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Thread: 5150 motherboard no boot diagnosis

  1. #41
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    OK ... I've made some more progress in troubleshooting.

    I'm quite positive there is no problem with the ROM chips. I'm fairly certain Ruud's diagnostic has a bug in the ROM checksum test.

    SO, that narrows this problem down to something with the keyboard controller. I will look through the BIOS code to be certain, but I think the non-boot behavior is likely due to interaction between the BIOS and keyboard. This board also has some noise in the speaker and that might be related.

  2. #42
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    Well, finally the board boots to cassette basic, but only with the keyboard unplugged. Cassette Basic is also reporting 12252 bytes free instead of 16k ... but all good clues. RAM chips are all known good chips, but I may pull them and verify. BUT, looking at the circuit diagram, U24 (LS322) might be worth a closer look as it connects with both SW2 and keyboard.

  3. #43
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    Quote Originally Posted by chris_nh View Post
    Cassette Basic is also reporting 12252 bytes free instead of 16k ...
    I see the same on my fully-functional 16KB-64KB motherboard, when switches SW1:3 and SW1:4 are off.
    The 12252 figure is an 'available' one; bytes available after BASIC has taken off the amount that it has reserved for itself (i.e. what is now free/available for program use).

  4. #44
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    Switch 3 and 4 are indeed off because this board is fitted with 64k of RAM. The number I'm expecting to see for this board/bios combo is 61404.

    The 12k number is what appears when SW2:1-5 are ON. I get 48k when SW2:1-4 are on. Neither is correct. I will be testing this more, but it points to a possible fault in the logic between SW2 and 8255 IC, or possibly the 8255 itself. I have not covered this part of the board yet with the oscilloscope, as my previous probing efforts were focused on the address bus. Now that the address bus issues are resolved, I will dig deeper into the 8255, keyboard and speaker circuits. (Noise is present on speaker, which also indicates a problem in this area)

    Thanks

  5. #45

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    Also check the DIP switches themselves; they could be faulty as well.

  6. #46
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    Stone, you were right ... I hadn't noticed, but somewhere throughout all the handling of this board, sw3 had become switched ON, which of course was the cause of the ram issue. With SW3&4 OFF I'm seeing the 61404 number in casette basic. I think it's really just down now to the speaker noise and keyboard problems.

  7. #47

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    Quote Originally Posted by chris_nh View Post
    Stone, you were right ...
    Sorry, but it was not I that mentioned that.
    PM me if you're looking for 3" or 5" floppy disks. EMail For everything else, Take Another Step

  8. #48
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    haha sorry, I meant modem7

    I discovered the speaker noise was due to power supply, and my other bench supply also decided it would be a good time to stop working, but I found another working power supply and the speaker noise is gone .. So this board is down to just the keyboard problem, which is no boot with keyboard plugged in.

  9. #49
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    OK, FOUND IT! Finally

    Keyboard socket on the motherboard, pin 1 is physically missing, so there it is. I guess I need to find a socket

  10. #50
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    I want to just give a quick review of the problems discovered on this board, just if anyone would find it useful:

    The board: 16-64k early revision 5150 motherboard presenting as "dead board" with no boot. Board had corrosion on several components due to a clock battery leak at some point in the past. Diagnostic roms working.

    Problems & fixes:

    1) Corrupted display, mixup of characters, random characters on screen. Problem was ultimately a bad ram chip (bit 6) in bank 0. You might not expect display corruption caused by a bad ram chip on the board, but it is possible in certain modes of failure. Fixed by replacing bad ram chip.

    2) Broken CPU socket. The CPU socket contained broken wipers and resulted in intermittent no-boot behavior. Replaced socket.

    3) Keyboard DIN socket pin-1 missing. Results in cursor blink on screen, but will cause IBM BIOS to not boot if keyboard is plugged in. The pin must have broken off at some point in the past and was physically missing from inside the socket.

    4) Corrosion of U13-U17 ICs ... ICs were likely still functional, but were replaced due to concerns of possible failure.

    So there you have it. An early 5150 board saved from the scrap heap. Thanks for following along with my rambling and thanks to everyone who offered advice and help. I appreciate it greatly!

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