background noise in Sansui AU 719

Discussion in 'Exclusively Sansui' started by SansuiJavier, Dec 19, 2017.

  1. SansuiJavier

    SansuiJavier AK Subscriber Subscriber

    Messages:
    134
    Location:
    Valencia (Spain)
    Hello everyone, I would appreciate your help:
    I have recap my AU 719, all the electrolytic capacitors (except the big ones 12000/63), black flags, components damaged by the glue and zener diodes. No transistor changed at the moment. I tried it and it comes out protection, I adjusted the bias and DC to correct values. The amplifier works but has a constant electric noise, not very annoying but audible, even with the volume at zero. If I raise the volume increases.
    Can someone guide me to find the fault?
    I have checked the connections to ground, change the plug, connect it without any source, invert the plug ... still present ..
    I have also noticed that the resistors R634-R635 in F3101 seem to have overheated (the amplifier has been in operation only for tests, in total about 15 minutes). These resistors are marked at 68 ohms / 0.5W in the scheme, I replaced them with 68 ohms / 1W of metal film, and it seems that they will not withstand those temperatures. I do not know if it's a problem different from the background noise.
    I would appreciate help, what is the probable cause of the noise and how to eliminate it? What points should I check first?
    I bought it recently, it had volume crunches and they have disappeared after deoxit, but it also had a "strange" capacitor that was added in some previous careless repairs..

    Thanks in advance

    Javier

    F3101 resistors:
    IMG_20171219_184917.jpg

    my 719 in the operating room:
    IMG_20171217_214547.jpg
     
    Last edited: Dec 19, 2017
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  2. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Those resistors do get VERY hot, I haven't found a cure as such - I just used physically larger ones (than shown above - despite what they are supposedly rated at) - the ones I used have a dark red body and don't show the scorching so much :) - they are a little larger than the 'standard' resistors on that board. I think they were an attempt by Sansui to reduce the heat in the series regulator transistors (which also get extremely hot in my opinion), and provide some current limiting protection to those regulators.

    As for the noise, you'll need an oscilloscope I think.
     
  3. SansuiJavier

    SansuiJavier AK Subscriber Subscriber

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    134
    Location:
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    The ones I replaced were big and ended like this ... 3101 10.jpg


    My God, that's new for me, I do not have
     
  4. ManyMoonsAudio

    ManyMoonsAudio AKA paul79 Sponsor Subscriber

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    Both channels I assume? Was the symptom there before you started work? You cannot separate this amp from its preamp via switch, so that makes it a bit more interesting to diagnose. Verify your grounds are all present and check all your voltages. I suspect a zener, but it could be a transistor also. Look for heat damaged transistors, and replace them with correct replacements.

    A scope helps, but you may be able to tease the symptom with freeze spray. If it is in both channels, you need to be looking at the supply board.
     
    Last edited: Dec 19, 2017
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  5. Robisme

    Robisme Sansui Enthusiast Subscriber

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    I suspect the preamp if the level of the noise increases as you turn up the volume.

    Both channels? Could be power supply related.

    Unplug the preamp feed from the amp boards and see if it goes away.

    Rob
     
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  6. SansuiJavier

    SansuiJavier AK Subscriber Subscriber

    Messages:
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    Location:
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    Merry Christmas to all, that first thing:)


    thanks, I unplugged as you say the preamp from the amplification boards (please correct me if it is not correct) and the electrical noise disappears. Does this mean that the noise comes from the tone or preamp cards? It would be a relief because they are much more accessible for review ... if so, what can be the most common cause?
    IMG_20171224_171724.jpg

    On the other hand, there is something that worries me in this amplifier, and I do not know if it can be a symptom of bigger problems: the auxiliary rear plugs show only 183 volts when I turn on the amplifier, when the right thing should be 220 ...:idea: what can happen here ?
     

     

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  7. Robisme

    Robisme Sansui Enthusiast Subscriber

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    There are six 2sa726 that should be replaced in the preamp. They can be replaced without removing the board.

    So yes, the noise is coming from the preamp, but it could be passed along from the power supply.

    Is the noise a static, hum, hiss?

    Rob
     
  8. petehall347

    petehall347 the brandy coffee man Subscriber

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    if there is 220v going in suspect power switch or bad connection
     
  9. SansuiJavier

    SansuiJavier AK Subscriber Subscriber

    Messages:
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    Location:
    Valencia (Spain)
    hum, with any input, with tone deactivated or activated, plus a slight hiss that increases with the volume but I can consider normal by the age of the amplifier, since I have not changed any transistor, but the buzz is still present at any volumeo_O
     
  10. SansuiJavier

    SansuiJavier AK Subscriber Subscriber

    Messages:
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    Location:
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    I update this post, I am still looking for the problem, I have reviewed the power supply and I have reinstalled the original resistors (bigger), I have verified that the voltages are correct, Now go to disassemble the preamplifier card, if it is disconnected, the noise disappears..

    This symptom is caused by the DBT, the voltages are correct connected directly to the grid:confused:
     

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