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1998 JCM2000

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  • #1

    1998 JCM2000

    I'm having a rough time just figuring out what schematics to look at to work on this amp. Here's pics with the board numbers. I just can't seem to put together a unified picture of the recifier/OT/HT fuse setup

    Customer brought the amp in with no tubes, and blown HT fuse. I replaced HT fuse, it burns up, with no tubes. I disconnected the primaries from the OT, and the amp is stable, w/ no tubes.
    Before I buy an OT, I'd like to rule out any other issues.

    The HT fuse in this amp is F2, and this is the only schematic I can find that has that labeled F2 but I can't find another one that shows everything else I need.
    Click image for larger version Name: IMG_6562.jpg Views: 171 Size: 2.83 MB ID: 1006430 Click image for larger version Name: IMG_6564.jpg Views: 139 Size: 4.05 MB ID: 1006431
    Tags: None
  • #2
    Here's the schematic

    I just subbed in an OT and it's still blowing HT fuse
    jcm2000_revmain_in.pdf

    The main board also has an F2 fuse (1,2,3,4) that are filament fuses, and that F2 is also blown. That corresponds to this schematic:
    Click image for larger version Name: JCM2-60-00 (1997) iss1.gif Views: 126 Size: 649.1 KB ID: 1006439
    Attached Files

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    • #3
      Wow so I replaced the Heater fuse F2 and the HT Fuse F2 is steady. How is that?

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      • #4
        Can you access both sides of power tube sockets to check for any arcing or miswiring?
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        ​

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        • #5
          Where is the HT fuse on that schematic?
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


          ​

          Comment

          • #6
            Never mind, I see the HT fuse on the pdf which is different from the schematic in the image. But it does not show where W14 or W15 connect, so impossible to guess why the heater fuse would factor into it.
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


            ​

            Comment

            • #7
              Originally posted by g1 View Post
              Never mind, I see the HT fuse on the pdf which is different from the schematic in the image. But it does not show where W14 or W15 connect, so impossible to guess why the heater fuse would factor into it.
              Exactly, I'm piecing together things.

              Well, the sockets don't sow any arcing, but it does look like the one with the blown heater fuse was replaced, it's clearly a different color.

              I biased up 2 El34s (ignoring the possibly problematic socket for now), and have sound, but R63 on this schematic( Finally found one that matches my main board) started to burn up. It's the PI dropper.
              I'll replace that and check the plate resistors and tube for PI, I hope it's not that e- cap...

              https://www.drtube.com/schematics/ma...98)%20iss5.pdf

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              • #8
                Originally posted by LarBal View Post
                Wow so I replaced the Heater fuse F2 and the HT Fuse F2 is steady. How is that?
                That is because the heaters draw a lot of current at power on and keep the high voltage surge to a minimum. No heater load means maximum charge current on the HT fuse and pop, the fuse has done what it is designed to do.
                Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
                If you can't fix it, I probably can.
                • Likes 1

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                • #9
                  Ok great! Thanks for that explanation.

                  even with the PI tube pulled, the R63 PI dropper is burning up.

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                  • #10
                    Well, I replaced all the filter caps (except the first bank, didn't have any large radials), and R63 is still burning up. I even replaced the 50p cap across the plates of the PI while I was in there. Plate resistors read in spec.
                    But R63 is burning up even without a PI tube in. I'm using a 1W, as that's what the schematic calls for.
                    Now I'm really stumped

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                    • #11
                      I've isolated it to the CF stage. With no tube in there, R63 doesn't burn. and any tube I put in there gets ungodly hot

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                      • #12
                        Well I seem to have gotten it resolved. I wish I could for sure say what the issue was, but I started with no tubes in the amp, and went one by one . I followed one of the molex connectors to yet another confusing schematic, off the cathode of the CF, and isolated all the parts, cleaning the connectors as I went. R63 never burned up. I'll monitor it for a while but think I'm in the clear. I did lift all the 10n caps around the filter caps in the preamp, but that wasn't an issue

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                        • #13
                          Wow this amp is a nightmare. It's stable with all tubes in, producing a little hiss through shop speaker while I burned in the power tubes and watched for drift for about an hour . all good.

                          Plugged a guitar in, and super weak signal. Like a whisper. Reverb control brings the most hiss.

                          Plugged into FX return, nothing.
                          Took FX send to another amp, and just got some hum ,hiss, and static
                          Gains seem to work on both channels but not volume of each channel

                          I'm going to guess that mute circuit is causing problems before the FX send, and it seems it can't be a coincidence that the FX return also doesn't work, but I can't seem to find anything they have in common (low voltage relays, etc)

                          Any ideas? Both schematics below


                          https://www.drtube.com/schematics/marshall/DL50-61-00%20(1997)%20iss1.pdf CON10 bottom right
                          https://www.drtube.com/schematics/ma...97)%20iss2.pdf CON 3 top right

                          and maybe this one for CON7/11 that feeds the PI
                          https://www.drtube.com/schematics/ma...98)%20iss5.pdf
                          Last edited by LarBal; 11-03-2024, 01:21 AM.

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                          • #14
                            Not sure if it applies to your version but here is a list of all connections on the JCM 2000.
                            Attached Files
                            • Likes 1

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                            • #15
                              Thank you. When I get back to the amp the first thing I'm going to check is all the molex connectors are in the right spots

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