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Marshall JCM 800 50W

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  • Marshall JCM 800 50W

    I have a JCM800 that somebody has attempted to put a new power transformer in. They have cut and splice wires and it’s a total mess and does not work. I want to make sure on some wiring before I install the new tranny. My schematic shows on the secondary side the two red wires go to the standby switch, The Black wire (B+ 0v) goes to ground, which is where they had the wires. The orange and black wire to pin 1 & 6 on first power tube. The brown wire was grounded.
    The primary side has the orange, yellow, and black wires to the voltage selector and the blue to main power switch.
    What am I missing? Seems like it’s correct. The transformer hums really loud and no sound.

  • #2
    Originally posted by pnut5150 View Post
    The orange and black wire to pin 1 & 6 on first power tube.
    Perhaps you counted the pins wrong? The heaters should be pin 2 and 7. Otherwise it seems correct. Also check the fusing (correct rating?) and use bulb limiter and no tubes when firing up after replacement. I'm thinking if there's excessive current draw in the amp. Without proper fusing the power transformer might go out for the third time.

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    • #3
      i will check and see if I’ve looked at the number wrong, the wires on there look like factory solder, they cut the wires and spliced them with black tape.
      When I remove the black wire they have going to ground, the tranny quits humming so loud. I also forgot to mention that the red LED for the gain is really dim until that wire is unhooked. Maybe tranny shorted internal?

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      • #4
        An internal short on any of the windings will cause excessive current draw, whether or not any load is connected. Usually the mains fuse will blow. Humming can be down to a number of reasons - high load current and/or loose laminations. If the transformer is the correct type for the amp but poorly fitted, there may be nothing wrong with it. Either use the neon test trick, or isolate the secondaries and power up the transformer and measure the primary current draw and secondary voltages.

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        • #5
          2 & 7 are the heater wires, I’ll change that and post back

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          • #6
            Sorry I’ve been covered up at work. The heater wires were correct to start with. The transformer schematic shows on the secondary the two red wire 360v going to the stand by switch, the B+ wire (red/black)0V going to ground. There is no B+ voltage, that don’t seem right that wire to ground?

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            • #7
              There will be no B+ voltage out of the transformer. It's still AC at that point (until it's rectified). Do you have ACV across the two red transformer wires?
              "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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              • #8
                "The transformer schematic shows on the secondary the two red wire 360v going to the stand by switch,"

                I don't know what schematic we are working from, but this is confusing. The two red 360v wires should go to the rectifier, not the standby switch, that comes after the rectifier.

                "the B+ wire (red/black)0V going to ground. "

                Again hard to understand. Usually a striped wire such as red/black is a center tap and does indeed go to ground. This is not where we look for B+, that would be after the rectifier.
                It's weird, because it WAS working fine.....

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                • #9
                  The JCM800 does switch the transformer wires to the rectifier diodes. Here's a schematic (maybe not the best one, but nonetheless...).

                  Click image for larger version

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                  "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                  • #10
                    Originally posted by pnut5150 View Post
                    I have a JCM800 that somebody has attempted to put a new power transformer in. They have cut and splice wires and it’s a total mess and does not work. I want to make sure on some wiring before I install the new tranny. My schematic shows on the secondary side the two red wires go to the standby switch, The Black wire (B+ 0v) goes to ground, which is where they had the wires. The orange and black wire to pin 1 & 6 on first power tube. The brown wire was grounded.
                    The primary side has the orange, yellow, and black wires to the voltage selector and the blue to main power switch.
                    What am I missing? Seems like it’s correct. The transformer hums really loud and no sound.
                    Have you verified the rest of the amplifier works (you should try measuring DC resistance on all points where high voltage taps connect to make sure there are no low resistances there)?

                    If someone attempted that hack job, they probably didn't diagnose the real fault in the first place.

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                    • #11
                      Originally posted by The Dude View Post
                      The JCM800 does switch the transformer wires to the rectifier diodes. Here's a schematic (maybe not the best one, but nonetheless...).

                      [ATTACH=CONFIG]55460[/ATTACH]
                      Always better to switch AC than DC (less contact arcing). But pulsed DC is ok as well.
                      Last edited by Helmholtz; 10-08-2019, 09:20 AM.
                      - Own Opinions Only -

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                      • #12
                        That’s the same one I’m using
                        So do the red wires to standby should be reading AC? The tranny I have is a Mercury, show the red/black wire with the two red wires as 0v and the two red 360v. The striped wire reads -362v but I have no high voltage on the tubes

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                        • #13
                          Originally posted by pnut5150 View Post
                          That’s the same one I’m using
                          So do the red wires to standby should be reading AC? The tranny I have is a Mercury, show the red/black wire with the two red wires as 0v and the two red 360v. The striped wire reads -362v but I have no high voltage on the tubes
                          The red/black wire should be connected to ground, so 0V.

                          So do the red wires to standby should be reading AC?
                          Yes, each 360VAC wrt ground.

                          Is this your PT?

                          https://www.mercurymagnetics.com/201.../P50JM-475.pdf
                          - Own Opinions Only -

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                          • #14
                            Yes sir that is the correct transformer I have.
                            The old transformer somebody had hooked up was smoking when powered up, when I unhooked the 0v B+ wire it would quit smoking and the lights would come back on bright. I don’t want this Mercury to smoke lol.

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                            • #15
                              I will check when I get home

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