Icom IC 781 Memory Battery Replacement

Discussion in '"Boat Anchor" & Classic Equipment' started by N5DUP, Jul 12, 2019.

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  1. N5DUP

    N5DUP Ham Member QRZ Page

    I have an Icom 781 that I bought brand new and still works reasonably well. Do any of you know of any documentation, video or otherwise that may help me understand how to get to the Logic A board where the 3V lithium coin battery for the memory is located? The service manual does not show how to get to it. I know it is under the Logic B board but have not figured out how to actually get to it. Any information you might be willing to share with me would be much appreciated.

    Paul Bilberry
  2. W9GB

    W9GB Ham Member QRZ Page

    The Icom IC-781 is an “aircraft carrier” radio.
    Icom’s expensive attempt to mimic the Signal One radio of 1970s.

    Sounds like you already have the Icom IC-781 Service Manual.
    SECTION 5 : Mechanical Disassembly (All Parts Shown)
    5.17 through 5-19 show Modules, Connectors, and PLL.A and PLL.B removal
    No Color by Numbers here — Icom shows how parts are assembled (bench tech experience)
    ICOM AMERICA may still have the PAPER Service Manual Available (large fold outs)

    Larry Hinton, K7YBZ provided this information on the ICOM Reflector in February 2015.
    As noted, there are two (2) lithium coin-cell batteries in the IC-781 (BR2032-1T2).
    Sanyo was the original OEM supplier. Panasonic bought out Sanyo, a decade ago, and changed the suffix used for Solder Tab models. Panasonic BR2032/F2N
    One is on the LOGIC-A board and the other is on the LOGIC-B board.

    On the LOGIC-B board, the battery + (BT1+) terminal, Service Manual page 131 of 224, is just to the upper right of the area for the OPTION UNIT UT-36 SPEECH SYNTHESIZER and just above J10.

    The battery is located on the LOGIC-A board. It is a BR2032-1T2 (Sanyo Part Number) but available through Digi-Key under Panasonic (BR2032/F2N).

    IC-781 Parts Layout on the Logic-A board found on page 130 (Service Manual).
    The battery (BT1) has two solder locations. The first located just to the bottom left of IC6 (between IC5 and IC6) and the second is just to the upper right of IC6 between D1 and Q8.

    Purchase and replace with the proper solder tab lithium coin cell.
    BAD SERVICE ADVICE: Soldering a button battery case and using drug store coin cells.
    Electronics Bench PRO's don't recommend that HACK.
    Last edited: Jul 13, 2019
  3. W9GB

    W9GB Ham Member QRZ Page

  4. N1RND

    N1RND Ham Member QRZ Page

    I just replaced the two br-2032 cells in my ic-765. I got them from Mouser. I don't think the 781 has a volatile memory system but check. Google search. A volatile memory means that if the battery dies or is removed with out applying voltage across it the operating system for the radio is lost and now the radio is a door stop. There is lots of info on the internet about this, go to the PIEXX board website for more info.
  5. KP4SX

    KP4SX XML Subscriber QRZ Page

    I don't know the 781. I replaced the whole battery/module in my 745 back around 1990 and it was about $30. I was browsing around ebay today and see the same modules around $80.
    If that is what it needs then its better than becoming a doorstop.
  6. N4MTB

    N4MTB Premium Subscriber QRZ Page

    When I changed the clock battery in my 781 I installed a holder for a CR2032 that I got from Parts Express, now every couple years I just pop in a new one.
    KB2FCV likes this.

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