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EDC 521 DC calibrator blowing fuses

Discussion in 'Electronic Equipment' started by JW, Feb 17, 2011.

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  1. JW

    JW Guest

    Here's one I haven't seen before. Bought an EDC (now Krohn-hite) 521 DC
    calibrator on the surplus market that was blowing fuses. There were two
    power supplies that seemed to be causing the problem +-150V. Turns out
    that there were two 22K 2W carbon comp resistors that are used as bleeders
    across the two 470uF filter caps. Both had changed resistance - one was
    about 2K, the other was 150 ohms! I've seen carbon comps drift, but never
    saw ones that drifted *that* far. Also, in circuits like this where they
    are dissipating over a watt, they usually drift high; at least in my

    Well, after replacement it seems to be working well. May need a
    calibration though.
  2. N_Cook

    N_Cook Guest

    If you still have them , try a neat 0.5mm thick Dremmelgrinding disc cut
    across the middle and see if there is a gradation of resistance developed
    across the material , and so along the length. Just under the surface
    coating you may find the high conductivity path.
    I keep a "black museum" of such oddities , don't know if anyone else does
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