Sansui AU555a space invaders sound effect.

Discussion in 'Exclusively Sansui' started by Sebastien, Dec 17, 2017.

  1. Sebastien

    Sebastien AK Subscriber Subscriber

    Messages:
    105
    Hi everyone,

    Appreciate any help on this one.

    I've got an AU555a with an intermittent noise, independent of input or volume/ tone controls.

    The best way i could describe it is kind of like an arcade machine special effect- sort of like a ticking distortion that comes and goes, sometimes it's distant and faint but distinguishable, other times it's very noticeable.

    Actually does anyone remember the interference you'd sometimes get through a speaker when you're receiving an SMS on your mobile phone? Maybe something similar to that.

    I've separated the pre and power amp section and i can still hear the noise through headphones. I've also unplugged the tone control board to find the noise still there. (though this probably serves no purpose as the pre amp was seperated anyway but thought i'd mention it).

    I'll also note this though it's likely a separate issue - the amp isn't silent when idling. There's a kind of background hiss, very faint, but i notice that it is amplified when increasing treble.

    Thanks for any help - the amp is on the bench i've got basic stuff to tinker with, soldering iron, solder sucker, component tester, dmm etc.
     

     

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  2. slimecity

    slimecity Super Member

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    2,520
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    Because of the age of this amp, its only capable of low resolution sounds. I think that it will specialise in space invaders sounds, but it could also perhaps also do donkey song or tetris sounds, depending on the input you choose.

    Do you get the sound on both channels? It sounds possibly like AC line noise to me. Some of that old gear picks up interference very easily.
     
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  3. Sebastien

    Sebastien AK Subscriber Subscriber

    Messages:
    105
    Hahah.

    I've got a variety of San amps of this vintage none with this issue.
    It seems to move around a little, strangely. Sometimes faint but definitely the same sound, and sometimes very obvious and loud, sometimes nothing at all.
    It's in both channels but not always at the same time
     
  4. slimecity

    slimecity Super Member

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    2,520
    Location:
    New Zealand
    Do you have a smart meter or net router nearby? I recently got a some noise from the latter on a preamp Im recapping - the noise was very regular and had other similar symptoms to your mystery sound
     
  5. Sebastien

    Sebastien AK Subscriber Subscriber

    Messages:
    105
    Hmm, i've got a WIFI router maybe 15m down a hallway. Would my AU7900 be suspectible to this too? It's even closer but doesn't have this issue.

    I'll switch it off and see if it does anything.
     
  6. LBPete

    LBPete Rolling Along Subscriber

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    It could also be noisy transistors. This amp has the notorious "flying saucer" transistors that are known noise makers. It's best to just replace them.

    Please post the serial number for the database. Click on the link in my signature block.

    - Pete
     
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  7. Sebastien

    Sebastien AK Subscriber Subscriber

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    I suspected these too after reading about them being problematic some time ago- i hit them with freeze spray without any luck. But i'll replace them anyway.

    Actually i wouldn't mind going through the whole amp and recapping it too. Does anyone have a parts list for the best cap/size options?

    Cheers
     
  8. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

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  9. smurfer77

    smurfer77 Super Member

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    Hi Sebastian!

    So the noise is heard on headphones when you separate pre-main so you have confirmed it is in the main amp section (or power supply). (a) is it on both channels or just one channel? (b) are there an switched mode power supplies, like a USB charger, plugged into the same outlet?, (c) the hiss may or may not be normal. When listening by headphones I hear hiss on my 555A when pushing past volume 5 or 6 or so, but this is hardly audible from speakers, (d) does the amp bias up ok?
     
  10. Sebastien

    Sebastien AK Subscriber Subscriber

    Messages:
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    Hey Mate!

    I'm pretty confident it's in both channels but not necessarily always in both, it comes and goes in severity and moves a little.

    There's an iPod charger hooked up to the same board, though it did make this noise when hooked up in my main system off a different board with no USB charger.

    In the end i was only able to measure off the emitter resistors like you said and adjust to 10mv. The left one (if you're standing in front of the amp with the face panels closest to you) i couldn't get a reading off either side of the emitters when i grounded to the chassis, i could get a reading with a probe on each side.

    The other side was the opposite, i couldn't get a reading off each side of the resistors but i could when i grounded one probe to the chassis.
     
  11. smurfer77

    smurfer77 Super Member

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    Sorry, I know you probably already have, but double check that emitter resistor resistance. If the voltage is the same on either side (?) it may have gone short.
     

     

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  12. Sebastien

    Sebastien AK Subscriber Subscriber

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    Ah that's right! No i haven't done that i'll report back on it this evening -thanks :)
     
  13. smurfer77

    smurfer77 Super Member

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    kind of unlikely, as a short emitter will causes bigger problems than you have... but best to rule it out. But we do need to figure out why you can't get a voltage over that resistor. If everything is fine there, we will have to start looking up stream to see why that side isn't getting bias. When you get a chance pull those big TO3 output transistor out and use your component checker and see what it says. May be something more upstream though...
     
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  14. Sebastien

    Sebastien AK Subscriber Subscriber

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    Sorry about the delay, needed time for Christmas and new years to pass.

    So i went to get a voltage reading on either side of that emitter resister and either it's behaving strangely or i'm doing something wrong.

    Scenario 1: ground probe to chassis, red lead to one leg of the emitter with DMM on autorange mV. No reading.

    Scenario 2: a lead on either side of the emitter gives me a reading +5.0mv

    Scenario 3: Moved to the back of the board, ground on chassis, probe on the emittor and the Right channel fuse blew. This is the second time that's happened.

    Thoughts?
     
  15. smurfer77

    smurfer77 Super Member

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    just to clarify, when you say emitter, do you mean emitter resistor?

    Ok. if you are blowing fused and having trouble measuring bias (you should be able to measure that by putting probes on either side of one emitter resistors), then it's time to power off, and measure emitter resistor values, and if they are ok, then to pull the output transistors and check with a component tester.
     

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