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Thread: Roland kc-550, no output?

  1. #1
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    Roland kc-550, no output?

    hey guys,
    i've got 2x roland KC-550's here. my friend asked me to take a look at them for him.
    both amps have the same symptoms, and he swears they were never abused.
    here's the deal: everything powers on, but no sound to the headphone jack or speakers.
    i pulled out the speaker and examined the power amp in each one and i found that both had burn
    damage marks behind R41. i can't find a schematic anywhere, so i'm not sure what R41 does.
    i tested the Left and Right power transistors in each amp for shorts and found nothing.
    all fuses are also intact.
    my plan right now is to replace R41 and see what happens... but, i'm wondering why it would've blown.
    any suggestions? i'll report back w/ any new developments after replacing R41.

  2. #2
    Supporting Member Jazz P Bass's Avatar
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    Call Roland.
    They charge for schematics, but if you have two to work on, it may be well worth it.

  3. #3
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    thanks for the suggestion. i'm finding now that R41 still reads on a meter @ 25 ohms. so i'm not convinced that replacing it is going to fix anything.
    hmmmm.....
    thoughts? is this a common thing for these amps? google search is not getting me anywhere, so far.

  4. #4
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    just got off the phone w/ roland. they will only sell their schematics to authorized service people. but i was able to get info on R41 out of them. it's listed as a 27 ohm resistor. so the one i have has seen some heat, but otherwise it's in spec. open to suggestions.... thanks guys.

  5. #5
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    What does R41 connect to? Power supply? Speaker output?

    Check all of the simple things first, break jacks, speakers, etc. Try one of the line out jacks any signal there? Like anything else, try and isolate the problem to a specific section of the amp. Is the main power supply working? How about the power supply to the preamp?

  6. #6
    Senior Member mozwell's Avatar
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    ROLAND KC-350 KC-550 Service Manual free download, schematics, eeprom, repair info for electronics
    you need to register, but its free, you need to answer basic electronics questions to register.....

    mountains of info on this site, heartliy recommended....

  7. #7
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    i replaced R41 in both amps, one of them is working normally now. although, maybe a little quiet. the other is still mute.
    i suspect the resistor replacement was less of a fix then perhaps re-seating some of the wiring harnesses.
    i'll try again tomorrow.
    and yeah...there's two sandbox resistors, one coming off each power transistor, at the point where these resistors
    meet, R41 goes from there to a pair of caps. i'm not sure...

  8. #8
    Supporting Member Jazz P Bass's Avatar
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    Could be the zobel network.
    If it is, replace both of the caps that attach to R41.
    Last edited by Jazz P Bass; 01-13-2012 at 12:50 AM.

  9. #9
    Supporting Member Jazz P Bass's Avatar
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    Roland KC 350-550 Service Manual

    Show nuff, it's the zobel.
    Replace the caps.
    Check the bases of the ouput transistors to see if voltages are correct.
    The outputs should barely be turned on at idle.
    I would scope the output, looking for oscillation.
    That may be where the power is going.
    Attached Files Attached Files

  10. #10
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    thanks for posting the service notes.
    to be clear, the caps I was referring to are C26 and C27.
    are those the ones you're referring to also?

  11. #11
    Supporting Member Jazz P Bass's Avatar
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    Quote Originally Posted by methodofcontrol View Post
    thanks for posting the service notes.
    to be clear, the caps I was referring to are C26 and C27.
    are those the ones you're referring to also?
    Yep.
    Technically, it may be a zobel or it may be a stability network.
    I do not know what "took out" R41.
    I simply would not trust those caps. (C26 & C27).
    The output section must be checked real close.

  12. #12
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    well..turns out replacing the R41 resistor in both amps ACTUALLY DID fix the amps. the one amp that seemed to still NOT be working also had a bum speaker. i put my meter ont it just as an idiot check, and i got nothing. hooked everything up to an external speaker, and it's working! just need a new speaker now.

  13. #13
    Supporting Member Jazz P Bass's Avatar
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    Which begs the question "why did R41 burn up?"
    I would still replace those caps as insurance.

  14. #14
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    Where would I buy resistor R41

  15. #15
    Supporting Member Jazz P Bass's Avatar
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    R41 is a 27 ohm, 1/2 watt resistor.
    Digikey
    Mouser
    Allied Electronics
    Newark in One
    Or try a local repair shop.

  16. #16
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    BigCJ- Welcome to the place. As Jazz mentioned, that is a really common part and is available from lots of sources.

    You didn't mention why you need one. Do you have an amp that has the same burned resistor or is it just the fact that there is no output?

  17. #17
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    Yes, I have the same problem. No output or blown fuse and R41 decaded.

  18. #18
    Junior Member rfpowerdude's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    Show nuff, it's the zobel.
    Replace the caps.
    Check the bases of the ouput transistors to see if voltages are correct.
    The outputs should barely be turned on at idle.
    I would scope the output, looking for oscillation.
    That may be where the power is going.
    I know I am digging this up from the dead, but I am in need of the Roland KC-550 schematic and the attached file Jazz P Bass posted above (Roland_kc-350_kc-550.pdf) seems to be no longer available. Anyone have an archived copy of this somewhere? I can't find it anywhere on the 'net at the moment. Maybe Roland got wind of the file out here and doesn't like it?

    Also, great forum! I wish I had found this place a few years ago. I have a few pieces of gear I need to fix...
    joe vitek
    Melbourne, Fla

  19. #19
    Supporting Member Jazz P Bass's Avatar
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    ROLAND KC-350 KC-550 Service Manual free download, schematics, eeprom, repair info for electronics

    At the bottom of the page, where it says 'need progs', that will change to 'download file' after a short period.

  20. #20
    Junior Member rfpowerdude's Avatar
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    Thanks John, however, this never changed for me. The screen shot below is what I get (file not found). Were you able to go to this link recently and get the file again?
    roland_kc550_not_found.png
    Could you email me that file if you have it? joe.vitek [aaht] l-3com.com. Thanks again!
    joe vitek
    Melbourne, Fla

  21. #21
    don't forget the joker g-one's Avatar
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    Appears to have been removed from elektrotanya.
    Try the one on this page:
    roland kc-550

    edit: if you can, please unzip and repost attachment to this thread
    rfpowerdude likes this.
    "Thank you. Now on this next one , ladies and gentlemen, I'd like you to pay attention to my tone - not so much my singing or the band... " - JP Lepage

  22. #22
    Junior Member rfpowerdude's Avatar
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    Thanks Greg! That's it! However, I couldn't attach it. It is slighty too big. I got this error when I tried to upload it:

    "The following errors occurred:
    Roland_kc-350_kc-550 copy.pdf: Your file of 4.89 MB bytes exceeds the forum's limit of 4.77 MB for this filetype."

    Doh, just an RCH over the size limit...
    g-one likes this.
    joe vitek
    Melbourne, Fla

  23. #23
    don't forget the joker g-one's Avatar
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    That would explain why it was posted as zip, thanks for trying!
    "Thank you. Now on this next one , ladies and gentlemen, I'd like you to pay attention to my tone - not so much my singing or the band... " - JP Lepage

  24. #24
    Junior Member rfpowerdude's Avatar
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    Here it is as the zip reposted here:
    Roland_kc-350_kc-550 copy.zip
    joe vitek
    Melbourne, Fla

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