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Help with older computer Real Time Clock

Discussion in 'Electronic Repair' started by Salvatore, Oct 15, 2003.

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  1. Salvatore

    Salvatore Guest

    I pulled a pluggable RTC module due to a dead battery (internal) hoping to
    be able to replace same.

    I was able to remove the cover but the package had the soldered on
    crystal, caps and battery completely encapsulated in dense and abundant
    black material.

    I managed to separate most of it and can replace the battery with a more
    current cell. The crystal is still embedded in potting material and
    cannot be removed to view without destroying the IC.

    In the process of de encapsulating, one of the oscillator components
    crumbled. It is a ceramic disk (typical yellow) and there is no marking
    remaining. I need to know the value of this cap to resolve the issue.

    The IC is a VIA VT82885N 24 pin DIP package

    the crystal connects to 2 Xtal input pins.

    each pin has a capacitor wired to it and they form a junction which
    connects to Batt minus. I am fairly sure of this wiring.

    One cap is a 10 (line under) and the other is virtually identical in size,
    but destroyed. I am not sure about the 10 value being mmf or nf, or ??
    Is it common to form this arrangement using identical values.
    should either be neg temp coeff caps??

    The caps are about 3/16" dia and thin.

    SO: Is the circuit as I see it correct?? What should the cap values be??

    Thanks for any suggestions (other than tossing the MoBo)
  2. ph12

    ph12 Guest

    These people may be able to help

  3. Salvatore

    Salvatore Guest


    I do not need a battery

    I need to know what the capacitor values are and if one might be a
    negative coefficient one.
  4. Franc Zabkar

    Franc Zabkar Guest

    According to the following app note, the caps should be identical:

    Using the formula on page 3, the capacitances should be of the order
    of 10pF each.

    Here is a datasheet for a similar (?), but discrete, RTC:

    - Franc Zabkar
  5. Salvatore

    Salvatore Guest

    Thanks Frank.

    The VIA VT82885N Datasheet included very little, and did not even identify
    the ground pin on the IC where the ext caps and batt - connect.
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