Binary Dinosaurs - STC Executel 3910, Dismantling

These things weren't designed to be easily serviceable. The order for taking one apart is this:

2 screws either side underneath to remove the keyboard. Disconnect the autodial cable (rainbow ribbon) and keyboard data (grey ribbon).
2 screws either side of the cowl for the display, it slides off backwards. **MORE VOLTAGE WARNINGS! MAKE SURE YOU HAVE SUITABLE TOOLS AND ARE AWARE OF THE DANGERS**. This is a good time to discharge the anode. There's a bleeder resistor on the board that means the tube is discharged as soon as power is removed, but you can never rely on this to be working. I use a BIG screwdriver with a wire wrapped and taped round the shaft with a croc clip on the end. This clip goes to the aquadag (metal strap) running across the back of the CRT. With it fastened (and with one arm behind your back, this is important!) slide the blade of the screwdriver under the rubber cover until it touches the anode clip. That's it. Unclip the the croc from the aquadag and separate the anode by pulling it away from the screen or pushing it away with the screwdriver.
Unplug from the mains. Remove the screw roughly centre rear to take off the top panel which also pulls away and up from the unit.
2 screws either side of the PSU at the rear. This has to be removed to get the board out, which you will need to do. Also remove grounding strap from the top of the PSU. Rotate the PSU towards the rear of the unit and lift up a few cm. Remove the multicolour output cable, ground spade connection and incoming mains.
Removing the display is difficult and takes some force. Remove the top cover of the analogue board under the CRT (2 screws). Under the base of the CRT you will see a thick plastic clip that holds the CRT in position and allows it to be rotated. Push this towards the rear of the case while attempting to lift the whole display. Eventually you'll apply enough pressure for the clip to clear the rotation ring and the screen will suddenly pop up. At the rear of the CRT is the yoke board, this is held to the frame with 2 cable ties which need to be removed. Gently push the yoke board away from the CRT and make sure to remove the thick black grounding cable that runs from the aquadag to the yoke board. Lift up the CRT and push the now discharged anode cap through the only available space down the left hand side, again this will need a bit of force and you have to deform the rubber cap. Undo the multiway cable going from CRT yoke to the analogue board. Put CRT to one side on a safe place.
Using a knife blade remove the top cover of the brightness control, then use a matching screwdriver (flat blade with a hole in the middle of the tip) to remove the knob. This is necessary to get the analogue board out.
Undo the screw at the rear left of the analogue board as well as the nut on the right. The board is held in place by 2 plastic uprights and needs to be prised up from those to pull forward. Also unthread the anode cable from the clip in the top of the display enclosure. Pull board towards you and unclip the display ribbon cable at the back. Store the board somewhere safe.
Dialler board: make a note of the colours of the phone wires (red, yellow, green, blue) and unscrew them. Unclip the rainbow cable and lift the whole board up over its clips. Keep safe.
(Optional RGB board on the right) Remove single screw and lift away. Disconnect the 2 connectors.

This is enough to allow full access to the component side for troubleshooting once the PSU is reconnected. You don't need a display attached because you'll know when it works as the tape drive will start up on a successful boot. If there's a tape in it, obviously. To fully remove the board:

Unplug cassette unit (12 way connector at centre rear), secretarial unit link (5 way next to battery), and printer port if it's connected.
Remove the 6 screws holding the board into the case, remember the one at the left rear is also an earthing point and your screwdriver WILL rub against the dialling board ribbon cable.
Unplug speaker from the front centre.
Lift board up at the front, enough to clear the lip of the case and the mountings for the keyboard and pull it towards you. It flexes, don't worry about it.
Put case to one side. Because of cable lengths and the requirement for the keyboard to always be attached, after any repair you need to refit the board to power it up. Note that you only need to attach the grey keyboard cable. The dialling board and its associated cable can be left out of the machine.