I went on a scavenging hunt on the web. I got some pictures collected in mij dropbox.
Take note that none of those pictures are mine, they were all found.

I hope you can use them:
https://www.dropbox.com/sh/h2wgy8qkk899pb6/AADTlRkLVcLQrAf9pNE9tXkGa?dl=0

If you like the images download them, I periodically remove folders to make room for new stuff.

Hello,
this doesn't look like a ROM problem. I suspect an address line stuck. But after reading the website you suggested you could start by cleaning the socketed chips.
Here is a nice guide on how to do this: https://www.youtube.com/watch?v=RkSCh53fO6Y
If this doesn't do the trick and you have some patience I'm willing to take a look at the PCB, I am from Belgium. Send me a PM if you are interested.

Have you any means of checking the signals on the boards? I think the CPU doesn't start. That's why the test LED's don't blink.
First I would check the clock signals (pg12) and the reset circuit (pg9) the manual can be found on the KLOV http://www.klov.net/manuals-videogames/D/Defender.pdf

Ok understood   

I interpreted your comment wrong 

of topic:
Engels is dan ook niet mijn moedertaal, de jouwe ook niet denk ik. (English is not my native language, and it isn't yours, I think.)

Nice job. Looks clean and simple.
Why buy something if you can make it yourself. Thanks for sharing.

Looking at other manuals from the same manufacturer I see them using odd voltages being : +20v, +10v +5v and -10v? I have a Invasion and condor pdf manual, but they both use different connections. But if you want I can share them.

pinout as tested on working board

G connector
                 -----------
          +5V |  A | 1  | +5v
          +5V |  B | 2  | +5v
          +5V |  C | 3  | +5v
          +5V |  D | 4  | +5v
                 |  E | 5  |
2P pot input |  F | 6  | 1P pot input
      2P serv |  H | 7  | 1P serv
           KEY |  J | 8  | KEY
         Coin2 |  K | 9  | Coin 1
     2p Start |  L | 10 | 1P Start
                 |  M | 11 |
                 |  N | 12 |
                 |  P | 13 | 
                 |  R | 14 | Composite Video
                 |  S | 15 |
          GND |  T | 16 | GND
          GND |  U | 17 | GND
          GND |  V | 18 | GND
                 -----------
Use 5k potentiometer between GND and +5v, connect wiper to pin 6 or F
2P controles only used in coctail mode. (put big switch 3 on)

N connector (BOTH SIDES EQUAL)
         ------
         |  1 | GND
         |  2 | GND
         |  3 | GND
         |  4 | GND
         |  5 | AUDIO+
         |  6 |
         |  7 | GND
         |  8 | GND
         |  9 | GND
         | 10 | GND
         | 11 | KEY
         | 12 |
         | 13 | +12V
         | 14 | +12V
         | 15 | +5V
         | 16 | +5V
         | 17 | +5V
         | 18 | +5V
         ------
Use GND as AUDIO-

I wired up the 5v and gnd to both the top and bottom board. Then I checked for out coming signals on the bottom board. On the screen of my scope I found only one pin with a signal. I was expecting more? Then I took a closer look at the signal and it appeared to me it could be composite video.
Did they use composite video on arcade boards? A quick search on the net taught me they did. Mostly on B&W cocktail tables so this should be it.
Now my test rig isn't capable of playing composite video so I hooked up a small video device in MacGyver style. And there was an image  .
The ball stops moving as soon as the image is steady, but with the crappy connection the picture goes on and of having the ball move jerky.
I also discovered the big switches on top don't respond. You should be able to put the game in play or test mode, but it seems it's only in test mode (large padel). Also the flip screen does nothing? So there is definitely some work to be done on this board.
But this solves the mystery, it is a zunzun block board. Now I have to find a way to display the composite video on a crt so I can try to get this game up and running like it did in 1979.