NEC 2SC959 Replacement

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

  1. petehall347

    petehall347 the brandy coffee man

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    they will be fine . i bet if you deduct the error in the meter leads they will be more like the spec .. to do this touch the probes together and deduct the reading from the 0.4 0.5 ohms .
    now see what the base voltages are at the output transistors .
    you never said if other channel measures good or not ?
    on another note be sure all grounds are intact .
     
  2. LBPete

    LBPete Rolling Along Subscriber

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    The left channel is fine. The right channel has the issues. Base voltages - bad channel J711 - 11mV, J721 0.0mV. The values are exactly the same on the good channel at J712 and J722.

    The ohmmeter leads measure 0.1 ohm.

    The grounds to the board should be OK. To make these checks I had to extend a number of the leads between chassis points and this board. I have a ground run directly to the board and I have the heat sink for the output transistors grounded as well. These are both done with alligator clip jumpers.

    - Pete
     
  3. petehall347

    petehall347 the brandy coffee man

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    base voltages of the drivers would be good to see . especially h709
     
  4. LBPete

    LBPete Rolling Along Subscriber

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    The base voltage for H709 is 143mV as measured at the anode of H715
    The base voltage for H711 is 19mV as measured at the anode of H717

    The good channel, H712 is 3mV and H710 is 14mV as measured at their corresponding diodes.

    The diodes provide an easy access point. I'd have to be spot on with the probes on the foil side of the board otherwise.

    - Pete
     
  5. petehall347

    petehall347 the brandy coffee man

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    r713 3.3k r721 2.2k . check those .
     
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  6. LBPete

    LBPete Rolling Along Subscriber

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    Sorry, I didn't have it plugged in when I took those last measurements. The actual values are much different.

    H709 is 9.5 VOLTS, H711 is 10 volts.

    The good channel is 29.5V and 31.3 volts.

    - Pete
     
  7. LBPete

    LBPete Rolling Along Subscriber

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    R713 is open! The only 3.3k resistor I have on hand is a 1/4 watt. I'm going to put it in to test and pick up a 1 watt tomorrow.

    - Pete
     
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  8. LBPete

    LBPete Rolling Along Subscriber

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    That did it! I have a clean signal now. Pete you are the man. Thanks for your help with this.

    For reference, the resistor has no obvious discoloration. It's not burned or damaged in any way.

    - Pete
     
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  9. petehall347

    petehall347 the brandy coffee man

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    nice one . :thumbsup:
     
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  10. LBPete

    LBPete Rolling Along Subscriber

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    Just a quick followup. It's back together and sounding great. I came up with a simple solution to getting those snowflake heat sinks to fit on the smaller diameter transistors. The contact surface ID of the heat sink has a split in it to allow it to stretch over and fit tightly on the transistor case. I squeezed it smaller but it was still too loose. I wrapped a layer of aluminum foil around the transistor and forced the heat sink over it. Now they are on tight, are still removable and will have good heat transfer.

    - Pete
     
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