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Roland JC77 problems

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  • Roland JC77 problems

    I've been having issues with a JC77. The first was the chorus selector switch which has been dealt with but now its blowin' fuses. Originally there was a lack of volume, before the selector switch repair. I plugged the pre-amp out into a good amp and it still had very little volume, before fixing the switch. I'm thinking the transformer is bad but I'm at a loss. Any ideas and/or help on what might be wrong would be great. Thanks in advance,Greg

    PS How do you test a transformer any way?
    Attached Files

  • #2
    The 'blowing fuses' may be the transformer.
    My first suspect would be the output transistors.
    The four bolted to the chassis.
    As this is a stereo amplifier, you can check one side to the other.
    With the power off & the power supply drained, check each leg from one to the other with an ohmmeter.
    If you find a dead short on any of them, there is your fuse problem.
    Last edited by Jazz P Bass; 06-14-2012, 02:11 AM.

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    • #3
      Don't I need to take the transistors out of circuit to get a reading?

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      • #4
        Originally posted by TarheelTechinTraining View Post
        Don't I need to take the transistors out of circuit to get a reading?
        Not for a quick test for shorts. You may need to remove from circuit for more accurate tests.

        Which fuse is blowing, the main fuse or one of the fuses on the pc board?
        Do you have a light bulb limiter?

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        • #5
          I sure do Bill. Its the main fuse thats blowing. I didn't use the light limiter at first because the amp worked before the selector "fix". After it blew the fuse I did use the limiter and it glowed Very bright, a dead short somewhere. I did check the outputs and there is no short. Thanks to all,Greg

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          • #6
            Whenever you are working on ANYTHING, and it develops a NEW problem along the way, it is almost always because of something you did. Look closely at your work. Make sure when you fixed your switch or whatever that you did not reconnect cables to the wrong connectors. Make sure anywhere you soldered you did not inadvertantly leave a solder blob shorting two points. Come up with a missing screw or nut? Could be wedged under a board, shorting out the supply.

            The transformer was not bad the day before, why would it be now? WHy do you think the transformer is the problem, as opposed to just about everything else in there? Transformers can fail, but they are almost always the last thing on a failure list.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              Well I thought the same thing " worked before but not now what did I do" When I got the amp it had little volume but played, hence maybe the transformer. I know you're right enzo but for the life of me I can't seem to find what is wrong. I've triple checked my wiring with the schematic and I don't have any missing screws.

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              • #8
                I'm with Enzo here, you always suspect what you did before the problem developed.

                Try pulling the two fuses on the pc board, does the limiter still light up? Have you checked the power supply diodes for shorts?

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                • #9
                  I been soldering for well over 50 years now, and that STILL is my first thought in these situations, "Oh lord, what did I do NOW?" We all have done it, and it ain;t like it will never happen again.

                  We look for something we did, but ultimately if we don;t spot anything, then we have to just approach it as a new problem the same as if it just now came in through the door.


                  You replaced the chorus switch? It is on the cirtcuit board, yes? Had to dismount the circuit board to do it? I see the main rectifier bridge is off the board, so ther are four wires that could have gotten confused. DO what Bill said, pull those two secondary fuses, main still blow? By now I hope we are using a light bulb limiter instead of actually blowing fuses.

                  The four power transistors are off board, any of them get disconnected? Potential for problem there. Or did you pull the traqnsistors off the chassis at all? ANY chance there is a missing insulator under one? Power off - duh - and go down the row of four, are any of the collectors shorted to chassis?

                  This is a shortcut to me, but it may be more complicated than you care to make it, but... A quick way to look for shorted power transistors is to measure for continuity from the speaker hot lead to the two DC terminals of the main bridge rectifier. The output transistors are each wired directly to a V+ or V- supply, and those originate at the bridge. SO a shorted output transistor will show as a short from V+ or V- to the speaker jack.


                  Try all that stuff.
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #10
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ID:	825196I couldn't agree more.Plus there aren't two fuses on the pcb , like the schematic, looks like they were never there. Also do you mean the zener diodes or the BRD?
                    And the lack of volume was there before the "fixed selector switch".

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                    • #11
                      And yes after the first fuse blew I used the limiter.
                      No continuity from bridge to speaker hot leads, V+ or V- Thanks fellows

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                      • #12
                        Originally posted by TarheelTechinTraining View Post
                        [ATTACH=CONFIG]18801[/ATTACH][ATTACH=CONFIG]18802[/ATTACH]I couldn't agree more.Plus there aren't two fuses on the pcb , like the schematic, looks like they were never there. Also do you mean the zener diodes or the BRD?
                        And the lack of volume was there before the "fixed selector switch".
                        Check the main bridge rectifier for shorts.

                        Looking at the photos, have you completely removed the pc board from the amp? It looks like you have unsoldered the power transformer leads and others as well?

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                        • #13
                          I agree check the bridge for shorts perhaps if you do it this way by unsoldering the +'ve and -'ve pins on the pc board and measuring the DC voltage across them.
                          The circuit you provided is a JC120 which shows 80 volts DC but the JC77 will probably be less.
                          That way you can also test the transformer (which I don't think will be faulty) the bridge rectifier and the way the transformer secondary connects to the board.
                          The pins have red and brown wires wrapped to them.

                          With everything mounted back in the chassis have you checked what Enzo suggested ?
                          Or did you pull the transistors off the chassis at all? ANY chance there is a missing insulator under one?
                          Power off - duh - and go down the row of four, are any of the collectors shorted to chassis?
                          If the insulating washers under the output transistors are perforated or if there is any connection between the back of the transistor (collector .|. middle pin of transistor) and the heatsink this will blow fuses.

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                          • #14
                            SOmehow it completely escaped me that the schematic was not for this amp. Howefver, looking at the photo, I can see two wire jumpers where the fuses would have been. I see the holes for the fuse clips in the board.

                            The lack of volume was there before the switch was changed, but the blowing fuses was not. I could be wrong, but I bet the two problems are not related.

                            Zeners? No, I am more concerned for the main bridge rectifier. The four heavy wires at the bottom of the photo go to it.

                            No V rails shorted to output is good, but still need to check if either V+ or V- is shorted to chassis.

                            You have unwired the transformer? OK, before connecting it back up, see the three terminals? Center is CT/ground, the other two are AC. Make sure the two AC ones are not shorted together. And are you sure which wire goes where of the three?
                            Education is what you're left with after you have forgotten what you have learned.

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                            • #15
                              Yeah enzo not the correct schematic but when I searched for the 77 thats what I got . Most of the componets are the same, or close enough. The brd tested perfect and its mounted to the chassis from the factory. I'm positive about the transformer to pcb connections, I drew a picture don't laugh and I took pixs plus on the schematic it shows the connection numbers for the three wires. I do have continuity between the ac wires, both orange wires, at the transformer unhooked from the pcb. Thanks so much for everybodys patience with me ,Greg

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