Blowing fuses in 919.....Any ideas?

Discussion in 'Exclusively Sansui' started by ghamilton, Mar 31, 2017.

  1. ghamilton

    ghamilton Super Member

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    I have been getting static in right channel of my 919 MC section. I have looked at the board many times to find where same trace may be arcing. I couldn't find anything. Well,...Today I turned up the volume a bit higher than normal in the MC position. The stylus was not on record serface because I was listening to the noise. Well, I heard a little "zzzzt" noise and the amp went dead. I found the 3a 250v white fuse was blown. I replaced it with same value. I put the amp in aux position and turned it on and it blew it again. I'M so sad. Now what? Thank you for any help.
     
  2. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    You are going to have to go through the painful process of checking for shorts with your multimeter, at all the power entry points of the PCB's and if nothing found, then disconnecting the power to all the boards so you can get to a point where the amp powers up without blowing the fuse. You then need to check all the PSU voltages and reconnect the boards one-by-one until you find the culprit.

    As you have been finagling with the MC Phono Amp, is it possible you may have trapped a wire during re-assembly - it's very easy to do. :(

    Good luck.
     
  3. ghamilton

    ghamilton Super Member

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    . Which board would you start with? And can I check it without taking it out?
     
  4. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    You won't be taking the boards out - just removing power from them to isolate the short, start with the MC Head Amp. You can also do the same to the Flat Amp and the MM Head Amp. You could pull the AC power input to the Small Regulated PSU - IIRC this will remove power from the aforementioned boards, this is the 3 push connectors on terminals 01, 02 and 03 with wires coloured White and Yellow - (Mark them before you take them off!).

    Remember to leave a generous delay between 'power off' and 'power on' cycles while you are plugging and un-plugging connectors to allow smoothing capacitors to discharge. And never reconnect anything with the power on - this is important.

    This is just phase 1 - finding out where the short is - there is a rocky road ahead.
     
  5. ghamilton

    ghamilton Super Member

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    This is going to be slow. I will start when I get around to it and let you know the progress. Maybe tonight or tomorrow. Thanks so much.
     
  6. ghamilton

    ghamilton Super Member

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  7. ghamilton

    ghamilton Super Member

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    OK, I'm looking at the MC board (2833) to figure out which two wires are the pwr (+ -). When I figure it out I'll put the multi meter to it to see if it's shorted. I,m unplugging 1-2-3 on the from 2844. And then what?
    Thanks.
    Greg
     
    Last edited: Apr 2, 2017
  8. ghamilton

    ghamilton Super Member

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    Ok, I unhooked the 3 plug-in pwr pins from pwr board 2844. I then turned it on and it blew that fuse again. I also unplugged the 3 plug-ins on the driver board 2839. It still blew the fuse. :(
     
    Last edited: Apr 2, 2017
  9. rasenthiran

    rasenthiran Well-Known Member

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    AU 919 is a very nice amp. Better leave it to the Pro to check it rather then getting in unwanted problem.
     
  10. petehall347

    petehall347 the brandy coffee man

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    if power board is now isolated you need to be looking at the diodes and capacitors on the power board for short circuit .
     
  11. slimecity

    slimecity Super Member

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    You may soon get more than blown fuses - sounds like something is badly amiss here - how about testing this via a DBT to reduce risk?
     
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  12. ghamilton

    ghamilton Super Member

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    HYPERION,...ECHOWARS,....Where are you?
     
  13. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    If you haven't found the short yet you have to keep going - unloading things one-by-one until you find it.

    At this point I would suggest you disconnect the output transistor power feed, by disconnecting 4 (or possibly 8) wires from the main PSU board in the centre underneath - the wires should be Red and Blue IIRC - insulate the ends so they can't come to any harm - as I said, it's a long and rocky road. :(

    I think you are probably into Pro or Tech territory anyway, for the repair if not the diagnosis, so maybe best to ship it off to someone? - sorry I am not closer.

    I think EW is having a time-out after the recent loss of his Mother, best to respect his privacy at this time, he will return when he is ready.
     
    Last edited: Apr 3, 2017
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  14. ghamilton

    ghamilton Super Member

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    I'm so sorry to hear about EW,s mother.:(
     
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  15. blhagstrom

    blhagstrom Mad Scientist, fixer. Subscriber

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    Isolate the power supply and use a dim bulb tester, 60-100w bulb.
    If the bulb is bright, its something in the power supply.
    You may need to isolate the transformer and verify that is producing AC on the matching color secondary wires.
    If that is OK, then the bridge rectifier and/or filter caps are suspect.

    The "zzzzt" noise may have been the switch, but I've never seen one short out and blow fuses.

    You said in the PM that you re-capped it a while ago. How good was your work?
    Given that its run this long, I sort of doubt its a bad solder joint, but its possible.

    With a unit blowing fuses, you need to start either at the AC cord and work your way in verifying AC and DC or start at the output transistors and work your way back verifying things are OK.

    If you don't have a dim bulb tester, build one. You will need it.
    If you don't know how to build one, google it.
     
  16. ghamilton

    ghamilton Super Member

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    Thanks. I'll check on building one. In the mean time,... My amp has always worked perfect,..except for the scratchey sound from the MC section. It had gotten worse lately though. When I turned up the volume past 12:00 with the stylus off the record just to listen to the noise that's when it went out. It was a arcing shorting noise. I should have known better.
     
  17. blhagstrom

    blhagstrom Mad Scientist, fixer. Subscriber

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    There is something wrong with it.
    Turning an amp up, even all the way, with no input should not blow it up.

    I just fixed an AU-717 that had noise on the phono section.
    Once the bad actors were replaced, I cranked up the volume and watched the o-scope hooked to the speaker lines to see if there was any noise left.

    I suspect you have known trouble maker transistors, there are lists around here on AK.
    Maybe fusistors that have gone bad. Sansui liked those.
    Probably VD 1212 diodes, Sansui liked those.
    Is the AU-919 the unit with "black flag" caps?

    And once all those possible areas to investigate are addressed, It now sounds like you blew up an amp.
    How were the adjustments after the re-cap. You did check them right? But even if they were good. it could have still let go.
     
  18. ghamilton

    ghamilton Super Member

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    I replaced all black flags with silver mica. The adjustments after recap were fine. However, the adjustments on the MC board were not touched. You need a scope for that one. (See SM) The biggest part of my amps life has been the MM section and Tuner. Only the past 2 yrs has the MC been used. I'm glad I got some of those ST outputs that EW told us about before they sold out.
     
  19. AUD101

    AUD101 Parsley The Lion!

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    If you are referring to MC Head Amp - Step 5 - Balance Adj - Scope not required - as per service manual
     

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  20. ghamilton

    ghamilton Super Member

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    Mine doesn't have VRO3-4-5-6. There are resistors in those places. VRO1-2 NEED SCOPE. Without scope VRO1 should be set at 2/3 from max. VRO2 to be set at 2/3 from min.
     

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