J5MK1 blown fuse

  • Hi guys, my J5 suddenly stopped working after a couple of months: I checked the fuse and it's showing no continuity.
    Before blowing the only replacement fuse I have at the moment I wanted to take some measurements.
    The power transformer used to get quite hot after an hour of use, so that's the first suspect.
    I could still keep my hand on the top surface without burning myself, but it sure was uncomfortable. (Is it supposed to run that hot?)


    This is what I measured.
    Primary (pins 3-7): 38.6 ohms
    Heater (pins 16-18): 1.0 ohm (unsoldered leads)
    HT secondary (pins 10-12): 100.4 ohms (leads still on, standby switch open)
    Shorted test leads: 0.9 ohms


    How do these ones look?

  • Hi Rigel,
    sorry for the late reply but I finally found an unused Pt to do the measurement ;)
    I measure quite the same values: 38 ohms for 3-7, 0,1 Ohms for 16-18 and 100 ohms for 10-12 so the PT seems to be OK. The reason for the blown fuse could be a broken tube with a shortage, so you should test the amp (after replacing the fuse) w/o tubes first. And yes, the PT gets quite warm, as long as you can touch it it's fine.
    Regards,
    Martin

  • Hi Rigel,
    sorry for the late reply but I finally found an unused Pt to do the measurement ;)
    I measure quite the same values: 38 ohms for 3-7, 0,1 Ohms for 16-18 and 100 ohms for 10-12 so the PT seems to be OK. The reason for the blown fuse could be a broken tube with a shortage, so you should test the amp (after replacing the fuse) w/o tubes first. And yes, the PT gets quite warm, as long as you can touch it it's fine.
    Regards,
    Martin

    Hi Martin, thanks for your help! Glad to hear my PT is working as designed.
    Looks like it was some momentary short that caused the issue: I did some thorough visual inspection on the amp, checked all the soldering, bent the tube sockets pins more outwards to make sure they don't touch, then started the amp through a current limiter, without and with tubes. Everything looked fine, so I fired the amp up at full voltage and managed to play without a problem for now. Hopefully it'll stay that way :)

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