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Sansui AU-D907 needs some recaping...

Discussion in 'Exclusively Sansui' started by JoseHH, Dec 1, 2017.

  1. JoseHH

    JoseHH Member

    Messages:
    82
    Location:
    Now In Bangkok
    I always wanted to have a Sansui AU-919, now I am in Bangkok and here found one AU-D907 for about 275 USD. The seller imports second hand audio equipment form Japan. Before I payed for it, I listened to it and asked the seller to remove the top cover. Although i spotted that the L-CH output transistors were the expected NMA/NMC1012, i saw that the right channel had transistors whose lettering indicated a different batch. I knew that there has been a previouds repair on the unit. I saw the s called "Black flag" capacitors in the driver board. The volume potentiometer is ALPS but it does not have the incremental "clicks" upon rotation. The amplifier powered up ad come out of protection, and sounded ok at low volume. I tested the front panel functionalities that i could and they seemed to work fine. Considering that the amplifier worked, and that its cosmetic appearance was acceptable and that i wanted to have this model, i decided to buy it even if the price is a bit to high. Now I am at home and i took a closer look at the amplifier and i was shocked to see the previous repairs. When i loosen ed the driver board with the heat sin to have a better look at the output devices i saw that the transistors replacements were incorrect. I am surprised the unit works at all!. Expecting to receive guidance from the experts in this forum, I decided to open this thread because i did not see many threads on the Japanese version of the AU-919. I think it will be a nice challenge to get this unit some preventive care to get it working safely. I have read all i could about the AU-919 and i have printed the service manual and an enlarged copy of the schematic. Now I am up for some fun removing the bad components. Front.JPG Output_Transistors.JPG
    The transistor with the tape is PNP type 2SA-747A !!!!, and is supposedly complementary is a 2SA1116, while it should have been 2SC1116. Please tell me if I am wrong but i think that these two transistors are either not connected, open or just by pure luck dont do anything. The Fuse F-02 blew (probably at the time the right channel failed) and it was bridged with a wire. Also, the driver transistor 2sd-382 has been replaced by a 2sd-525. I will replace all the black flags and try to get a better set of driver and output transistors.
    The question is, should I get the expensive set of TO-3 2sa1116/2sc2607 from vintage sansui or should i go with the TO3-P modification suggested by Ilimzm and Echowars?. I have a set of the 2sa1294/3263 and enough MJ21195/6 to put on the unit. I will appreciate it having some input from other members of this forum.
    Thank you.
     

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  2. ghamilton

    ghamilton Super Member

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  3. JoseHH

    JoseHH Member

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    82
    Location:
    Now In Bangkok
    Thanks Conrad, I have already read your page. Very useful information. I got a set of silver-mica for the driver board and I have some for most of the amplifier but i need to order some 22 and 3 pf if i want to get all the black fags replaced. Today removed the driver bard. It was not so difficult actually. First removed the 3 connectors, next I unsoldered wires 21, 22, 31 27, 28 from the driver board. Then I unsoldered the +/- 56 V wires from under the capacitor bank. Then i flipped the board/heat sink assembly 180 degrees which now it may be done since there is only a bundle of ground wired connecting it to the chassis. Then I unscrewed the board from the heat sink (six screws, 2 on the driver transistors heat sink). The last step was to unsolder the 4 ground wires labeled as 07 (two white wires) and 08 (two black wires). I will clean and replace the black polystyrene capacitors and I may replace the few electrolytic capacitors, the multiple junction small-bead-diodes and the 4 trimmers.
    Is there any other component that needs to be replaced?. I have a few questions based on the transistor set that is currently installed. Namely, the drivers in the right channel are currently unmatched they are 2SD525+2SB537.
    IMG_1181.JPG
    I have as possible replacements the complementary pair MJE15032 and MJE15033, fT=30MHZ, V_CEO=250V, and 10 Amps. They are supposed to be used as audio drivers. In a post in this forum ilimzn said that the MJE15032 is not suitable. But it is not clear to me why he thinks so. Perhaps there there is some detail of the datasheet that i am not looking at?.
    For the output transistors, right now the right channel looks pathetic:
    IMG_1181.JPG
    I think I will initially install a set of MJ21195/MJ21196 on both channels. Once i get this amplifier working again i will probably replace them by the modern TO-3P.
    This is all for today...
     

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  4. JoseHH

    JoseHH Member

    Messages:
    82
    Location:
    Now In Bangkok
    I looked at the output transistors and, sure enough there were only 4 of them working, two on each channel. Instead of the parallel transistors set, the amplifier was left working with a simple complementary stage on each channel. Whoever was the technician that did the previous repair converted this beautiful AU-D907 into an AU-D707 without Loudness switch!. He clipped the pins of two of the transistors so the sockets did not make an electrical connection with them. I tested all the transistors and the ones with cut pins were shorted were shorted.
    ClippedNMC1012.JPG
    The emitter resistors and the sockets were intact. Also, besides a poorly matched driver transistor the driver board was in good shape.

    After labeling all the wires and taking tens of pictures, I decided to start working on the driver board F-2839. Another problem that this amplifier had, is that the right and left channel output wires were exchanged. I did not notice this at the beginning and I labeled the white wire as 27 and the blue wire as 28 (more on this latter...). Here is the driver board before I started replacing the black flags capacitors:
    F2839_before.JPG
    Then I removed all the electrolytic capacitors all the "black flag" types and the 4 driver transistors. Next I cleaned the glue and installed
    mica capacitors in the places that I found the "black flag" types. Then I installed new electrolytic capacitors, using WIMA stack types for 4 1uF. The rest were Panasonic FC or Nichicon FG 470uF 100V and two KZ 47uF 50 V. I installed 25 turn trimmmers for bias and offset.
    For the driver transistors I installed the MJE15030/MJE15031. I read a post by ilimzn in this forum where he explains why these transistors are not ideal for this amplifier. However, I used them anyways to get this amplifier working again with a complete set of output transistors.
    I cleaned the heat sink and installed a set of MJ21195/96. Then tested with a dim bulb test (got the correct behavior bright then dim). Removed the bulb tester, and used full mains. Bias and offset initially were zero and not responsive, while the DC offset were 350mV for the right channel and 170mV for the left. Because the output wires 27 and 28 were exchanged, the behavior was very confusing at the beginning, but it became clear after trying adjustments on both channels. After correcting this issue i could adjust both DC offsets and bias to the values required in the service manual!. Now is time to rework the power supply and then continue with the rest of the black flags.
    IMG_1374.JPG

    Merry Christmas to everyone!!

    Jose
     
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  5. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Looks like a very neat job Jose - nicely done. :thumbsup:
     
    Last edited: Dec 24, 2017
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  6. Overundr1

    Overundr1 AK Subscriber Subscriber

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    Did the pre-drivers test bad or did you change them as preventative (mje15030/1)
    If you have a hot enough soldering iron a spot of solder on each of the four transistor mounting nuts to the ground plane is suggested.
    Working on a 919 at the moment myself
    So far so good :)
    -Lee
     
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  7. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Yep, lightly file the sides of the nut to clean them (2 faces is enough), tighten them up in position - then solder the 2 cleaned sides of the nut to the track.
     
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  8. AUD101

    AUD101 Parsley The Lion!

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    "The Big Smoke" - UK
    Touch of flux may help also(?)
     
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  9. JoseHH

    JoseHH Member

    Messages:
    82
    Location:
    Now In Bangkok
    Thanks Lee,
    I had the impression that the right channel drivers were mismatched. But the transistors checked ok. In fact the amplifier worked before i took it apart to replace the capacitors. The left channel appears to have the original 2SD382(L)/2SB537(L). The right channel had a 2SD525 and a 2SB537(L) which I think are not a good match. What do you think?. Here is the picture:
    IMG_1438.JPG
    I may try to buy a 2SD381 but there are fakes all over the place. Here in Bangkok i found 2SB537 and 2SB536 but I do not know if they are counterfeit. All the other possible substitutes for these TO220 transistors turned out to be unavailable or too suspicious to be used.
    For example I have transistors marked C2238 (O) which appear to be KEC, and 2SA985 marked NEC, and some without brand, and I also have transistors marked C2275 and one marked A968 with no brand. If you can tell by looking at the picture which may be genuine, here is what I have:
    IMG_1435.JPG
    Perhaps someone in this forum knows these parts well enough to tell?. To me Digikey MJE devices seemed the most conservative option. Please keep in mind that I do not have the output transistors. But If I can avoid damaging other components and the printed circuits, I would stay with reliable replacements. By the way, the diamond differential transistors were not 2SC1985/2SA992 either. They were 2SC2383/2SA1013. Are you aware of any AU-919 or AU-D907 which carried these from factory?

    With regards to the square nuts, you mean the ones on the collectors which are the +/-56 volt rails?. I can try to put solder on one side like John and you suggest. So far, before I reassembled this part I cleaned the solder on the area where the nut lands and used solder wick to leave a clean, flat thin layer of solder for the nut to sit flat against the soft tin layer to make the connection. But I do like the idea of soldering the nut.
     
  10. JoseHH

    JoseHH Member

    Messages:
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    Location:
    Now In Bangkok
    Thanks John, I actually used several of your recommendations, and particularly like your threads where you show your work. Very valuable stuff indeed!
     
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  11. ghamilton

    ghamilton Super Member

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  12. Overundr1

    Overundr1 AK Subscriber Subscriber

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    Sheesh I must be tired ground plane obviously a typo however everyone got the idea anyways :)
    I have used those mje parts in the past when the originals were bad on other x1x series amps. The 907 might not like them so check for oscillation.
     
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  13. JoseHH

    JoseHH Member

    Messages:
    82
    Location:
    Now In Bangkok
    Thanks Lee,
    I will try to borrow an oscilloscope and a signal generator. How do you normally test for oscillation? I was thinking on grounding the input of the driver board, and probe directly at the points where the speaker wires come out of the driver board. If the unit oscillates, i should see it in the oscilloscope, but this may not be very professional.
     
  14. Overundr1

    Overundr1 AK Subscriber Subscriber

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    Start at 1vac peak to peak or less square wave at say 1000hz watch for overshoot at the start or end of the pattern. If none observed work your way on up the frequency curve to at least 50khz if your scope and signal generator will handle it. Since you do not have original outputs yet there should be no oscillation but once you secure the fast ouputs this becomes a very important step.
    It also is good bench practice to have the device under test on a current limiting device such as a dim bulb tester until you are sure the amp is stable.
    Note the 50k upper limit, this series of amps will amplify signals at that level and even higher so we want to catch any poor behavior on infrasonic frequencies.
     
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  15. JoseHH

    JoseHH Member

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    82
    Location:
    Now In Bangkok
    Thanks Lee,
    When i do this tests I will post the plot of the amplifier response to a square wave at a couple of frequencies. It will be interesting to see how the amplifier performs in this sense.

    Jose
     
  16. JoseHH

    JoseHH Member

    Messages:
    82
    Location:
    Now In Bangkok
    Hi everyone, I have a good question that I would like to ask.
    I am still trying to find the best substitutes for the driver transistors 2sD382/2sb537 for an AU-D907 (Japanese domestic version of the AU-919). I installed the MJE15030/1 but when I read the post of ilimzn, it is not the best solution:
    http://audiokarma.org/forums/index.php?threads/au-919.291461/page-4

    EchoWars mentions that the 2SC2238 and 2SA968 could be replaced by the 2sa2140/2sc5993 which are TO-220F:
    http://audiokarma.org/forums/index.php?threads/sansui-au919-parts.120187/

    Appear to be a very good match for the 2sD382/2sb537 but these can not be used because they do not have the collector metal tab to be connected via the mounting screw.
    Then I it occurred to me that there must be a metal tab inside the plastic case of the transistor, so I sanded the plastic in the tab zone for a pair of 2sa1837/2sc4793 that i had, and sure enough there is some sort of metal tab that is actually connected to pin 2 of the transistor:
    AddaptingTO_220F.JPG

    The lead for the collector is part of the heat sink as in the TO-220 devices but it is bent in a strange way. I believe that it it possible to make a reasonable electrical connection with the head of the screw to use these more available devices as drivers in the AU-D907.
    I do not know if the Panasonic 2sa2140/2sc5993 also have the metal tab inside but for mechanical reasons it must be there. If so, they could be a lot easier to find and are perhaps one of the best substitutions for the original drivers in these fast amplifiers.
    Even the ones I sanded look promissing, but a bit shy in current (only one amp, 230V, ft=70Mhz)
    Would this be a viable way to replace bad driver transistors? Anybody can criticize
    :)
    Thanks,

    Jose
     

     

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  17. RobinTW

    RobinTW Member

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  18. JoseHH

    JoseHH Member

    Messages:
    82
    Location:
    Now In Bangkok
    Hello Robin,
    I am glad your unit has all the original parts in it. Enjoy taking care of it. If I were you I would not use it until all the problematic parts have been replaced. You can get almost any component in Taiwan. That is a bonus for you. I have been to Taipei and Hinshu. Nice!!
    As for me... I will find the way to get it working well again. No hurry.
    Regards,

    Jose
     
  19. JoseHH

    JoseHH Member

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    82
    Location:
    Now In Bangkok
    Hello, I would like to update that after reinstalling the driver board with the heat-sink and the output transistors the AU-D907, and checking to be OK once again with dim bulb tester, and readjusting the offset and bias the amplifier works with the MJE15030/1 drivers and MJ21195/6 outputs. The offset and bias stayed practically where i left them in my previous test with the power stage out of the enclossure. I played music with a CD player and it sounds pretty good. There is only a bit of hiss at high volume when the toe controls are not dissabled. I only tested the Aux and tuner inputs. I did not bother with completely reassembling the unit. Just the bare necessary.
    Now I took the amplifier apart again to change capacitors in the power supply boards. I will post photos soon. Thanks to all the reccomendations.

    Jose
     
  20. RobinTW

    RobinTW Member

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    Location:
    Taipei, Taiwan
    I will replace those black flags eventually and do a full recap when I have time. I am thinking if mine can survive 39 years, it must have better black flags. Those with bad black flags are already dead or half dead. So, when they are replaced, I'd like to have them checked to confirm my theory. However, I don't have the equipment to do the testing.

    If you are to replace the outputs with TO-3P modifications, will you get both channels done or just the bad one?
     

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