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Crate VC 50- New filter caps / blowing Fuse

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  • #1

    Crate VC 50- New filter caps / blowing Fuse

    HI,

    So I recently replaced all of the larger caps (they were bulging out) on a newly purchased used VC50 (http://www.blueguitar.org/new/schem/...ge_club_50.pdf). Replaced all the tubes as well, fired her up and sounded great and clean.

    After a couple songs it blew the HT fuse (F2 T250ma) and I have not been able to track down why. I double checked every new cap for correct polarity, replaced a coupling cap here and there that looked a bit suspect and checked for DC all over the grid. The only other issue is slightly high voltage on V1 (190v pin 6) which I've temporarily "corrected" with a 21au7.

    Voltage reading from fuse to ground (tp21)is .35A as of now, VDC at same point is 376.

    Ran a cap parallel to c32 with no change. Checked Voltage Divider R83/R58, checks out fine but in circuit r58 reads OL

    Any ideas where the fault could lie?

    Power tubes Plate Voltage: 360v
    Power tubes Grid Voltage: -15v
    Tags: None
  • #2
    Since my experience with new manufacture tubes is so poor, and they are so easy to swap, I would try it with the old power tubes.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


    ​

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    • #3
      Does it repeatedly blow that fuse, or was it just a one time thing?
      It's weird, because it WAS working fine.....

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      • #4
        Since my experience with new manufacture tubes is so poor, and they are so easy to swap, I would try it with the old power tubes.

        Tried with two quads, one JJ one Sovtek both pull .35 across that .25 fuse. With no tubes, it pulls .37.

        Originally posted by Randall View Post
        Does it repeatedly blow that fuse, or was it just a one time thing?
        I believe if it's pulling .35 across the fuse (I have a 500ma installed for testing w/ lightbulb limiter when needed) then it will pop again before long, right?

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        • #5
          I read that fuse as 1/2 amp, not .25.
          It's weird, because it WAS working fine.....

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          • #6
            Agree^^^, schematic clearly says 1/2A slow fuse (500ma), not 250ma.


            Voltage reading from fuse to ground (tp21)is .35A as of now, VDC at same point is 376.
            Um... this makes no sense, can you restate it? TP21 is B+ voltage, I'd believe your 376v. But voltages are never ".35A", that says current, which you would never measure to ground from a supply anyway.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              Fuuuuuuhh......

              Thanks guys. Enzo, Yes I'm sorry I meant Amperage. And I did not know you couldn't measure ground to supply, but that shoulda been obvious as current would be different at at nearly every point in the amplifier right? You can only measure current across.... I will keep this in my addled mind.

              As for the fuse, I'm guessing they use the same boards for VC30's because it is clearly labeled on the PCB as T250MA I will measure the current ACROSS the fuse and install the correct one as per the VC50 schematic hahah.

              Just got back from tour I think I smoked too much in the van....

              As always you guys are the best, I appreciate this forum so much. I never had an amp tech teacher (as I'm sure you all can tell) so a resource like this is invaluable.

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              • #8
                Well , just as long as the smoke remains in the van and doesn't start appearing in your Crate !

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