SAFETY
Very high voltages are generated inside the casing, around the cathode ray tube.
Before opening the casing of the computer switch off and leave for a few minutes
to allow static charges to leach away. Then remove the electric plug from the
mains socket. Even then, high voltage static charges may remain for a time which
can administer a shock. Whilst such shocks are unlikely to cause injury they
can be unpleasant and are best avoided, so I recommend that the machine is left
for at least an hour, preferably overnight, after disconnection before removing
the casing back.
METHOD
Remove the printer, keyboard and any accessories connected to the expansion
port. leaving the unit bare. Place a folded towel or similar soft cloth on a
working surface and lay the computer face down on it. Remove 6 screws from the
back and sides and place in a safe place. Remove the two small knobs from the
side of the case; they simply pull off. Carefully lift the back off. You may
need to insert a knife blade in the joint to start it, but it should lift off
easily. If not, find out why before proceeding.
Turn the machine right way up and examine it. Just below the tube is the power board, and beneath the power board is the CPU board and drive(s). These machines come in various layouts, so only general instructions can be given. It is recommended that notes and sketches be made whilst dismantling to ensure that all connections are remade correctly. In all cases that I have encountered plugs and sockets have been colour-coded, or marked or will only go on one way.
Unplug all connections between the power board and CPU board and between the power board and the tube. These are:
1) Small printed circuit board plugged into neck of tube.
2) Single black wire from front of tube plugged into this small board.
3) High voltage cable from line transformer attached to tube. Fold back the rubber protector so that the two springy connecting wires can be seen. These wires pass through a hole in the tube and have their ends bent over to prevent them pulling out. Push the wires to one side together and they will slip out. No tool should be needed.
4) The white 6-way connector (carrying only 4 wires) from the deflection plate connections of the tube to the power board. This connector has a plastic clip one side to secure the connector. Insert a blade between clip and plug to release it before unplugging. It should come off easily when this is done.
5) The white 5-way connector at the RHS of the CPU board from the power board. It has a similar clip
6) The blue 4-way connector at the LHS of the CPU board from the power board. It also has a clip.
7) The single black wire from the CPU board to the LHS of the power board. It is secured by a screw.
The power board should now be free to be pulled out of its slides. There is a clip on the RH slide which has to be released first, then the board can be removed. It may be stiff but ease it out; don't use too much force, and check all the while that no more connections remain. To free the mains cable either remove the plug or take out the screw which holds down the CPU board.
Replacing the board is the reverse of the above. Check off the connections as they are made; it is easy to overlook one. The machine can be checked before replacing the back but don't make any revisions without switching off and leaving the machine for a time as described earlier.
Ron King
E-Mail address........ron@king27.freeserve.co.uk