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Thread: 4xAA BIOS battery replacement not working

  1. #1

    Question 4xAA BIOS battery replacement not working

    I have a tall tower 386 I got over summer 2020. It came with a barrel BIOS battery soldered to the motherboard. The battery was completely dead and the motherboard was not holding any BIOS data after shutdown (date, time, floppy drive settings, and hard drive type). So I unsoldered the battery and put 4 AA batteries in the battery case that is on the floor of the computer case. I got the proper wire to reach from the battery case to the pin header on the motherboard for the external BIOS battery(ies). The AA batteries are brand new, non-rechargeable. I verified with a multimeter that the contacts that touch the pins have the correct voltage (1.5 V. * 4 = 6 V.).

    Problem is, the BIOS still does not hold any data after shutdown. Why?
    Sattinger’s Law: “It works better if you plug it in.” 🤯 Corollary: “It works even better if you plug it in correctly.” 🤯🤯
    "The simplest solution is the most likely solution." --My paraphrase of Occam's Razor
    "You can get [a computer] like yours at a garage sale for, like, fifteen dollars," --Strong Sad, sbemail #33

  2. #2
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    Quick question: Did you use a diode in series with the battery pack? You need something to block the charging current.

    But that shouldn't have let the smoke out of anything. So next question: What does your 386 board use for a clock/memory chip? (e.g. DS1285, MC14686, etc.)

  3. #3

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    Quote Originally Posted by Chuck(G) View Post
    Quick question: Did you use a diode in series with the battery pack? You need something to block the charging current.
    No.

    I do??

    Good grief
    , the more I get into fixing my vintage computers, the more I realize how clueless I am on even the simplest things about it, even after having used computers since 1990, done some programming, knowing how to solder, and and being an automotive engineer.

    Can you please give me a step-by-step guide to follow (or link) of how to properly connect the batteries, then? Thanks.

    But that shouldn't have let the smoke out of anything.
    Do you mean that it is safe to assume the 6 V. at whatever current would not have broken any components in the computer? I have been using fully charged Duracell non-rechargeable batteries.

    I have disconnected the batteries from the motherboard. I was having a repeated symptom of argumentative booting, where I would often have to restart the booting because the hard drive buzzed, or the floppy drives were not accessed, etc. Could such a voltage and current cause that sort of thing?

    So next question: What does your 386 board use for a clock/memory chip? (e.g. DS1285, MC14686, etc.)
    The clock chip on the motherboard says "DOC-20NA, 14.31818 MHz., KDS-9M, JAPAN."
    Sattinger’s Law: “It works better if you plug it in.” 🤯 Corollary: “It works even better if you plug it in correctly.” 🤯🤯
    "The simplest solution is the most likely solution." --My paraphrase of Occam's Razor
    "You can get [a computer] like yours at a garage sale for, like, fifteen dollars," --Strong Sad, sbemail #33

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    I mean the so-called "CMOS" chip (it's really the RTC with a bit of memory). I think you just read off the legend on a crystal or XCO. Photos would help.

    The idea behind a blocking diode is to prevent the computer from charging non-rechargeable (i.e. primary) batteries.


  5. #5

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    6V may be too high. The original battery was likely a 4.8V NiCad, not 6V.
    Dwight

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    I've powered 386 systems (have two so configured now) with 6V AA supplies, so probably not a big issue--however, they all use the 4-pin external battery header on the CPU board.

  7. #7

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    The OP said he is using the external batt header. He doesn't need an extra diode.

    The actual problem could be one of the diodes on the mainboard failed though. I had that happen recently, board used two 1N4148s in series and one had failed open.

  8. #8

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    One thing that it is worth watching out for when on board pcb batteries get replaced by a battery holder with batteries and flying wires is the type of plug or connector used and whether or not the pins on the flying wires are exposed.

    I saw a case where the battery pack, still had its negative wire connected and the + terminal got unplugged and got dragged across PCB connections. An external battery pack especially with Nicads or AAA or AA sized batteries can source a large short circuit current and can destroy IC's with an accident. At least most times there are diodes on the pcb which will prevent a reverse polarity accident.

    But since very little current is taken from the battery, I have become inclined to put a series 1k resistor with one of the battery wires inside some heatshrink sleeving. At least this way if the battery pack wires gets shorted out or the is an accidental contact elsewhere the current is limited to a very safe value.

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    Quote Originally Posted by maxtherabbit View Post
    The OP said he is using the external batt header. He doesn't need an extra diode.

    The actual problem could be one of the diodes on the mainboard failed though. I had that happen recently, board used two 1N4148s in series and one had failed open.
    Missed that--I got all wrapped up in the "I clipped the old battery out" Okay, Emily Litella.

    However, I have run into a board where the battery (obviously failing) was left intact and a DS1287 was substituted for the MC146818. An external battery would have accomplished nothing.

    You never know...
    Last edited by Chuck(G); December 30th, 2020 at 07:12 PM.

  10. #10

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    That's actually a good approach if you can make it work. I tried to swap my already socketed MC146818 with a dallas DS12887 in lieu of repairing the diode, but the BIOS would just halt with a CMOS bad error. I know the dallas module worked on another system so it must have just been something incompatible. A similar ODIN RTC module did pass POST on the board, but I ended up just repairing the diode

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