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Roland KC 550 keeps blowing fuse 1

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  • Roland KC 550 keeps blowing fuse 1

    Hi all, I’m new to the forum and new to keyboard amps, but not new to AC and DC electronics. Although it’s been awhile and I’m rusty in those areas. I have a Roland KC 550 that a friend gave my wife to use, there complaint was that it would not power up. So my wife says, oh my husband can fix that, so here we are.

    I believe the main problem is in the power supply circuit board. Can anyone tell me what the output AC voltages should be coming off the secondary winding of the transformer. I can post the voltages I’m getting tomorrow. I believe the DC voltages coming out of the rectifier is supposed to be +15V and -15V.

    I did have a problem with the primary winding side of the transformer, the thermal fuse was blown. I jumped it out and now I have output on the secondary winding side. No issues (Fuse blowing) with the secondary winding when it is unplugged from the PS circuit board. But when I plug it in And turn the power on the #1 fuse blows. It’s the correct fuse 5A slow blow 5mm x 20mm. In addition the 2 fuses on the PS circuit board do not blow.

    I also found a blown 27ohm resistor on the amp board that is in a circuit with the output to the woofer. I replaced it. I will also add that I do have the correct schematic for the amp.

    Any thoughts would be appreciated.

  • #2
    Schematic below for reference:

    1) It's ok to use the transformer with the thermal one-shot fuse bypassed for testing, but you should install a new one before you return the amp to operation.
    2) You should build a light bulb limiter for testing and to save fuses.
    3) If that 27 ohm resistor was burnt, you should check that C26 & C27 aren't shorted.
    4) You should unhook the speaker(s) until the amp is repaired and do not reattach them until you verify the amp is not putting out DC voltage.
    5) The fact that F2 and F3 do not blow before F1 likely means that your short is in the -VCC1/+VCC1 supply or the output/amp section. It looks like you can disconnect WP25-WP26 to see if it's supply or amp.
    Attached Files
    Last edited by The Dude; 10-20-2020, 02:40 AM.
    "Yeah, well, you know, that's just, like, your opinion, man."

    Comment


    • #3
      Thanks, I will check those things tomorrow and see what I get.

      Comment


      • #4
        1) fill the Country field in your member panel, service reccomendations depend a lot on where you live.
        We donīt even know your mains voltage.

        Originally posted by Victor A View Post
        . I have a Roland KC 550 that a friend gave my wife to use, there complaint was that it would not power up.

        I believe the main problem is in the power supply circuit board. Can anyone tell me what the output AC voltages should be coming off the secondary winding of the transformer. I can post the voltages I’m getting tomorrow. I believe the DC voltages coming out of the rectifier is supposed to be +15Vand -15V.
        No. Please read the schematic.
        Your power transformer has 5 taps.
        #3 centertap plus #2 and #4 give youn +/-24V which are then down regulated to +/-15V
        #3 + #1 and #5 give you +/-54V for the main amplifier.
        But when I plug it in And turn the power on the #1 fuse blows. It’s the correct fuse 5A slow blow 5mm x 20mm. In addition the 2 fuses on the PS circuit board do not blow.
        No, correct fuse is either 4 A or 2A, as shown on schematic.
        Read it.

        I also found a blown 27ohm resistor on the amp board that is in a circuit with the output to the woofer. I replaced it.
        If you mean R41 please say so.
        If itīs blown it means the amplifier oscillated badly, which may have blown the power transistors Q7 and Q17 and highly likely the Tweeter.
        Probably damaging driver transistors Q9 and Q16.
        I would replace all four and any resistor directly connected to them which is toasted or worse.

        After vreplacing them turn amp on ONLY through a lamp bulb limiter and without speakers connected, check for no shorts lamp bulb shining bright) and DC at speaker out (sholuld be almost zero)

        I will also add that I do have the correct schematic for the amp.
        Ok, YOU have it, post it here.
        Check version, it may or may not match the one kindly offered by The Dude.

        Juan Manuel Fahey

        Comment


        • #5
          1) USA, Southeast Alabama, 110V Single phase

          2) Fuse 1 - for 100V/117V - T5AL 250V (PS1 Board)

          3) Yes R41on the AMP Board

          4) Will go through the other suggestions later today

          5) The schematic I have is the same one (Aug 2005) that The Dude posted, thank you for that. The file I have was saved at a higher resolution and the file size is too big to upload.

          Thank You

          Comment


          • #6
            Good, if you replace those 4 transistors and anything visibly broken/burnt you have good possibility of solving this.
            Juan Manuel Fahey

            Comment


            • #7
              For “The Dude” - WP25 and WP26 are hard wired in, will have to desolder to check that.

              Juan - The only burnt thing I have found is the 27 ohm resister (R41)on the amp board which I have replaced. And I still need to check the capacitors at C26 and C27. So you are saying to just go ahead and replace the transistors at Q7, Q17, Q9 and Q16, that these are the likely culprit. Will have to order those, do you have a preferred place to order from? Spent part of today also building a lamp bulb limiter.

              I was able to pull the voltages off the secondary winding of the transformer today, they were Taps #2 and #4 = 34.6V, #1 and #5 = 86.5V, and #3 being the neutral/ground.

              Thanks for all the input and advise, it is helping me remember how all this stuff works. Will keep you updated as I proceed.

              Comment

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