My Sansui 400 just stopped working

Discussion in 'Exclusively Sansui' started by matmat, Apr 6, 2017.

  1. Mr. Nick

    Mr. Nick New Member

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    Personally I would use DBT with 60W bulb. 100W bulb may not lit up bright enough.
    I think you can test DBT with any cheap electrical device. Place it on concrete floor and see there's no flammable material nearby for example to be absolutely sure. Also you can check the wiring with DMM continuity mode.

    It's good to replace output pair for channel. Not all 4, if one channel works fine.
    I think majority here are to learn :)
     
  2. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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    Well I just plug in the bugger... If the bulb stays on... you have a short somewhere. Turn it off quickly.
    I usually do.
    Use your DBT afte the new outputs are in place, remember to use mica insulators with a fresh non conductive thermal paste. (you can find some tutorials of that on youtube)
    Remember to test the power supply section for shorts, (rectifier & semiconductors).
    Please keep asking and sharing some pics :thumbsup:
     
  3. matmat

    matmat New Member

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    Thank you both!

    How do I do that? Do I also do it before the DBT test?
    Also, do you second Mr. Nick's recommendation for a 60W instead of 100W?

    I will post pictures of the DBT when I put it together, and of the Sansui once I've put the new outputs in!
     
  4. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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    Check on youtube "how to test diodes, and rectifier". Great videos there.
    On your unit, Since I don't have an unit on hand I'm looking on the schematic. And the parts that you have to look, and (may serve you well for practice and get to know your amp) are this.

    Capture.PNG

    On how to read schematics, take a look on this.
    https://learn.sparkfun.com/tutorials/how-to-read-a-schematic

    And just in case the transistors 2SC458 are well know troublemakers.
    If you can post some pics I probably can help you better since I'm not an expert and my english is kind of lame
    The 60w bulb will do.
    Cheers
     
    Last edited: Apr 19, 2017
  5. matmat

    matmat New Member

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    Update: I DID IT!

    Well, mostly...

    I replaced the output transistors, and built a dim bulb tester.
    Then I plugged the receiver through the DBT, and everything seemed perfectly fine (no shorts).
    So I plugged it back on the regular current, and again, everything seemed fine for a few days: clean sound on both channels, with different sources (Phono and Aux). I was very happy!

    But today when playing from my computer (on Aux), I noticed a distortion, mostly on the left channel at first, but then on both channels. The distortion is only audible when I turn the volume up about 1/4 of the knob, and gets worse as volume goes up.
    I hear it on speakers and headphones alike. I tried switching from Aux to Tape Monitor, but the problem is the same. I tried playing from my phone instead of the computer, and I tried a different cord, but still the same.

    Interestingly, I am not hearing that distortion when playing a record on Phono.

    Does anyone have an idea what the problem might be this time?
     
  6. matmat

    matmat New Member

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    I spoke too fast: the distortion problem is also occurring with Phono, same way as every other input, on both channels.

    Please help if you know where I should look! I miss music in my house :)

    Thank you!
     
  7. slimecity

    slimecity Super Member

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    Ok - this is likely to be a separate issue as because you are hearing sound - that means the outputs are still going. Did you adequately heatsink-paste the new outputs and are the mica washers all in place?

    Do you have the service manual? If so, check the DC offset and bias settings using the manual procedures.

    I don't think there are "pre out" and "main in" RCA sockets on the back of you unit - can you confirm this?

    Did you end up replacing the 2SC458's as mentioned above? If not, do that next with say, KSC1845.

    Once you have taken all these measurements, report back.
     
  8. matmat

    matmat New Member

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    Location:
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    Thank you for your help, slimecity. And thank you Hipocrates as well.

    To answer your questions:
    • Yes, I cleaned the original mica washers thoroughly, then applied thermal paste on both sides, and put the washer back in place.

    • I do have the service manual (from here), although I must admit it's a bit hard for me to understand where the DC offset test instructions are. Is it the part that says "Current Adjustment"? If yes, it seems that the part they are referring to is on the F-1150 board. Does that mean I have to take it out of the receiver to test it? Also it seems there are several F-1150 boards, how do I know which one?

    • I tried to test DC offset at the speaker terminal (using general instructions found online, not specific to this receiver), and I am not getting much of a reading, but I only have a cheap analog meter with 2.5V as lowest setting. Do I need to invest in a proper DMM?

    • No, I don't see any "pre out" or "main in" RCA sockets. Only Tape Rec & Monitor, Aux, and Phono 1 & 2.

    • I have not replaced the 2SC458's yet. I suppose I could order the replacements and do it, if you think my problem indicates this might be the cause. There seems to be two of them on each F-1150, do I need to replace them all?

    Thank you for your patience!
     
  9. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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    • I have not replaced the 2SC458's yet. I suppose I could order the replacements and do it, if you think my problem indicates this might be the cause. There seems to be two of them on each F-1150, do I need to replace them all?
    They usually distort the signal, and yes is better to replace all of them on drivers board.
    On my HK330b (which is full of them) made the trick.
    http://www.mouser.com/Search/ProductDetail.aspx?qs=UMEuL5FsraDCmxWozTypUA==

    Take notes about the pinout, KSC1845 are ECB and the 2SC458 are BCE.... Once we replace the usual suspects we can move forward.

    BTW how is the offset and bias?
     
    Last edited: May 9, 2017
    slimecity likes this.
  10. matmat

    matmat New Member

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    Thanks. Ok I will replace them first and see what happens.

    Like I said earlier, I am not sure I understand how/where to test DC offset and bias. Is it on the speaker terminal? Or is it on the driver board? I read the service manual attentively but I'm afraid it's a bit too advanced for me.
     
  11. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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  12. matmat

    matmat New Member

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    Thank you. I did the offset test on the speaker terminal, and I'm not seeing much, but I suspect it's due to using a basic analog multimeter with 2.5V setting, probably not precise enough. I probably need to get a better DMM.

    I found the same page that you posted in the Service Manual, but I don't really understand all the instructions. Would be great to see a tutorial with photos/video...
     
  13. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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    Can your DMM test current? Remember that we are looking for current (milliamperes) not voltage (volts)
     
  14. matmat

    matmat New Member

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  15. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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  16. matmat

    matmat New Member

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    Update:

    • I checked the DC offset, both channels look good (around 10mV)
    • I can't figure out how to measure or adjust bias for the life. I keep getting negative readings, I'm not sure what I'm doing wrong.
    • I replaced the 2SC458's with KSC1845's
    The distortion on the left channel is gone, but it's still there on the right channel.

    And I just realized that one of the quick-acting fuses is blown. When I switch the fuses, the distortion goes to the other channel. I am hoping that replacing the fuse will solve the problem!

    To be continued...
     
    Last edited: May 31, 2017
  17. slimecity

    slimecity Super Member

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    Yes, many Sansuis have fuses in the biasing circuit, so these will protect that.

    Do you have a digital or analog meter? If you are getting negative readings, sorry dont want to sound patronising - but the probes are around the right way?

    It sounds like you are nearly there. Are the trimpots original or have you replaced these? If the former, are readings jumping around while you adjust, or adjusting smoothly?
     
  18. matmat

    matmat New Member

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    No offense taken, I am a beginner indeed! But in this case, I am pretty confident are around the right way (following the picture in the service manual). In doubt I also tried the other way around but it wasn't making sense either.

    By trimpots do you mean the DC bias adjustors on the driver boards? (with a little wheel that you spin with a screwdriver?)
    I did not replace these, but I don't know if they were replaced by the previous owner. They do look original... at least they look old.
    The readings do jump around quite a bit, but they do react to the my adjustment somehow... Just not in the range that I'm supposed to be reading.
     
  19. Mr. Nick

    Mr. Nick New Member

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    So have you polished the fuse holder inside and outside contact points? Try deoxit spray there and brush them clean.
    From time to time fuses go bad over time. If you remove the fuse and test both of them there should be good solid continuity.
    Trash them if they do not.

    Trimpots are the bias adjustors and you spin them with the tip of your screwdriver, but very carefully.
    If the reading is jumpy, say reading jumps just by touching the top of the trimpot, they are oxidised.
    POWER OFF, then memorize (take a photo before) which direction the trimpot is facing and use deoxit spray there and then turn the trimpot from side to side 30 times to get better contact, then leave the trimpot at
    direction where it was facing in the first place and try adjustment again according to the service manual.

    Don't worry about the negative readings. To my understanding it doesn't matter, if it's negative or positive reading just the millivolts/amps count there.

    While adjusting the bias you must power the amp normally - not through DBT, because the bulb in series will mess up the readings.
     
    Last edited: Jun 1, 2017
  20. matmat

    matmat New Member

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    Location:
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    Thank you for the advice, Nick. I just cleaned the fuse holders and bias adjustor with deoxit.

    Now the reading aren't so shaky anymore, but not consistent with those in the service manual:
    On the left, I get a range from -1.00mA (one end of the trimpot) to -1.26mA (when I spin to the other end)
    On the right, I get a range from -1.57mA (one end of the trimpot) to -1.67mA (when I spin to the other end)

    The service manual says I should adjust to about 10 or 15mA. That's way out of the range I am reading. What should I do?
     

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