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JMI VOX AC30 Motor Boating on Normal Channel - Help

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  • JMI VOX AC30 Motor Boating on Normal Channel - Help

    Hello Everyone,

    I have a JMI re-issue VOX AC30/6 which is a recreation of the old AC30 with the top boost mounted on the back panel. Very nice amp but as of late the "normal" channel will start Motor Boating if I turn it up. The other channels are fine, it's just the normal channel. I tried replacing the tubes but it's still there. I tried poking around with a chop stick to see if it is a lead dress issue but no luck. Can anyone here help me track down this issue?? Appreciate the help.

    Joseph

  • #2
    By "Motorboating" do you mean it oscillates on its own? Do the tone controls have any effect?
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    • #3
      Not sure of the amps age but it may well have old power filters. The symptom your having could be a lack of decoupling. You can take a big cap (say, 22uf/450V) and clip the negative end to ground. Then clip positive end to the positive end of each filter one at a time. If the motor boating stops at any time then you probably need new filter caps. CAUTION!!! These will be very high voltage contacts so you must use wire that rated for 600V or more, be careful not to let loose leads dangle dangerously and do not make connections unless the amp is turned off and most of the voltage is bled from the caps.

      The other possibility is a loose or oxidized ground connection. This can also allow signals to join where they shouldn't and cause oscillations.
      "Take two placebos, works twice as well." Enzo

      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

      "If you're not interested in opinions and the experience of others, why even start a thread?
      You can't just expect consent." Helmholtz

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      • #4
        In this schematic, C37 (top right) would be the most likely suspect http://schems.com/schematicheaven.ne...x/ac3093pr.pdf
        My band:- http://www.youtube.com/user/RedwingBand

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        • #5
          Thanks so much. this is not an old amp. it is only a few years old and the components should not be old but they may be defective. It motor boats only on the normal channel and the tone cut has no effect on it. the top boast addition on the back has treble and bass control but that is only for the top boost itself and has no effect on the normal channel. It's not oscillation, like in the high pitch type but a low rumbling, distorted thud like motor boat chugga-chugga sound (like a trem with a maxed out intensity). I have to turn up the volume to induce the motor boating but if i reduce the volume, it goes away. The weird thing is that my trem channel which is suppose to have trem, doesn't. I tried the foot switch but is doesn't effect anything. again, I checked the tubes but they are good. something in the circuit is messed up.

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          • #6
            Originally posted by pdf64 View Post
            In this schematic, C37 (top right) would be the most likely suspect http://schems.com/schematicheaven.ne...x/ac3093pr.pdf
            thanks for that tip. how would i test it? replace it? Is there a quick and dirty way to test that cap? thanks again.

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            • #7
              Originally posted by cluster View Post
              this is not an old amp. it is only a few years old and the components should not be old...


              ...The weird thing is that my trem channel which is suppose to have trem, doesn't. I tried the foot switch but is doesn't effect anything. again, I checked the tubes but they are all good...

              This just became more complicated. I think.

              On the off chance that one of your young filters IS bad and the trem problem is a different issue, try the parallel cap across C37 just in case. That's the quick and dirty test. Another cap, 10uf to 100uf rated for 350V minimum connected parallel to the suspected cap. Make sure to observe correct polarity. These caps have a +/- requirement. Remember these are high voltage circuits. Turn the amp off but leave the standby on and let the amp sit for ten minutes like that. Unplug it before working. Failure to do this and a clumsy slip could kill you.

              EDIT:Ok... I went and had a smoke (Not the whacky kind) and thought on it...

              There have been several reissue AC30's ever since the mid 80's. There's no knowing (for we on the forum) that this wasn't old and unsold inventory when it was sold. So for us the age of the amp, and therefor filter caps, remains a mystery. So a cap failure is still on the table, but...

              You have two problems that could both be caused by a bad ground connection. If the same ground is used by both circuits, and is faulty, that might be something to look for too. You could check this by testing the resistance from the - end of C37 to the chassis. If it's more than a few ohms over your meters default reading there could be a loose or oxidized contact.
              Last edited by Chuck H; 07-20-2015, 12:57 AM.
              "Take two placebos, works twice as well." Enzo

              "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

              "If you're not interested in opinions and the experience of others, why even start a thread?
              You can't just expect consent." Helmholtz

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              • #8
                Originally posted by Chuck H View Post
                This just became more complicated. I think.

                On the off chance that one of your young filters IS bad and the trem problem is a different issue, try the parallel cap across C37 just in case. That's the quick and dirty test. Another cap, 10uf to 100uf rated for 350V minimum connected parallel to the suspected cap. Make sure to observe correct polarity. These caps have a +/- requirement. Remember these are high voltage circuits. Turn the amp off but leave the standby on and let the amp sit for ten minutes like that. Unplug it before working. Failure to do this and a clumsy slip could kill you.
                Thanks so much. I will try that tomorrow since the amp is at the studio. This is a traditional AC30/6 which has no stand-by switch. I've worked on amps before, mostly Marshall amps. AC30 amps are foreign to me, so i need to look study a layout so I can figure out how they work. I know how to drain the caps, etc.Click image for larger version

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                • #9
                  It looks the be made in Doncaster by resurrected JMI brand owned by the Harrison felons, of Music Ground infamy Posh music shop owners who hob-nobbed with celebrity clients convicted for part in 1million stolen guitar plot | Daily Mail Online
                  My band:- http://www.youtube.com/user/RedwingBand

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                  • #10
                    Originally posted by pdf64 View Post
                    It looks the be made in Doncaster by resurrected JMI brand owned by the Harrison felons, of Music Ground infamy Posh music shop owners who hob-nobbed with celebrity clients convicted for part in 1million stolen guitar plot | Daily Mail Online
                    I didn't know about that. Got it used (but like new condition) from an ebayer. Suppose to be a recreation of the real thing.

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                    • #11
                      Yes, a friend has got an AC15, it looks to be very well made.
                      What's not mentioned in that article is the amp faking business they ran, tarting up old Marshalls etc to look like pristine versions of more valuable models.
                      The forgery of such amps was reputed to be a sizable cottage industry in South Yorkshire, and generally a good job was made, to such a point that it takes an expert to tell fake from genuine. There's even certificates of authenticity signed by Marshall staff for them.
                      They also own the current Hiwatt brand, amongst various other classic 60s names.

                      Whatever, as Chuck suggests, tack a good cap across one then the other of those blue 450V 10uF electrolytics at the right hand end of the lower tagstrip, one of which will likely be 'C37'.

                      The internal metal screening plate on the tone cut pot looks to have been turned 90 degrees; I think it should be screening the tone circuit from the adjacent line power switch. Looks also like the wires to the tone control have been 'untwisted' from their original condition.
                      My band:- http://www.youtube.com/user/RedwingBand

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                      • #12
                        Just jumper a new one where the old one is connected with test leads or tack it on with wire.
                        Watch polarity and be careful as it will have high voltage on it.

                        The old one may just gave a bad connection.

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                        • #13
                          Might have found the culprit. An ungrounded 33uF cap in the top boost add-on bracket. I jumped the negative shield to ground and the motor boating seems to have gone away. The bad news is my trem still doesn't work. These components look too new to fail. Any hints on what look for to get this trem to work. Are their voltages I can check to see where the problem might be? Like I said, the Vox circuit is new to me. Looking at that tag board makes my head spin. See pictures.

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                          • #14
                            Humm.. that 33uf seems to have lost it's lead wire. That would need to be replaced, or clamped to the bracket if it has a good connection to ground too.
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                            • #15
                              What a shame, that's a very expensive, super long life / high spec Rifa cap; looks like the lead was bent too tight too close to the body.
                              May be able to solder on to the stump that's left?

                              Have you tried known good 12AX7 / 12AU7 tubes in that vib/trem channel?
                              My band:- http://www.youtube.com/user/RedwingBand

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