How to fix the Toshiba 486 laptop power-on problem

Barry Redmond, May 2001.
barry.redmond@dit.ie

Toshiba 486 (T-series) laptops such as the T1900, T2400 and T4500 have become infamous for a serious problem that prevents the machine from being powered on. Typical symptoms are that the machine turns itself off after a second or two, leaving a flashing LED. The problem is described on the Ultimate Unofficial 486 Toshiba FAQ .

I have repaired a T1900 that had these symptoms, so below are the details so that others may try the same fix. I don't give any guarantees with this. I just did it to a single T1900, so it might not work for your machine. However, my machine has been running continuously (24/7) without any problems since I made this repair (April 2006: it's still running, 5 years later).

This repair requires the complete disassembly of the machine, the replacement of a capacitor (by soldering it in), and the reassembly of the machine. If you're not comfortable with this then don't try it. Don't blame me if it goes horribly wrong!

The capacitor to be replaced is 1000uF, so you will need a new 1000uF capacitor. I recommend that it should be at least 25V, because it seems likely that the reason for the original failure is that the maximum voltage of the installed capacitor is too low. This will almost certainly mean that the new capacitor is larger than the old one and will make it difficult to fit in. See the photographs below to see how I managed it for my T1900.

If this works (or even if it doesn't work) for your machine please let me know so that I can add the information here. Corrections or additions to these instructions are also very welcome.

Procedure for a T1900:

  1. Disassemble the machine so that you can remove the board that contains the power supply circuit. On a T1900 this is the lower of the two system boards. Disassembling the machine is a long process, involving a lot of small screws. You have to completely remove the top half of the casing, the keyboard, the metal shield, the screen, and the disk for you can remove the two system boards. You can get detailed parts lists and assemby diagrams for all the Toshiba T-series machines in the Toshiba technical support bulletins. You can get instructions for removing the top half of the casing, the keyboard and the metal shield from the instructions for installing Apricorn hard drives.
  2. Identify the capacitor to be replaced, and note which way round it is in the board. On my T1900 the capacitor is 1000uF and is a squat brown cylinder standing on its end. Photo 1 below shows the location of the capacitor on the lower system board of a T1900.
  3. Remove the capacitor. It is very difficult to remove it intact, as the board is a multilayer one, so it is probably best to minimise damage to the board and surrounding components by not trying to unsolder the capacitor's leads. Instead, just cut or break the leads under the capacitor so that it comes away leaving the leads still in the board.
  4. Now insert the new 1000uF capacitor. You will probably have to find a way to make the new one fit in the space. Remember that the two system boards must be able to fit together properly. Check your positioning very carefully before deciding how you will do it. Photo 2 below shows how I did it in my T1900. I used some plastic sleeving from some old cable to insulate the leads of the capacitor to make sure they could not touch each other or anything else, and I just soldered the leads to the top of the board where the stumps of the old capacitor leads were. Make sure you connect the capacitor the correct way round. On a T1900 the + lead must be connected to the point furthest from the power input connector.
  5. Check your work carefully to make sure there are no solder shorts or other problems.
  6. You should be able to test your work before you reassemble the machine. Carefully connect the two system boards together, attach the power connector, and press the power-on switch. If everything is ok the LEDs should indicate that the machine is attempting to start up as normal.



Photo 1: The location of the capacitor on a T1900.


Photo 2: The replacement capacitor in a T1900.

May 2001