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Thread: NEC PC-9821 S1 Repair

  1. #11


    Thanks TD-Linux for the write up.

    I am in a similar predicament with my PC-9801ES(a specification like yours but an earlier architecture with an i386SX (16MHz) CPU and a V30 (8MHz) CPU but no IDE interface). Depending on what DIP switch is set to determines which CPU is running.

    Mine arrived in rough shape, the monitor had numerous cracks and breaks in the plastic housing. The computer case was severely misshapen from landing on the back corner with a lot of force. Tried to power it up but very little came to life. Could hear that the monitor might be working but I did not have another system to try. The system had a high pitched hum similar to a bad power supply. Unplugged the connectors to the system board and found that there was very little voltage at the connectors.

    Digging around the internet and some help from Google Translate I find that this power supply for this computer, TDK PU471, is most likely suffering from bad capacitors. Went to a couple of the local shops and rounded up all the replacement capacitors. Now I thought it was a good time to build my ESR meter that had been sitting on the shelf, with all the components in a bag, it is a GM328. After a little diversion, only one damaged LED had to be replaced that was salvaged from an old telephone answering machine, I tested all the old capacitors and found a bunch were out of specification. Even for the fun of it I tested all the new capacitors too. After I replaced all the capacitors in the power supply the system came to life. (looking at the CRT face down and everything stripped from the damaged case I could see the memory test. I am surprised, especially with the state the packaging arrived in, that any life was left in the monitor or the computer. I now feel like I am caring for an injured, abused and neglected animal that is barely hanging on. I have some hope at this point and I decided it was worth my time to rebuild the broken plastics for the monitor. I used Q-Bond to put everything back in place for good. I felt like a MAD SCIENTIST When you make the plastic repair compound too thick it makes a creaking sound, like something is compressing and a small poof of vapor is emitted from the compound. A few minutes later I reassemble the parts and now that the CRT monitor is back together I can try some more trouble shooting)

    The system would get stuck on trying to find a SCSI device in an endless loop. I expect that this system at one point used to have an external SCSI disk.

    After removing the C-BUS SCSI card I can get the system to the NEC N-88 BASIC prompt but still can not get into dos.

    I have tried everything to get the system to boot on the integrated 80MB HDD/controller but it is just not responsive. On reset or power on I got a mix of no read, try to initialize and the occasional DISK ERROR(system halt). I even tried a freeze and thaw cycle (actually got the HDD LED to illuminate). Took HDD out and tried to read it on a win XP laptop machine with PCMCIA SCSI adapter through an external SCSI DVD-ROM drive enclosure but did not have any luck(it just did not even attempt to initialize). It could of been a lack of proper SCSI termination. So, I gave up on the HDD.

    Tried some real floppies written with a PC-9821V13/S5C3 system and formatted a couple with 1.2MB setting and made system disks with windows 95 system files. Put system floppy disks in the P-9801ES trying both disk drives independently with each disk (each had a different set of sounds and timing) and for the heck of it I even tried both at once simultaneous(odd behavior both drives were now acting identically, like they were twins. I would have expected the system to try to boot only one disk drive at a time) and still no boot and dumped me to the NEC N-88 BASIC prompt. Maybe the PC-9821V13/S5C3 disk drive was not making the disks properly.

    So, I tested making the boot floppy disks on actual floppy drives from the PC-9801ES transferred to the PC-9821V13/S5C3 and confirmed the boot disk works on the PC-9801ES disk drives hooked up to the PC-9821V13/S5C3 system and all seemed promising. So I moved the floppy disks and drives over to the PC-9801ES and no boot, just attempt to read and it just ends up in NEC N-88 BASIC prompt like before.

    Tried swapping the ribbon cable between the two systems. Same outcome.

    Disassembled the whole system and removed the battery and cleaned the damaged PCB from the leaking battery. Looked for broken wires with my eyes and my trusty Klein Tools MM2000 multimeter. I could not find a broken wire(yet). (I might revisit looking for broken wires in the near future.) I soldered on a lead and socket for a replacement battery(will install a replacement battery if I can ever get it to boot)

    I put it all back together and ended up with a red tinted screen upon showing the NEC N-88 BASIC prompt. On this unit the dip switches are manual. I set the "clear memory on reset" dip switch and the red tinted screen returned to normal color on next boot.

    I am using hi-res mode on a CRT monitor.

    Finally, I tried to use a GOTEK USB floppy drive emulator flashed with FlashFloppy and no luck. Disk images boot on PC-9821V13/S5C3 machine but do not boot on PC-9801ES.

    Next try is to get a cable and terminator for the C-BUS SCSI card and see if I can get it to boot with a CD ROM.(Always like finding new hardware)

    And/Or keep looking for broken traces on the main PCB.

    I may end up getting a SCSI2SD if all else fails. Or a RaSCSI setup from GIMONS.

    Or I could have the Wife approve some more purchases for a Scope and Logic Analyzer and follow your lead.

    Wish me luck

  2. #12


    Whoa! What a journey. I feel relatively spoiled with my Japanese finds - the worst I've had to do so far is replace some shorted tantalum capacitors and hose off battery acid. Do you have any pictures of your Frankensteined CRT monitor? Plastic repair is definitely not my forte, so I always take an opportunity to learn from people further along on the journey.

    I know some people have had trouble with the Gotek/HxC if their 98 has a floppy controller without an internal variable-frequency oscillation circuit, they have to go build their own - but it sounds like if the drives work OK on the V13 (which I'm pretty sure has VFO in the controller) that must not be it.

    I should set my Ap2 to boot into BASIC instead of just making a painful screech when I accidentally leave a non-bootable disk in the drive. Does the ROM BASIC have the N88 Disk BASIC stuff in it? Perhaps you could check to see if you can read files from floppies at all from there ("FILES," etc). I've got lots of experience with it from my PC88s but have not done anything more than check it out on my 98.

  3. #13
    Join Date
    Jan 2007
    Pacific Northwest, USA
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    There were a couple of small manufacturers of external (to the drive) VFO/data separator boards for the PC98 boxes that needed the facility. It enabled the use of commodity floppies.

    There is documentation on the drives, so it should be possible to fab something up. The only impediment is that PC98s aren't exactly common outside of Japan.


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