through Main Ins
    NOT present when no source and rear switch is on "separated."
    DC offset is negligible
     
    Last edited: May 4, 2018

     

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  7. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Bad ground contact somewhere?

    You should check ground paths in and around the PSU (in other words not oxidised, contaminated with flux, switch cleaner or anything else). And also check that the regulators (capacitance multipliers) TR604 & TR605 are functioning correctly. Check their output voltage - and compare it to their input voltage (see schematic), if you have an oscilloscope to check the ripple on their output so much the better. Have C11 & C12 + C05 & C06 on F-2596 been changed? ZD01 and ZD02 ok? - correct voltages???
     
  8. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

    Messages:
    4,549
    Location:
    Milwaukee, WI
    Thank you for the suggestions. I will check these out and report back.
     
  9. Overundr1

    Overundr1 AK Subscriber Subscriber

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    More food for thought, if the main caps are tired you will see increased ripple and probably some hum. Be careful checking around those main caps, they can bite you big time
    -Lee
     
  10. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

    Messages:
    4,549
    Location:
    Milwaukee, WI
    I'm getting 13 volts to both zeners. C11 & C12 + C05 & C06 have been changed--every electrolytic in the power supply has been replaced. I put a bipolar cap in where one was required. I tested the main caps in-circuit with a little Chinese all-in-one tester and they showed extremely low ESR but 2.9% voltage leakage.

    On the schematic, TR604 looks like -51v in and -52v out, and TR605 looks like 51v in and 49v out. I am getting different readings for those--I'm seeing 44v and 43v for TR605 and -42.8/-42.2v on TR604.

    I checked the zeners out of circuit and they both read open line in one direction and like 25 meg ohms in the other direction. I'm guessing they're bad, from that. Should I replace TR604/605 as a preventative measure too, or no?
     
  11. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

    Messages:
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    Location:
    Milwaukee, WI
    Just replaced the zeners. No change. I replaced TR604/605 with modern equivalents, one by one. I think the hum dropped a little after I replaced the 2SB560, and when I replaced the 2SB438 the hum vanished entirely. So that was it. Whee! Thank you everyone!
     
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  12. sregor

    sregor AK Subscriber Subscriber

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    :thumbsup: (but my inner nitpicker tells me 2SD438
     
  13. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

    Messages:
    4,549
    Location:
    Milwaukee, WI
    Whoops! Yes, 2SD438. It's right here in front of me.
     

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