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Trying to fix Olivetti M24

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    Trying to fix Olivetti M24

    Hello all!

    First time writing here. I bought an Olivetti M24 from a local guy. It was sold as a non working unit, but it was from a family who had bought it new in the 80's. The guy told me that it had worked the last time he tried some ten years ago. Since then it had been in storage. It looked very clean and nice when I went to inspect it, so I thought that maybe with a little work I can make it run again. I have salvaged some other computers and have a basic diagnosing and soldering skills. No real education in electronics thou. I got a monochrome monitor and a keyboard with the machine and also manuals and diskettes. All original Olivetti.

    Now some months and a massive amount of forum reading and tinkering with the machine I decided it was time to see if I could get some help in troubleshooting this beauty. I now have three M24 motherboards and none working. Let me try to explain.

    General facts
    • I have tested the power source with a multimeter and according to the information in service manual it should be ok.
    • It has two 360KB floppy drives.
    • No bus expansion board.
    • Visually in great condition.
    "Original" motherboard
    • Came with the machine.
    • 512KB RAM soldered to the MB. Empty sockets for additional RAM.
    • Power LED doesn't work.
    • Battery has leaked at some point. Don't know the amount of damage. Removed the battery.
    • When powered on, keyboard leds stay light a second and then start blinking forever.
    • Oddly, the paraller diag board gives: 80 C1 C2 C3
    • BIOS v1.36
    • Visually the cleanest of the three MBs I got
    Replacement MB from eBay
    • Gave up with the original MB and just searched a working MB from eBay.
    • Sold as 100% working. Double checked from the seller before purchasing.
    • 128KB RAM soldered. 512KB on sockets.
    • Power LED works ok.
    • Battery has leaked at some point. Don't know the amount of damage. Removed the battery.
    • When powered on, keyboard leds stay light a few seconds and then go out. At the same time speaker gives a click-sound.
    • Paraller port diag gives: 00 41 42 7d
    • BIOS v1.42
    Second replacement MB from eBay
    • When told about the first replacement MB, seller kindly send another one to replace the faulty. No costs to me.
    • 512KB RAM soldered to the MB. 128KB on sockets.
    • Power LED not working.
    • Battery maybe has leaked at some point. Don't know the amount of damage. Removed the battery.
    • When powered on, keyboard leds start blinking endlessly.
    • Paraller diag gives: 00
    • BIOS v1.42
    • Wrote to the seller about this faulty MB too. No response yet.
    Questions that torment me at night
    1. Why these odd diag codes "80 C1 C2 C3" from the orig MB?
    2. What MB should I focuse my efforts to or do I throw them all to trash?
    3. How can I test RAM modules? I just have a multimeter. Should I buy a cheap scope of some kind?
    4. How can I test the graphics card? Or can one deduce from this information that it is ok?
    Things I have tried
    • Cross-switched bios chips between MBs. No effect. I assume that BIOS chips are ok.
    • Moved all socketed ICs from 1st replacement to original. No effect.
    • Piggy-Banked all socketed RAMs from 1st to soldered on orig. No effect. This was really tricky to do and I’m not sure that I succeeded with every chip.
    • Checked continuity with multimeter here and there.
    • Replaced socketed CPU on 2nd repl with the CPU from orig. No effect.
    Thank you for reading this far and sorry for the long post. I really would like to see this machine alive. Any help appreciated. Pictures of the motherboards:

    Orig_MB.JPG Original MB
    1st_replacement.JPG 1st replacement
    2nd_replacement.JPG 2nd replacement
    Last edited by MikkoS; August 17, 2021, 09:39 AM.

    #2
    Welcome, MikkoS! This is a very complete write up and some nice pictures.

    Hopefully someone else here has some experience with this particular system. I do suspect that all three boards are repairable, based on the pictures that you posted.

    - Alex

    Comment


      #3
      Here in the forum somewhere is a complete POST code list made by Trixter from reverse engeneering the M24 BIOS. That should help identifying the problems. Unfortunatelly I wasn't able to find that thread by a quick search.
      <album>

      Comment


        #4
        Is this the posting you were looking for? https://www.vcfed.org/forum/forum/ge...agnostic-codes

        Larry

        Comment


          #5
          Yes, that's the thread.
          <album>

          Comment


            #6
            Thank you for replying! Yes, I have read that thread and the info there really helps to understand where the problem might be. One thing is for sure, I need to test memory modules on these boards but I'm unsure what is the best way to do that. I ordered a cheap chinese oscilloscope and will try to use that. There are some videos on youtube about checking memory modules using oscilloscope.

            The replacement MB went suddenly from bad to maybe dead. Now LPT diag board gives just "00". Original board still gives me "80 C1 C2 C3" which is kinda odd. Maybe I just assume that it's "00 41 42 43". So then it hangs on the DMA channel test. Maybe I will replace the DMA chip and see what happens. I'll have to order first some sockets.

            Comment


              #7
              Major breakthru!

              While I was waiting for the oscilloscope and sockets to come from China I decided once more to fiddle with the original motherboard. I had already tried to piggybank memory chips on that MB (all of them at once) but this time I decided to do it again one by one. Whart the heck! Changes were slim for anything to happen, but lo and behold the paraller diag board went all the way to C5:
              "80 C1 C2 C3 C4 C5"

              My heart almost stopped! The monitor was still black as a coal but when I adjusted it (with trembling hands) this picture emerged:

              POST_diag.png

              I can't believe my good luck! Maybe I should continue my piggybanking testing

              Comment


                #8
                Ok, I now have a fully functional M24 with BIOS v1.42 and a full 640KB of RAM. A few notes here, maybe someone will find them useful in the future:
                • My M24 would not start without a working RTC battery when using BIOS v1.32. When upgraded to v1.42 it would start without the battery.
                • Original MB had one bad memory chip. Found it using biggybanking one by one. It was the second chip tested. Lucky me! Replaced it and everything started to work.
                • Original MB will print C1 C2 etc. to paraller diag. Don’t know why. Even uprading the BIOS from 1.36 to 1.42 did not change that.
                • Transferred newer BIOS and extra memory chips from MBs bought from eBay. So the faulty MBs were useful after all. Also they will provide replacement parts in the future.
                I would like to thank all the users who wrote to earlier threads (and this one) about diagnosing Olivetti M24 / AT&T 6300. Reading those threads really gave me hope that I would someday see my own system working. I feel damn lucky that the day came so soon!


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