1. Rest in Peace Paul (Kegger) If you would like to help the family in this time of great sorrow and need, you may donate on their GoFundme page: https://www.gofundme.com/mckechnie-medical-and-funeral-fund?
    Dismiss Notice

Help needed - Sansui AU-9900 transistor substitution

Discussion in 'Exclusively Sansui' started by mubs13, Jun 13, 2017.

  1. mubs13

    mubs13 New Member

    Messages:
    11
    Location:
    Melbourne, Australia
    Hi all,

    I am in need of a bit of advice with transistor substitutions in AU-9900. Mine is in need of repair, having issues with unstable bias and slight distortion in one channel. I was thinking of replacing the usual suspect transistors on the driver board (F-2583) as it still has original transistors there which might be giving up their ghosts now...

    I have prepared a list of transistor replacements primary based on this extrmely helpful substitution list here:
    http://audiokarma.org/forums/index.php?threads/once-more-sansui-transistor-substitutions.583009/

    But I have also noticed some alternative transistor substitutes being mentioned here and there, so I just wanted to get it reviewed by the experts and experienced people here to make sure I have got the replacements right (e.g. based on the location and usage on the particular board)

    For the F-2583 driver board, I have the following:


    Tr01, Tr02, Tr03, Tr04 2SA726 KSA992FA
    Tr05, Tr06 2SC1628 KSC3503
    Tr07, T708 2SA818 KSA1381
    Tr11, Tr12 2SC735 BC33716
    Tr13, Tr14 2SA562 BC32716
    Tr15, Tr16 2SC1124 KSC2690AYSTU
    Tr17, Tr18 2SA706 KSA1381ESTU

    Tr19, Tr20 2SD382 MJE15030G
    Tr21, Tr22 2SB537 MJE15031G

    Does this list look Ok? For easy reference, I have marked the replacements based on the position on the board, Please see this image here:
    [​IMG]


    I am also a bit confused with the Transistor list given in the service manual for F-2583, in particular for Tr05/06 and Tr07/08. The list has 2 different transistors printed for both set of positions, grouped by a big curly brace e.g for Tr05/06 both 2SC1124 and 2SC1628 are given (see image below). However, on the schematics Tr05/06 are given as 2SC1628. Similarly, Tr07/08 are given as 2SA818 only.

    [​IMG]


    And lastly, I may also need replacement Molex socket connectors for the driver board. Whoever serviced it before decided to solder the wires directly into the pins! :mad::mad::mad: I have searched and found this post here with a suggestion - http://audiokarma.org/forums/index.php?threads/au-9900-pcb-connector-blocks.398717/#post-9626896

    Is the pitch value of 5.08mm right for F-2583 as well? If anyone has replaced or fixed the sockets on this board, any suggestions or tips would be very helpful.

    Thanks guys. Really appreciate the help. :thumbsup::)
     

    Attached Files:

  2. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    37,446
    Location:
    Hertfordshire, UK
    That is correct.

    2SA562 normally replaced with KSA733 or BC327.
    2SC735 normally replaced with 2N3904 or BC337.
     
    Last edited: Jun 13, 2017
    mubs13 likes this.
  3. mubs13

    mubs13 New Member

    Messages:
    11
    Location:
    Melbourne, Australia
    Thank you! :)
     
  4. ManyMoonsAudio

    ManyMoonsAudio AKA paul79 Sponsor Subscriber

    Messages:
    2,963
    Location:
    Tulsa Ok (C-Ville)
    Have you replaced the fuse resistors?
     
  5. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    37,446
    Location:
    Hertfordshire, UK
    I think you will find there aren't any in this model Paul.
     
  6. LBPete

    LBPete Rolling Along Subscriber

    Messages:
    10,337
    Location:
    Long Beach, CA
    The AU-9900 is a great amp.

    Please post the serial number for the database. Click on the link in my signature block.

    - Pete
     
  7. mubs13

    mubs13 New Member

    Messages:
    11
    Location:
    Melbourne, Australia
    Great, one less thing to worry about :)

    I do plan to check and replace other components too as needed. I will also be recapping the driver board as well as the power supply boards. Finding the resistor and capacitor replacements are easy, its the transistor replacement I needed some help with. Just want to make sure I have the right substitutes. I really like this amp and would love to see it working well again.


    Any advice regarding the Tr05/06 and Tr07/08 transistors parts given in the list in service manual? Are they equivalent? Does that mean either of those can be used in those positions? Or should I just go by schematics and use the ones given in there?

    Cheers.
     
  8. ManyMoonsAudio

    ManyMoonsAudio AKA paul79 Sponsor Subscriber

    Messages:
    2,963
    Location:
    Tulsa Ok (C-Ville)
    Oh, right you are... It is the "A" that has them.

    I am a bit surprised to see this amp having an issue though. These things are bullet proof for the most part, unless the 2SA818 and 2SC1628 are present in your amp. Some came with these, some did not.
     
    Last edited: Jun 13, 2017
  9. ManyMoonsAudio

    ManyMoonsAudio AKA paul79 Sponsor Subscriber

    Messages:
    2,963
    Location:
    Tulsa Ok (C-Ville)
    While replacing all the transistors is not too bad of an idea, sometimes, I would rather try and find the best replacement for the actual offending parts.

    That said, if your amp has the 2SA818 and the 2SC1628, those are where I would point blame first. The rest of the transistors should be fine really.

    Unfortunately, there are not allot of options to replace these. KSC2690AYS KSA1220AYS or KSA1381ESTU is also good.
     
    Last edited: Jun 14, 2017
  10. mubs13

    mubs13 New Member

    Messages:
    11
    Location:
    Melbourne, Australia
    Hey all,

    Thanks for all the help. In between my kids and the job, I was finally able to get some time to work on my au-9900 this weekend. I replaced all the transistors on the F-2583 driver board as per the replacements given. I replaced them all because I do not have a proper transistor checker so I couldn't have confidently checked each of them. Also, there were mixed transistors on the board, some replacements and some originals. So I decided to just replace them all for consistency and reliability.

    Good news is that the amp powered up fine and the original issue of distorted channel seems to be fixed now. It played fine on both channel using my test speaker. Bias looks stable and responds well to adjustment.

    However, there is a small issue. DC offset is fluctuating on one of the channel. On first power on, I set the bias and DC offset on both channels and played some test music and it was all good. I turned the amp on again after an hour or so and this time protection kicked in after about half a minute. The amp does come out of protection on power on but eventually goes into protection after a short arbitrary amount of time. I tried to measure the offset before the protection kicks in and I can see it swinging between 2 digit to 3 digit number on my digital multimeter. Adjusting the DC offset VR seems to have some effect but I can not get the offset voltage to stabilize.

    Any clues what could it be please? Any particular area or bunch of components I should test and focus on?

    Thanks.
     
  11. mubs13

    mubs13 New Member

    Messages:
    11
    Location:
    Melbourne, Australia
    Did some more investigation as below.

    Tried to measure DC output on faulty channel before the relay, after the protection kicks in. Is the circled point in the diagram below correct to measure dc offset before relay? The 'brown' point had 38V DC measured with negative to the ground :eek::eek::eek:. Orange measured only around 4mv DC.

    [​IMG]


    Also, R channel Tr06 (replaced with KSC3503) on the driver board gets really hot within 10 seconds of turning on the amp. Too hot to touch comfortably. The Tr05 on L channel remains normal.

    Is this looking serious? :( I really hope its not anything major. Please, any help will be much appreciated.
     
  12. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    37,446
    Location:
    Hertfordshire, UK
    The KSC3503 - is it installed correctly, no pin mixups? - if it's getting hot it's probably being turned 'hard on' by a fault with another semiconductor, and if it's getting as hot as you say it won't last long before it too goes faulty.
     
  13. mubs13

    mubs13 New Member

    Messages:
    11
    Location:
    Melbourne, Australia
    Hi John, I have double and triple checked the pins, it's installed correctly. Also, the same transistor Tr05 on the other channel is installed same and is working fine. I feel it's probably something else as you have suggested. I am also worried that if I leave the amp on for too long, it might kill the heating transistor. So I do quick measurements and turn off the amp within a minute. This is making a bit difficult to diagnose too.

    I'll keep looking in the mean while. If I could get some clues as to where to look, that could help me a lot.

    Cheers.
     
  14. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    37,446
    Location:
    Hertfordshire, UK
    That's what a DBT is for, allowing you to keep the unit on for a bit longer so you can take voltage readings and troubleshoot. :)
     
  15. mubs13

    mubs13 New Member

    Messages:
    11
    Location:
    Melbourne, Australia
    I guess it's time for me to get one :)

    I'll head to my local hardware store to get the parts and make one asap.

    Thanks for the tip. Would a 100watt bulb do?
     
  16. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    37,446
    Location:
    Hertfordshire, UK
    I would go lower - 60watt might be better. - try to get both wattages - then you can see for yourself. ;)
     

Share This Page