Sansui stereo repair

Discussion in 'General Electronics Chat' started by inwo, Oct 1, 2014.

  1. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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    Bens schematic.jpg View attachment 73623

    A friend is somewhat attached to this old stereo.
    He bought it while serving special forces in Vietnam.
    The finals (lower) were replaced 20 years ago. Possibly by me. Seems familiar! They were generic.

    I found an original set of 4 on Ebay and replace the same two again. Also replaced one driver. (circled) with something similar from junk box.

    Each side (dc supply) is fed with an external 5A fb fuse.

    That seemed to fix the amp which is his main concern.During one "power up" while working on the AM/FM, both are very low sensitivity and output, speaker made a loud thump and blew one final again.

    It seems there are issues in the final amp circuit, and I only have one spare transistor.

    Will my mismatched driver be suspect?
    Output caps?

    If I don't get any good ideas here, I'll order a good set of 2sc756, and hope for the best.

    Originally I told him it was not worth repair, and he was OK with that. He has since given me something in trade, so I want to give it my best shot.
     
    Last edited: Oct 1, 2014
  2. JDT

    Well-Known Member

    Feb 12, 2009
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    After replacing the transistors, best not to connect the speaker to start with. Replace the supply fuse with a wirewound resistor. About 100 ohms or so. The resistor will limit the supply current to hopefully a safe value if there is a fault. With no signal, the supply current should be quite low and the resistor will only drop a few volts (all being well). Then power up with no signal and measure a few voltages.

    Positive end of the output cap should about 1/2 the supply volts.

    When everything looks good, connect the speaker and try with a low-level signal. Trouble is, if one transistor goes, usually takes a few others with it!

    Only when you are absolutely sure everything is OK, remove the resistor and put the fuse back in.
     
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  3. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the amps are direct coupled, that means problems if one of the lower level tgransistors shorts, it really messes up the bias on the outputs.
     
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  4. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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    It worked for a week or so on and off testing. Now it works again.

    I guess I'll try replacing the next level down from the output. I can't think of how to test when it's working.

    I will check for half voltage while current limited.
     
  5. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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    It looks like you saved me a pair.
    With 100 ohms and no in/out connected resistors are getting hot. Output transistors are still ok.
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Are the diodes D801 and D802 mounted on the heatsink?
    If so they are there to provide compensation for the bias with temperature change.

    Bertus
     
  7. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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    Yes, notice the corrosion with cap pulled. MVC-005F.JPG
    Two transistors in foreground pulled. One is bad. Again. :(
     
  8. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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    Would a newer design in 220 package be a good idea for the 2 X 2sc756 ?
     
  9. #12

    Expert

    Nov 30, 2010
    16,330
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    I think so. The package is only abut getting rid of heat. The transistor inside the package can be exactly the same...or better...but it's difficult to know what's, "better" without understand the design very well. Also, it's easier to get heat sinks for a TO-220 package than for a TO-5 package.
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Are there other marks on the 2SC756?
    There seem to be quite some variations in them.
    (see the attached PDF with a scan from my transistorbook).

    Bertus
     
  11. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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  12. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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  13. alfacliff

    Well-Known Member

    Dec 13, 2013
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    those 756's were used ad rf amp transistors for cb radios too, might be one went into oscilation when used as an audio amp.
    some of the newer to220 transistors are totslly insulated, would make it a lot easier to mount without an insulator on the collector.
     
  14. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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    I'm not qualified to choose. I'll see if someone will stick their neck out.

    Then the question Should I go back another stage? 2sc485/458? Think I'll quit first!!!!!!!
     
  15. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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  16. alfacliff

    Well-Known Member

    Dec 13, 2013
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    one big thing in your favor, its a stereo amp, you can compare signals on a good chanel with those of the channel thats bad.
     
  17. Lestraveled

    Well-Known Member

    May 19, 2014
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    Wow, just found this thread.
    In the 70s and 80s I did a lot of electronic repair work, and yes, I have repaired several Sansui amplifiers. I lived near Eglin AF base and the servicemen brought these back from Vietnam. These amps required that the output transistors be a matched set and the driver transistors be a matched set. I think I remember that ECG (now NTE) offered matched sets. There was also a color code thing with them. The transistors had to have the same color dot on them. It was a long time ago.

    Update: I pulled out an ancient ECG manual and the 2SD118 crosses to an ECG 280. They also list an ECG280MP which is a matched pair of ECG280.

    Update #2: NTE lists a NTE280MP, a matched pair of 280s.
     
    Last edited: Oct 2, 2014
  18. Lestraveled

    Well-Known Member

    May 19, 2014
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    The 2SD118 is a 7 amp part. The MJE15034 is a 4 amp part. It won't last.

    Here is a link to the only spec sheet I could find for the 2SD118:

    http://www.americanmicrosemi.com/information/spec/?ss_pn=2SD118

    The NTE website says the NTE280MP is available from several places. I didn't check the driver transistors.
     
  19. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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    I have the 2sd118s The MJE15034 is to replace the 2sc756 drivers.

    If they fail again, I may use a matched pair. Unless I lose interest by then. :)

    ps.

    Any hints on checking the am/fm problems? I'm not willing to dig to deep. All my old service monitor test equipment is long gone.
    At one time I could check uv for xxdb quieting.
     
  20. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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    Replaced the drivers.
    Standby current thru finals is about the same as the good side.

    Voltage at output is not centered. Closer to ground.

    Try matching transistors better?
    I have one more. Swap them?
     
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