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Thread: IBM 5150 continuous beep on powerup

  1. #1

    Default IBM 5150 continuous beep on powerup

    Hi,

    I have a battered old IBM 5150 PC (64-256k revision) that I am trying to resurrect. I cleaned up the mainboard and power supply, but when I came to test powering on the board, the C7 Capacitor at the top right of the mainboard (near J7 / COMP text) popped and smoked after about 10 seconds.

    After reading up I learned that I should be able to snip the Capacitor and the board would function without.

    After removing the failed cap, I re-tested the PSU voltages ( 11.67V, 4.93V, -12.14, -4.88 ) and retested the board, but after 8 seconds it emits a continuous tone, sometimes the tone sounds distorted. Or multi tone.

    I have checked the DIP switches for the memory config, and reduced the RAM to banks 0,1 (DIP’s set to 128k). I have even tested with the CGA GFX card removed and the CPU removed. In all cases after ~8 seconds the continuous tone starts.

    I guess the first question I have for any 5150 experts is are the voltages for the PSU in spec?

    Should I replace the removed C7 Capacitor before testing further?

    Is there any other area I should investigate?

    Any help much appreciated.

    Tricky

  2. #2
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    Do you have a XT specific keyboard plugged in?
    Thomas Byers (DRI)- "You'll have a million people using the A> [MS-DOS prompt] forever. You'll have five million using [nongraphic] menu systems such as Topview, Concurrent PC-DOS, Desq, and those types. But there'll be 50 to 100 million using the iconic-based interfaces."

  3. #3
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    Hello. Welcome to these forums.

    Quote Originally Posted by trickydee View Post
    ... but when I came to test powering on the board, the C7 Capacitor at the top right of the mainboard (near J7 / COMP text) popped and smoked after about 10 seconds.
    After reading up I learned that I should be able to snip the Capacitor and the board would function without.
    ...
    Should I replace the removed C7 Capacitor before testing further?
    So, per [here], the only capacitor on the +12V line, a line that only provides power to the expansion slots (no motherboard components).
    The absence of that C7 will not stop the motherboard from starting, and not cause a continuous/varying tone from the speaker.

    Quote Originally Posted by trickydee View Post
    In all cases after ~8 seconds the continuous tone starts.
    The ~8 seconds is interesting. From what I have read, the symptom usually appears very soon after power-on.

    Quote Originally Posted by trickydee View Post
    I guess the first question I have for any 5150 experts is are the voltages for the PSU in spec?
    See [here]. The POWER GOOD signal is important too, because it going above +2.4V is what triggers the motherboard to start.

    From what I have learned, a continuous tone (or variation) is a usually (usually, not always) a sign that the power-on self test (POST) is not executing. But for some 5150/5160 motherboards only. For example, if I remove the CPU (or BIOS ROM) from any of my 5150/5160 motherboards, I do not hear the continuous tone symptom. But some other members of these forums do hear the symptom when that is done.

    Quote Originally Posted by trickydee View Post
    I have a battered ...
    If you have not already, do a thorough visual inspection of the motherboard. There have been reported cases of the trimmer capacitor (see [here]) getting damaged due to poor motherboard handling.

    Have you re-seated all socketed chips (and whilst doing so, looking for pins bent up underneath the chip)?

    Have a spare known-working 8088 CPU ?

    Perhaps obtain a replacement U33 BIOS chip (EPROM and adapter per [here]) .

    I think that it would be worthwhile to try a Supersoft/Landmark diagnostic ROM.

  4. #4

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    The weird thing is, I cannot remember a place in the BIOS that outputs a continuous tone for what ever reason. But a corrupted ROM might do weird things that has this result. A broken 8253 could cause this tone. And it is possible that a broken 8253 does not refresh the on board DRAM, which means no working stack which on its turn sends subroutines into oblivion: the PC hangs.

    Regarding the voltages, they are within the specs of +/-5% so that should not be an issue.

    My first suggestion: if you have a board with ICs in sockets, replace them with ones you know that are good i.e. coming from a working board. Start with the 8253, if possible.

    My second suggestion: the Landmark diagnostic ROM or the one I created: http://www.baltissen.org/zip/diagrom.zip. Please use the instructions on the minuszerodegrees site for using the ROM. If the 8253 is broken indeed, both diagnostic ROMs should show that on the screen. If you have no have any screen at all, then the trouble lays much deeper, I'm afraid. Then you would need something like a logic analyzer.

    Success!
    With kind regards / met vriendelijke groet, Ruud Baltissen

    www.baltissen.org

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    Quote Originally Posted by Ruud View Post
    The weird thing is, I cannot remember a place in the BIOS that outputs a continuous tone for what ever reason.
    I have been through the POST/BIOS; I can confirm that there is no code that does that.
    My hypothesis for a continuous tone when the POST does not start, is in post #2 at [here].

  6. #6

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    Many thanks for the replies. I have sourced a replacement 8088 and a NEC V20 so I can test replacing the CPU.
    I am also going to get hold of a copy of the landmark diags to see if they provide further information.

    Once I get the parts Iíll update you all on my progress.

    Again many thanks for helping me out on this.

  7. #7

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    So an update, I installed a NEC V20 and still got the continuous beep (after 8 seconds). I have also tried reducing memory down to the single soldered bank of RAM, again. The same continuous beep. I also tried piggy backing other RAM chips onto the soldered bank, again no joy.
    @Ruud - Thanks for your suggestion I donít have a working board to swap components from (yet) so I am going to purchase an 8253.
    I am also working getting an copy of the Speedsoft diagnostics ROM and a new BIOS.

    Will update as soon as I make some progress.

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