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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.

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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; 01-12-2012 at 11:50 PM.

  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
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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.

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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?

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

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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
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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.
    Arch Stanton was here.

  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...
    g1 likes this.
    joe vitek
    Melbourne, Fla

  23. #23
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    That would explain why it was posted as zip, thanks for trying!
    Arch Stanton was here.

  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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    Do you know the specs of C26 & C27? My C26 exploded and burned the R41 and C27 and toasted my speaker too. Trying to fix it myself but can't find specs on those. Desperate for help...

  26. #26
    Supporting Member The Dude's Avatar
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    The schematic posted above says .068UF.
    "Nooooo-body expects their thread to get knocked sideways by Mindy Fleedleman.... Well! Excuse! ME! " - Leo_Gnardo

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    replacing a busted a cap

    Thanks!! I just ordered a film capacitor that meets the specs. Fingers crossed that it will work. I also ordered a pack of 'flame proof' resistors to replace the R41. Here's a link to the caps I just bought :

    https://www.westfloridacomponents.co...apacitors.html

    here's the link to the super resistors:

    27 Ohm 1/2W Flameproof Resistor 10 Pcs.

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    Why?

    I see someone here also has a blown R41 and/or blown C26&27. Why did this happen? Is this particular failure common? I don't know much about electronics aside from soldering, I ordered the new components but I'm worried that they are going to immediately blow again after I replace them, is that likely? I'm nervous about this. I hope my amp just magically doesn't have any problems once I replace the parts...

  29. #29
    Supporting Member Jazz P Bass's Avatar
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    The components that 'blew' are of the circuit type labeled "Zobel Network'.

    If one of the capacitors just so happened to 'go short', then that is what took them out.

    OR, the output section itself is damaged & 'took' the Zobel out.

    For test purposes, the amp should run without the network.

    If you want to check the output section, the briefest test would be to hook up a volt meter to the output jacks.(no speaker)
    Set the meter to read Volts/ DC.
    If you get a very low (millivolts) reading, it is a good indication that the output is o/k.

    There are much more indepth tests that can be applied, but this one will suffice for a start.

  30. #30
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    I'll add: (and this is mostly my opinion based on experience and not necessarily fact)

    The voltage rating of cheap caps needs to be de-rated at higher frequencies. I don't necessarily trust the rating in a Zobel Network. If it were me, I would have probably ordered higher voltage rated caps for piece of mind and not looked back. Usually when I've seen this happen, it's because the caps were poor quality and under-rated in the first place.
    "Nooooo-body expects their thread to get knocked sideways by Mindy Fleedleman.... Well! Excuse! ME! " - Leo_Gnardo

  31. #31
    Supporting Member Jazz P Bass's Avatar
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    I had a Class D output filter capacitor go up in flames.

    It went when feedback ran amuck through the PA.

    And yes, I went with a higher voltage rating too.

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    thanks for the reply. after the caps and speaker blew i put a secondary speaker in to see what would happen and it outputted my signal but at a low distorted volume with very broken sounding dynamics. does that mean the output is fine and the caps just randomly blew for no reason (except for the fact that i was pushing the amp hard when the caps blew)?

  33. #33
    Supporting Member The Dude's Avatar
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    It sounds to me like you've got problems other than the Zobel network. Have you checked for DC on the output as Jazz P Bass suggested in post #29?
    "Nooooo-body expects their thread to get knocked sideways by Mindy Fleedleman.... Well! Excuse! ME! " - Leo_Gnardo

  34. #34
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    Oscillation might cook the zobel, and could also cause weak distorted output.
    Arch Stanton was here.

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    How do I find post #29?

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