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Fender Deluxe VM blowing fuses immediately after taking off standby

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  • #1

    Fender Deluxe VM blowing fuses immediately after taking off standby

    Fender Deluxe Vintage Modified (VM) is blowing fuses immediately after taking it off standby. It belongs to a friend who says it happened all of a sudden with no warning. The amp was working one minute and then pffffttt, it blew the 3A mains fuse. He replaced the fuse with a correct replacement and it blew immediately after taking the amp off standby. He asked me to take a look at it, so I'm gonna try to track down the short.

    I have removed all power tubes and preamp tubes and it still blows fuses. Right now my focus is on the TUBE PCB and I have removed it completely from the chassis. I have checked all the electrolytic caps on the TUBE PCB and none are shorted and ESR and capacitance readings are reasonable for all the caps on the TUBE PCB board. Next I will begin checking all the diodes, but I wanted to get some input before digging too much deeper without a plan.

    Here is the schematic from which I am working.
    Attached Files
    Tags: blowing fuses, deluxe vm, diodes, electrolytic, short, tube pcb, vintage modified
  • #2
    The reason I suspect the TUBE PCB is that the fuses only blow when coming off standby. I have all the PT secondaries disconnected and the fuse hasn't blown, but the standby and TUBE PCB are now out of the mix. Here are the voltage readings I am getting from the PT secondaries:

    Mains Line Voltage - 121.9 VAC
    P7, P8 - 7.22 VAC
    P9, P10 - 342 VAC
    P11, P12 - 29.04 VAC
    P13, P14 - 6.92 VAC


    ON THE TUBE PCB
    OK, I've lifted one leg on D11, D12, D16, and D17 and all of them test good. Also, checked D13, D5, and D6 and they test good, but I did notice the forward bias for D5 and D6 is closer to 1.5 V rather than the 0.6V for the PS diodes.
    Last edited by Tone Meister; 08-04-2013, 04:00 AM.

    Comment

    • #3
      Without proper test equipment all that you can do at this point is disconnect the secondary.
      Power it up.
      If it blows a fuse then the transformer is bad.
      If it holds a fuse then you can connect, one at a time, the different secondaries.

      Comment

      • #4
        Originally posted by Jazz P Bass View Post
        Without proper test equipment all that you can do at this point is disconnect the secondary.
        Power it up.
        If it blows a fuse then the transformer is bad.
        If it holds a fuse then you can connect, one at a time, the different secondaries.
        Before I can begin connecting back the secondaries for test purposes, I need to solder the components back to the board. I am gonna call it a night and get a fresh start in the morning. Specifically, what test equipment do I need?

        Comment

        • #5
          A lamp limiter, an ammeter to measure line current, a variac, one or two volt meters.
          That is what I would refer to as proper.
          Can you do the job with less?
          Probably.

          Comment

          • #6
            Gotcha, I haven't built the lamp limiter but now seems like a perfect time. I do have a variac with volt and ammeter, but I am waiting for a current transformer to arrive so I can use the ammeter on 1A/10A scales, therefore, the variac is not available for troubleshooting at present. I do have several quality multi-meters and could use the ammeter function on any of those.

            Right now my challenge is to reconnect each secondary to the loose TUBE PCB in a safe manner in order to monitor what happens as each secondary is connected. As soon as I solder the lifted components back to the board, I will insulate and secure the board and connect P9 and P10 to see if the fuse blows. If the fuse does not blow with P9 and P10 connected, then I'll check to see if I get +438VDC B+ at TP1. You'll recall that I'm getting 342 VAC directly from P9, P10.

            Comment

            • #7
              OK, C30 (8200pF/1KV) is shorted. Should I change out all these caps or just the broke one? I am inclined to swap all four caps C30, C31, C41, C42 and all four diodes D11, D12, D16, and D17 in the power supply. What would you do and why?

              Thanks!

              Comment

              • #8
                Shorted C30 would have bypassed the diodes (except D11?) so they should not have been stressed. I would not worry about them.
                As far as C31, C41 and C42, I'm not sure if these particular caps are prone to failure, but they're probably cheap enough, so for peace of mind you could just replace all 4 if you like.
                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


                ​

                Comment

                • #9
                  Sounds like a solid plan to me. It's such a pain to get the PCB out of the chassis, in my mind it only makes sense to go ahead and rebuild the rectifier while I have it out and be done with it.

                  Thanks g-one!
                  Last edited by Tone Meister; 08-04-2013, 05:01 PM.

                  Comment

                  • #10
                    Replaced C30, C31, C41, and C42 and problem is solved. Thanks Jazz P Bass and G-One for weighing in.

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