This game has hardware precursor to the early Williams hardware shared by Robotron, Joust, etc. A few items of note include that there were two different versions of the interface board, and that Defenders interface board's inputs are mapped backwards according to the Robotron type ones. The Defender ROM board has 4 LEDs instead of a 7 segment display. ROM 5 is missing from ALL Defender ROM boards, so don't worry about seeing the missing EPROM in the middle of the Defender ROM board. The power supply for Defender was also different.
Defender's most common problem is having a loose joystick which begins to bang into the plastic overlay, eventually cracking it.
Remove the joystick. You will see that the joystick rotates on two pins which are mounted in holes in brackets on either side. The pins after a few years of abuse have worn the holes larger, and the holes in turn have worn down the pins.
Warning: This procedure requires welding. Only have experienced welders do this part of the procedure for you. I will not be held responsible for any damages to you or your parts if you decide to follow this procedure.
First, take the two brackets and locate the holes that the pins rotate in. Weld these holes shut; we are going to re-drill new holes. Also, there is a hole in one of the brackets where the joystick shaft goes through. This hole often get's too large from being banged by the throw of the joystick. Weld some fresh metal at the long ends of this hole to make the hole a little smaller, near what it was originally. When you are done welding, cool the metal and grind the welded areas so they have a flat surfact again.
Next, examine the pins that rotated in these holes. They should be cylinder shaped, but most likely they are more conical. That's fine. If the pin has "mushroomed" (head becomes larger than body), then you should file/grind the head of the pin until it's diameter matches the rest of the pin.
Measure the pin with a caliper, and then find a drill bit which will be slightly smaller than that. It is best to underestimate, as we will be using a round file to match the diameter exactly.
In the brackets, redrill the two pin holes again. The new hole will be directly between the two screw holes that connect the brackets together. When you have re-drilled the hole, check to see if the pin fits in the hole. If it is loose, you used too large of a drill bit and need to start over. Otherwise, use a round file to make the hole larger until the pin fits perfectly (but make sure it can still rotate freely).
Remount and assemble the joystick base onto the control panel, and insert the joystick. Check the throw of the joystick and make sure it does not hit the control panel overlay. Use the round file again to file the shaft opening until you get the desired throw.
Install the joystick shaft with it's washers onto the rotating block with the pins. There should be a spring washer among the washers for the joystick shaft. If not, you need one. If it is still loose after adding the spring washer, you will want to use fatter washers. You want the joystick shaft to be held as tight as possible to the block.
BTW, if you wanna avoid all this work, you can purchase repro bases at Mantis Amusements