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27679Re: [midatlanticretro] Re: Reforming Capacitors: EAI Analog Computer TR-48

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  • David Gesswein
    Oct 11, 2012
      On Thu, Oct 11, 2012 at 05:38:46PM -0000, joshbensadon wrote:
      > I read on one site that I should be reforming them for several hours.
      > Some suggested up to 40 hours. I think I'll go for the 40 hours but
      > at 25% rated voltage for the first 24 hours.
      >
      Normally you either want to ramp the voltage up or if out of circuit
      use a series current limit resistor to keep the reforming current low.
      Starting at 25% voltage may still stress them though much less than
      full power.

      The three items parameter shifts that will indicate the caps are bad are
      leakage, low capacitance, and high ESR. Sometimes you also get visual
      hints its time to replace (drive was still working fine)
      http://www.pdp8online.com/tu56/pics/caps_bad.shtml?small

      The leakage is easy to test with a resistor in series with
      the reforming supply if you have disconnected them from the circuit. If
      your doing the variac method all you can do is look for getting warm,
      bulging or leakage from vent. Capacitance and ESR can be measure with the
      proper equipment. If either the capacitance or ESR is bad you will
      get excessive ripple on the supply voltage so just measuring that is
      reasonable. Does the documentation you have specify the allowable supply
      ripple? If you have a fixed load current you can estimate what ripple you
      should have based on the capacitance.
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