Technics SU-CH7 Amplifier cutting out

Discussion in 'General Electronics Chat' started by T1m, Mar 3, 2015.

Am I likely to be able to repair this by myself, or will i need a qualified technician?

  1. Repair by myself

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  2. Repair by technician

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  1. T1m

    Thread Starter New Member

    Mar 3, 2015
    13
    2
    Hi guys

    I have been pointed in your direction as i am having issues with my old school Technics SU-CH7 Amplifier turning its self off/cutting out. I saw there is another thread regarding this model but the OP didn't reply to the suggestions mentioned or conclude what the problem was. (http://forum.allaboutcircuits.com/threads/technics-su-ch7.101224/).

    Although the seperates system comes with 4 units and 2 speakers am I currently only using the ST-CH7L (radio tuner with power button), the amplifier SU-CH7 and the speakers (2 x SB-CH7). I should mention at this point that I am using it for the sound coming out of my iMac, using a 3.5mm headphone jack to phono, with the red and white phono cables plugged into the 'DAT Play Back' section of the back of the amplifier (this is the middle 2 sockets of the 6 available) as it seems to produce the best sound. I then set my input as 'DAT' using the far left DAT button on the front of the amplifier. This normally works and plays back sound perfectly for a few minutes and then cuts out (returning to standby mode), I then wait for about 10 minutes and I'm able to turn it back on again manually (it doesnt turn itself back on).

    - I am unsure if I am using the correct input for a start, but as I said the other 2 options sound very muddy.
    - When the amp cuts out I have taken the cover off and found the component RTP1M5B006 (square shaped box middle left of the unit when viewed from the front) to be rather hot, the fan does seem to be working though.
    - Would using a higher quality 3.5mm to phono cable help this matter at all? I used the same cable in my previous setup with a Bose system and had intermittent issues with it, when it would seem to cut out all but the highest frequencies of the song until i tweaked and played with the phono connections at the back of the Bose (to be honest i thought this was an issue with the Bose phono connectors rather than the cable, but now I'm thinking otherwise), then it would work fine again.

    To complete the required info I am based in Sheffield Yorkshire, I have beginner/intermediate experience with fixing electronics, I don't have a soldering iron or voltage meter but can borrow one if necessary. Model number is SU-CH7EB-K and Serial number is FM1GB12364.

    Thanks in advance guys
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Welcome to AAC.

    Hmmm !
    Pictures always help.

    But try this first.
    Take the cover off connect the speakers L+R. Give an input of your favorite song...( compulsory ) o_O You will be hearing it while testing.
    Start the amp when it is cold ( heat sink or the whole amp).
    Check your time. Start the amp and play a music at moderate level.
    Check if the fan is working...if not increase volume. Before your ears start to hurt the fan should work.
    Check the sink temp by hand. Keep away from components and PCB.

    Post back.

    BTW nice amp.
     
  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    One more thing...
    If the fan does not work at all, keep the music playing at moderate level and feel the temperature on the sink continuously till Amp shuts off.

    Post back all the details. I will take it from there.
    If you got soldering and DVM skills I can help you fix it
     
  4. T1m

    Thread Starter New Member

    Mar 3, 2015
    13
    2
    Hi R!f@@, I was really hoping you'd get involved in this thread so that's a great start! I will be able to do the tests when I get home in a few hours, in the meantime I took a few pictures last night which I can upload for you to view. I'm unsure what you want to see in particular, but if you let me know I'll do my best to show you want you require.

    I noticed there was what looked like a burnt area on the PCB near the front left of the amp, which I've uploaded. I imagine this is part of the problem but will await your feedback.

    Let me know what further images you need. Thanks!
     
    R!f@@ likes this.
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I have the Service manual.
    Burnt area picture is something I like to see. Just give me an inside picture and I will point out where I need to see close ups.
    Resize the pictures if needed. I will tell if I need high res pics. Either good lighting is important without glare.
    Still you did not post abt the tests I asked you in the first place.
     
  6. T1m

    Thread Starter New Member

    Mar 3, 2015
    13
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    IMG_4869.JPG IMG_4877.JPG
     
  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Just curios..how ?

    I am here to help
     
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Burnt area looks bad.
    The components might still be good.
    Check the other side to see if there a dry solder joints.

    But first read my first post. Need to get the fan issue out of the way u know
     
  9. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I checked the burned area components. That transistor regulates 5V from 23V so it will get hot. Dry joints are possible but that area is working as without that 5V the Amp won't work at all I think.
    {ed}
    Correction.
    That 5V is used by the μC. So that area is working at least
     
  10. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    Hot places are a dead give-away for fractured solder joints. Resolder everything in that neighborhood.
     
  11. T1m

    Thread Starter New Member

    Mar 3, 2015
    13
    2
    My friend is a member of this forum and said you were the man!

    I decided since I've got the equipment that I would make a video of the testing process for you, it might be clearer to see what the problem is. The video is uploading as we speak to my google drive, I'll share it on here as soon as it's finished!
     
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  12. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    :) Wow ! That is a nice complement. Didn't knew I was so popular.

    Still I was hoping for the fan test u know. :rolleyes:
     
  13. T1m

    Thread Starter New Member

    Mar 3, 2015
    13
    2
    :D:D:D HAHAHA well you are! Here is the fan test results and video

    Fan Test Results:

    - Song played at 80% volume on my iMac
    - Plugged in via headphone jack to phono, in the DAT Play Back sockets, in DAT mode
    - played at -32 Db for 1 minute - Fan was working
    - increased volume to -26Db for 1:30 - Fan was working
    - lasted for 2 minutes 30 seconds before cutting out and fan stopping
    - Heat sink was not hot after cut out.

    See the video here: https://drive.google.com/file/d/0B7V-0q9AVeGuNnl3MS1RQzZqc0k/view?usp=sharing
     
  14. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    o_O You uploaded a 256MB file...that is too much for my bandwidth.:oops:
    You would need to compress it.

    So heat sink is cool to touch when shutting off and the fan works.
    This does complicate things.

    It is Midnight, 0030 Hrs for me. U should update your profile and add ur location.

    I will have to go through the Diagram tomorrow.
    So I will leave you to do something.
    Check for Dry solder Joints under the PCB. You need to reflow the weak joints and clean them too.
    I will check in tomorrow.
     
  15. T1m

    Thread Starter New Member

    Mar 3, 2015
    13
    2
    It is compressed! Its just 4 minutes long.:) If you use google Chrome to view the link you will be able to watch it like a youtube video, and select the quality (360/720 etc. )

    OK no worries I will do that.
    Thanks again speak to you tomorrow.
     
  16. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    One more thing.
    Like to know if it turns off even nothing is played on, with speakers disconnected
     
  17. T1m

    Thread Starter New Member

    Mar 3, 2015
    13
    2
    Hi, Yes I just tested it without the speakers plugged in and no music playing, and it still switches itself off, about 2:30 before it switched off into standby again.
     
  18. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    OK...!
    Did you try to check the soldering for cracks?

    Are the switching off times accurate to 2:30 mins
     
  19. T1m

    Thread Starter New Member

    Mar 3, 2015
    13
    2
    I haven't checked for cracks yet, you asked me to wait. If I do I will first have to remove nearly every PCB and component to take it off and look underneath it. I will test the switch off time again and confirm for you.
     
  20. T1m

    Thread Starter New Member

    Mar 3, 2015
    13
    2
    Ok I just tested it twice to see how long it lasted before switching off, the first time it took 2:10 and the second time it took 1:05. So it is not always the same!
     
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