JTM45 Clone troubleshoot

Discussion in 'Amp Tech Center' started by iSoft, Jun 2, 2019.

  1. iSoft

    iSoft TDPRI Member

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    Hello guys,

    I'm coming (again) for a troubleshot on a JTM45 clone i just built.
    Last time i had a problem on a kit you were all really helpful and was resolved really fast, thanks for that ! :)

    So i've just finished wiring all this up and the mains fuse (2A) blows right away as soon as i turn the amp on.
    Checked 4 times the whole wiring (still might have missed something, we never know) and didn't find anything wrong unfortunately :/
    Here are pics off the build, schem and layout (pdf files), it's a Tube Town PX45 kit (jtm45 with master volume)

    Thanks for helping me solving this thing out :p

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    Attached Files:

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  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    It looks like one side of the fuse holder is going straight to ground? That can't be right.
     
  3. iSoft

    iSoft TDPRI Member

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    Thanks for your answer :)
    On the HV Fuse one is going straight to ground, but not on the Mains fuse, those are going to the On/Off Switch.
     
  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Ah - I see that fuse is on the center tap.

    I can't see a clear enough picture of the mains fuse, which I'm assuming is built-in to the IEC jack?
     
  5. iSoft

    iSoft TDPRI Member

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    Yes it indeed is built in the IEC :)
    Here is a pic :

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  6. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Well, if there's shorts blowing the main fuse the first thing would be to disconnect the secondaries and see if the fuse still blows. That can be done by removing the rectifiter tube (schematic says 5V heater so I'm assuming it has one) and your 6.3V heater. If the fuse still blows it has to be something pretty early on as that only leaves the fuse wiring and the switch and the power light.

    Ideally you'd have a light bulb limiter or other current limiting device so you don't have to repeatedly blow fuses while you figure it out.
     
  7. iSoft

    iSoft TDPRI Member

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    Actually the amp here is Diode Rectifier, no Tube Rectifier on this model (the last JTM45 i built was Tube)
    The 5V Heaters are unused and isolated, the 6,3V are leading to the power tubes.

    (no tubes are plugged right now of course, doing the first tests)
     
  8. dan40

    dan40 Tele-Afflicted Silver Supporter

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    As Corliss mentioned, unsolder the high voltage secondaries and the heater filaments and then see if the fuse blows when switching on. If it does, the issue would lie in the PT wiring or power switch wiring. If the fuse does not blow, try soldering one set of secondaries at a time and retesting. This will help you to narrow down the possibilities.
     
  9. dan40

    dan40 Tele-Afflicted Silver Supporter

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    I notice a piece of bare wire between the two rectifier diodes on the terminal strip. Any chance that it is touching the center tab on the strip and causing a short to ground?

    Edit: It looks as though you snipped that tab off of the terminal strip so there should not be any problem there.
     
  10. iSoft

    iSoft TDPRI Member

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    Thanks for your answers :)
    So i did the test, when 6,3V heater unsoldered it still blows the fuse, but with HV1 and HV2 unsoldered, doesn't blow anymore.
     
  11. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Well that narrows it down to something in the high voltage. Re-check all those areas to see where the short is.
     
  12. Wally

    Wally Telefied Ad Free Member

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    As Corliss said, you need a current limiter. Since you identified an area of concern...the HT secondary....you can lift the bias circuit from that secondary and B+ from the rectifier. Does the fuse still blow? If so check out the rectifier. If not, reconnect the bias....if the fuse doesn’t blow, then you have problems in the filter section or downstream from that area because the fuse blows when the B+ connection is made...or so it would seem to me.
     
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  13. Nickfl

    Nickfl Tele-Afflicted

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    Are you doing all of this without a current limiter? How many fuses have you gone through this point? Building a current limiter would seriously cut down on your fuse budget and it might also prevent you from having to buy a new power transformer before this is all over...
     
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  14. dan40

    dan40 Tele-Afflicted Silver Supporter

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    I would look closely under the board at the b+ jumper wires and other areas of the b+ rail for solder blobs or bare wire touching the chassis. Is it only the mains fuse that blows or has the b+ fuse blown also?
     
  15. iSoft

    iSoft TDPRI Member

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    Thanks for the replies and sorry for the late answer, i was off those days :)
    So i gave it a try by desoldering the wire leading from the Rectifiers to the board, still blows.
    Then desoldering the wire leading from rectifier to B1, still blows.
    Then desoldering both : Still blows XD
     
  16. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    What secondaries do you have connected at this point? If none, can you get us some up-close photos of the primary wiring?
     
  17. iSoft

    iSoft TDPRI Member

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    Damn, i realized the B+ fuse was also blown without checking earlier, did all the tests with it...Doing it again by changing both
     
  18. iSoft

    iSoft TDPRI Member

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    None of the wire leading from rectifier to board and standby switch are soldered now, and mains still blows.

    Here are pics, solders are starting to look bad cause of the solder/desolders tests i'm doing :p :

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  19. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Are you sure your mains power switch works the way you think it does? You might want to check the switch terminals with an ohm meter while operating the switch to see what terminals connect to each other.
     
  20. Wally

    Wally Telefied Ad Free Member

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    Lift the secondaries from the rectifier circuit....what happens?
     
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