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    Tantalum capacitors, like C56, normally explode...

    Tantalum capacitors, like C56, normally explode open circuit. C56 is not required for 5160 motherboard operation (it only filters the +12V passing through to the expansion slots). You may discover...
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    Are you aware that if the IBM MDA card is the...

    Are you aware that if the IBM MDA card is the only video card fitted, the two video switches on switch block SW1 (see [here]) need to be set to MDA ?
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    And a photo of the 06/10/85 ROM set would be nice...

    And a photo of the 06/10/85 ROM set would be nice to add to [here].
  4. 16 = 00010000 48 = 00110000 So, bit 5...

    16 = 00010000
    48 = 00110000

    So, bit 5 toggling, but not bit 4. Per my earlier note, you would have had the speaker fitted.

    Referring to the diagram at [here], I see the major possibilities as...
  5. Additional to my previous post: Refer to the...

    Additional to my previous post:

    Refer to the diagram at [here].

    Two parts of the speaker circuitry are fed back to the PC4 and PC5 pins of the 8255 chip.

    Therefore, with a continuous tone...
  6. Assuming possession of a logic probe or...

    Assuming possession of a logic probe or oscilloscope:

    Step 1: Refer to the diagram at [here].

    Step 2: Power up the 5160 to Cassette BASIC. At this point, we know that the POST has already...
  7. Welcome to these forums. As a very first...

    Welcome to these forums.


    As a very first thing to look at, verify that the speaker's connector uses the outer pins, per the photo at [here].


    And just to be sure, verify operation of the...
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    Lots of IBM 5150 information at...

    Lots of IBM 5150 information at minuszerodegrees.net


    Careful. The IBM 5151 can be damaged if it is plugged into the wrong type of video card.


    Photo #1: The 16KB-64KB type of 5150...
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    Out of interest, is it one that you have just...

    Out of interest, is it one that you have just acquired, or one that was working for you but no longer working ?
    If the previous, it opens up other possibilities, e.g. wrong type of SIMMs fitted by a...
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    I have made an assumption (shame on me) that you...

    I have made an assumption (shame on me) that you are dealing with two problems, a power supply problem and a motherboard problem.

    If we go back to your original question, with the subject being a...
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    In theory, there is a timing consideration. In...

    In theory, there is a timing consideration.

    In many motherboard designs, after power-on, the motherboard is held in the reset state for a short period. One of reasons may be the 8088. Its...
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    Damage could result through over-voltage. For...

    Damage could result through over-voltage. For example, the power supply generating +6V on the +5V line.

    I expect that a decent power supply will contain circuitry to detect an over-voltage...
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    Some EU based services are included at [here...

    Some EU based services are included at [here].
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    The fact that your multimeter is beeping,...

    The fact that your multimeter is beeping, suggests to me that you have your multimeter in the 'continuity' setting. For future reference, note that the continuity setting can sometimes be...
  15. They appear in the results of an eBay search...

    They appear in the results of an eBay search using: laptop post tester
  16. Are you are aware that [these...

    Are you are aware that [these] are available on eBay ?
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    I panicked when I thermally observed the same...

    I panicked when I thermally observed the same with my cat, Jenny, but the cause turned out to be that she is very analytical/logical, i.e. very left brained. :-)
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    Random 0400 201 For the '64KB - 256KB' type of...

    Random 0400 201

    For the '64KB - 256KB' type of 5150 motherboard, the '04' in '0400 201' corresponds to address 16K, i.e. an address within RAM bank 0.
    I do not know how an 'uncabled' extender...
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    In the 'optional Boot ROM' of the diagram is a...

    In the 'optional Boot ROM' of the diagram is a note that includes, "RP2 always required, or 10K resistor between pins 1-2".

    Yes, the diagram would have been 'cleaner' if RP1 had been used instead,...
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    Maybe the ET4000 based card is faulty, faulty in...

    Maybe the ET4000 based card is faulty, faulty in a way that makes its I/O ports appear at multiple areas, with one of those areas being the same area used by the CMOS/RTC chip.
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    And does the error message reappear if you put...

    And does the error message reappear if you put back the original video card ?
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    So, 'CMOS inoperational SYSTEM HALTED' always...

    So, 'CMOS inoperational SYSTEM HALTED' always appearing at power on.
    That certainly sounds like the POST's test of the 'CMOS/RTC' chip (per [here]) is failing.


    Had the subject been an IBM 5170...
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    In my experience, resistor arrays are very...

    In my experience, resistor arrays are very reliable.

    Have you ruled out the power supply as the cause ?

    If yes, do you see 4.28V when the motherboard is the only device powered by the power...
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    To me, that is lower than expected. For example,...

    To me, that is lower than expected. For example, looking at the data sheets of the RAM at [here], 4.5V is usually the minimum. What does the data sheet for your fitted RAM indicate?
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    IBM PC ? If so, confirm for us that it has the...

    IBM PC ?
    If so, confirm for us that it has the the third and final motherboard BIOS revision (10/27/82).


    How do you know that?
    An 1801 error on-screen when the 5161 is disconnected?


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