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Toshiba T1900C

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    I'm fighting a failed S-100 system at the moment just because a single electrolytic capacitor in the power supply had one of it's leads break free from the circuit board. The net result was that the diode bridge output of pulsing DC took out the voltage regulator and the 5vdc output went to 24vdc and smoked the floppy drives and other circuits.

    You need to put new main regulator cap's in NOW and hope you haven't done more damage to the Toshiba. Removing a single bypass cap (like your tantalum) is not the same as pulling the main cap's in the power supply. For quite a few years a large number of TV's and other electronic devices were FUBAR just because the cap's in the power supply failed to regulate the output power. Replacing the cap's often returned the equipment back into operation. They can not be left out.
    Crazy old guy with a basement full of Pentium 1 laptops and parts


      That's hard to believe, unless the cap played some other part than a simple filter cap. I have no doubt that the 120Hz unfiltered PSU output made the system act up, but I can't see how it would blow out a voltage regulator or floppy drive.
      Reach me: vcfblackhole _at_ protonmail dot com.


        Any update for this?

        When all the caps are desoldered, clean the board off with isopropyl alcohol to remove any electrolytic fluid that may have leaked from failing capacitors.


          Originally posted by Chuck(G) View Post
          That's hard to believe, unless the cap played some other part than a simple filter cap. I have no doubt that the 120Hz unfiltered PSU output made the system act up, but I can't see how it would blow out a voltage regulator or floppy drive.

          But I don't wish to steal this thread, just commenting that things go FUBAR when main filter caps aren't installed. However, pwr. supply designer cheaped out, IMHO, and used one winding output transformer to supply +24 and +5 v circuits. Also no solid mechanical mount for the main filter cap. Main filter cap lifted free from the board. The +5vdc circuit using a 723 IC with 2N3055 driver trans went to full unregulated ~30volt and believe me the magic smoke came out of the floppy drive.
          Crazy old guy with a basement full of Pentium 1 laptops and parts


            Originally posted by DankEngihn View Post
            Any update for this?

            When all the caps are desoldered, clean the board off with isopropyl alcohol to remove any electrolytic fluid that may have leaked from failing capacitors.

            Thanks for asking, DankEngihn!

            I got back home today and wanted to check this again.
            I found five fuses, that were all ok. Which is kind of bummer, it would have been
            easier fix (but of course, if fuses were busted, something else probably would have been too...).

            I pulled suitable caps from old atx psu. Replaced the 1000uF cap and the two blue 220uF caps
            from used spares, since I got no new ones to try with. I then tried to boot, but the DC IN led blinking
            was the same as before.

            I measured RTC battery, it was now 2,91V when no charging and 3,25V while charging.
            The power button has 4,77V on one leg (when pressed down zero volts). I believe power button
            is working as it should.

            I got up 3am today and drove some 200 miles to my working town. It's now about 8pm
            here and I think I need to continue this tomorrow. Fighting against fatigue while trying to fix
            things might just make me mess things up more.

            granted, that I may well have done some damage to the unit myself by trying it without these caps.
            But I've been thinking about others who have fixed t1900c laptop with faulty capacitor(s).
            They reported, that the machine actually turned on for second or two, and then immediately shut down.
            This unit is just dead - apart from the DC IN led blinking.

            This has made me think, that there may also be something else wrong with the unit and not the caps after all.
            CPU ? rams ? I guess a number of things could go bad and make the unit just have black screen.


              I'm suspecting that it's the power supply.

              Try to get your hands on an OEM Toshiba 19v power supply.

              EDIT: the picture you posted showing both of the boards looks odd. On the power board, between the"Toshiba" chip and the "flash" chip looks suspicious.
              Last edited by DankEngihn; January 13, 2020, 10:21 AM.


                Good early morning!


                I think that suspicious area between toshiba and flash chips was due to light reflection.
                There is a black stamp there, I posted a better picture of that area.

                About the power supply - if it was the cause, why would the 12V to the battery leads
                give the exact same symptoms ? I did try the 12V again now that all caps are in place, but
                there is no change.


                  Don't know if the blink codes in this old thread match what you are seeing, but take a look:

                  Plus there's this one:

                  Plus there's this one:

                  I think a Google search might turn up more.
                  Crazy old guy with a basement full of Pentium 1 laptops and parts


                    Thank you DeltaDon for the links.
                    I don't exactly see the same symptoms. I see long - dot - dot - dot and then pause.
                    That is curious, it would make 8 hex.

                    even if it is long - dot - dot - dot - long - dot - dot - dot (to make 8 digits), 10001000 -> it would become 88 hex.
                    Am I thinking it the wrong way ? is it 00010001 binary -> 11hex ? or 01hex if it is only 4 digit code.

                    But I think there is long wait after every 4 digits, therefore it seems to me it is only 4 digit long code.

                    In the T1900C maintenance manual there is one 08h code:
                    08h HDC overrun (DRQ)

                    But I don't have HD installed. The code applies to HDD test program in the diagnostics disc, so it is not the right code here.

                    I'm thinking to buy better multimeter. Would this be useful/reliable to check ESR from electrolytic capacitors ?


                    Not a bad price for multimeter, if it allows me to check capacitor failures.
                    Drawback is to wait easily 6 to 8 weeks before it would arrive from order.



                      Heh, when all else fails, replace the lower system board. if that fails, replace the upper one .
                      If only it was that easy to come up with working replacements. The maintenance manual was obviously
                      meant for service technicians to make warranty repairs. They did not have to worry about replacing
                      capacitors or faulty chips on boards.


                        Thought I would comment on this thread.
                        I too am fighting to revive a T1900C.

                        Issues that I have found so far:
                        1) several large electrolytics on the FATSL1 (PSU) board were really bad/ leaky. Replaced.
                        2) one trace was damaged from the leaked acid
                        3) several (most) of the small surface mount electrolytics were also bad/leaky. 10uF/6.3V and 33uF/6.3V. Replaced.

                        I now have a PUS board that puts out +5V, +3.3V on all the right places, and the system seems to try to start. The rails come up, the drive spins up, the CAPS and NUMLOCK LEDs flash, ... then nothing.
                        The LCD never comes on, and I think that the machine is not surviving POST. So, I never get anything on the screen, no beeps nothing.

                        I've been doing all sorts of little debugs here and there but without a schematic, it's hard. Also, I know nothing of the history of this computer... maybe it died for deeper reasons than the PSU.

                        It's always a challenge taking a machine that is known dead for unknown reasons.
                        I think I probably have a good drive out of it however.

                        If anyone has any comments on what to check if all the power rails are up, but the machine never "boots", please chime in! I know there could be a million reasons though.

                        to the OP - what happened with your boards?


                          Originally posted by Twospruces View Post
                          Thought I would comment on this thread.
                          I too am fighting to revive a T1900C.

                          to the OP - what happened with your boards?

                          Hi there, and happy new year!

                          I had to give this one up. I ended up buying an ESR meter, but could not find the culprit from the psu.
                          I also found 19V power source, but it too failed to bring life to this unit. I still have the laptop - board and
                          everything in pieces in a box. It's been almost a year since I fiddled with this, I probably could not remember
                          how to put the puzzle back together.

                          Later, I ended up buying defective Toshiba T4500C. It too had failed psu, but with that unit I got lucky
                          and was able to repair it. Reason was defective electrolytic cap. Made a video of repairing that unit
                          to youtube her:



                            I might be interested in the parts; I got to a dead end and eventually damaged the board a bit. Set it aside. Without a schematic it is a real struggle to repair!