NEC 2SC959 Replacement

Discussion in 'DIY' started by LBPete, Jun 24, 2017.

  1. Alan0354

    Alan0354 Super Member

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    Did you put it in and try the 2N3440. All talks are cheap now, just do it. If it works, who cares about the hfe.
     
  2. LBPete

    LBPete Rolling Along Subscriber

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    It's in and works. But the amp still has distortion in that channel that I'm chasing.

    - Pete
     
  3. Alan0354

    Alan0354 Super Member

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    Last edited: Jul 12, 2017
  4. LBPete

    LBPete Rolling Along Subscriber

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    It certainly is. I replace the transistor because it was open. Now I'm seeing the distortion. I'm feeding it a 1k Hz sine wave and looking at it with a 'scope. The slope of the positive curve is distorted and does not increase at the same rate as the negative curve when the volume is increased. Here are a couple of pictures. The one on the right is at a higher volume then the one on the left.

    Clipping 1.jpg Clipping 2.jpg

    It's in the right channel only and the input from the preamp is clean. If you are interested here's the schematic. I've replaced all the transistors on the board and checked all the diodes except the bias diodes.

    [​IMG]


    - Pete
     

    Attached Files:

  5. petehall347

    petehall347 the brandy coffee man

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    set dc balance ..make both clip evenly .
    keep an eye on idle current as it might change when setting dc balance .. i can sometimes find i need to keep trading one for the other . 2 dmm.s helps here .
     
  6. Alan0354

    Alan0354 Super Member

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    I need time to read, question is whether your scope show the dc offset. Can you adjust so the line on the scope is EXACTLY in the middle before you connect to the output, AND also tell me the V/div setting of the scope.
     
  7. petehall347

    petehall347 the brandy coffee man

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    this amp unless mistaken is cap coupled and appears out of balance to me . balance and current can be related so both need setting carefully .
     
  8. Alan0354

    Alan0354 Super Member

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    Yes, I am surprised this amp is a single rail design where the output is at 40V!!! That make it even more important to read the DC voltage at the output. The coupling cap might be leaking or shorted.

    Ha ha, I never seen one like this!!!
     
    Last edited: Jul 12, 2017
  9. Alan0354

    Alan0354 Super Member

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    Hey LBPete

    Important, do not hook up to the speaker. Using a resistor as a load across the output terminals, use like a 5K resistor to ground and measure the output with scope and with a meter to read the DC voltage. There is a possibility the coupling cap C0006 is leaking.
     
    Last edited: Jul 12, 2017
  10. petehall347

    petehall347 the brandy coffee man

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    the collector of the lower shown output transistor should be half of the rail voltage . this can easily be checked at the outputs cases .
     
  11. LBPete

    LBPete Rolling Along Subscriber

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    Thanks guys. All the electrolytic caps on the board were changed out by the owner before it came to me. Your suggestions have turned up another problem that's probably related. There is no bias on that channel measured with a voltmeter connected across the emitter resistor. The trimmer position makes no difference. The other channel behaves normally. I have the unit powered through a 60w bulb for safety so I haven't been too concerned about setting bias up to this point.

    As far as looking at offset with the 'scope, the pattern jumps off the screen when the scope probe touches the test point. I have to bring it back with the vertical screen adjustment. Both channels do this but to different degrees. the probe is grounded and it's set to 0.2v/div.

    - Pete
     
  12. petehall347

    petehall347 the brandy coffee man

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    what voltages are you seeing at outputs collectors ?
     
  13. LBPete

    LBPete Rolling Along Subscriber

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    Using the schematic I posted as a reference, Pin J709 (H001 collector) - about 66v, Pin J717 (H002 Collector) starts around 8.5v and slowly climbs. I stopped watching it when it got to about 11 volts.

    - Pete
     
  14. petehall347

    petehall347 the brandy coffee man

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    see if it will adjust to 33v . then check bias again .. or as i said earlier hook up 2 meters .one for bias and other for dc balance otherwise known as centre voltage .
    ps r723 for balance
    r729 bias
    or r724
    r730
    if other channel .
     
  15. LBPete

    LBPete Rolling Along Subscriber

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    It adjusts down but up. About 10.5v is about as much as I can get out of it.

    - Pete
     
  16. LBPete

    LBPete Rolling Along Subscriber

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    The big coupling caps are original. I don't see any physical damage to either of them.

    - Pete
     
  17. petehall347

    petehall347 the brandy coffee man

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    be absolutely certain the drivers are in the right way round .
    does the other channel adjust to spec ?
     
  18. LBPete

    LBPete Rolling Along Subscriber

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    I'll check but it's unlikely they are in wrong.

    - Pete
     
  19. petehall347

    petehall347 the brandy coffee man

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    also check emitter resistors .
    and check h703 or h704 if other channel
     
  20. LBPete

    LBPete Rolling Along Subscriber

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    The drivers are in the right locations. When I installed them I verified the lead orientation of the replacements were the same as the originals. The emitter resistors measure 0.4 and 0.5 ohms. They are marked 0.24 ohms. My ohmmeter doesn't seem to indicate hundreds of ohms.

    - Pete
     

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