Try to start normaly with EXOS, and if you press keys then hear keyboard clicks in headphones? If yes then the machine started just problem with video output and internal speaker. If no then probably problem with the video RAM accessing. And if no right LPT table for the Nick then it is display just garbage which are "no signal" on modern TV.

Yes the Beep test also setting the border color, white when silence, and coloring when beeping.

Yes accessing problem with the onboard memory are the main suspect.
RAM chips, CPU, or the bridge chips (U5,6,7) between the Z80 and Nick bus also are possible.

The Beep test write 00h, FFh, 55h, AAh to the FFh segment, starting the 0. address then continue with the next bytes. You can hear the readed value: low sound are 0 bit, high sound 1 bit.

Ok, so I hear --

00000000
11111111
10101010
01010101


So it looks like Z80 is reading / writing RAM ok?

So points to Nick not reading from RAM?

Also the 64 has a broken coil, any idea where I might get a replacement?.

It is a transformer. I not have other idea than remove from other dead motherboard
Alternative way: buy a 5V to 12V DC-DC converter (Step Up) module and replace the entire 12V generator circuit.

What's the difference between the 2.4 ROM beep test and the TESZT.ROM? Wondering why I get a good picture with the one ROM and not the other...

What's the difference between the 2.4 ROM beep test and the TESZT.ROM? Wondering why I get a good picture with the one ROM and not the other...

The 2.4 is the same as the TESZTOLD, it doesn't try to initialize the display, the Nick running free as it is powered on. I selected this is because with the forced initialization method (TESZT.ROM) on a many defective machine nothing can be seen, also lost the border color notification.

Now thinking about the next version: it will run for a few bytes without initialisation and then try to make a display.

The 2.4 are same with the TESZTOLD, it is don't try initialize the display, the Nick running free as it is powered on. I selected this is because with the forced initialization method (TESZT.ROM) on a many defective machine don't see anything, also lost the border color notification.

Now thinking about next version: it is will run for a few bytes without initialisation and then try to make a display.

I tried the EXOS ROM from my other Enterprise, and now it is working. At least, I get the Enterprise logo flashing on my screen. Many thanks for your help tracking this down Zozo, you have been invaluable help as always!  

So reversing the caps must have damaged the EPROM in some way. I'm just blanking it and will see if its still usable, if not then it's the easiest part in the Enterprise to replace, so I've been very lucky... 

Hopefully with the new set of caps (installed the right way round) EXDOS will work with this EP and I can then start my SD card prototype. I am going to do a wired prototype on a eurocard first this time...

Hmm, I can boot the EXOS ROM from my old EP64, but cant use the 2.4 ROM. I tried blanking and reprogramming the EPROM, same problem with not booting. I tried another EPROM, same problem. But still the old EXOS boots fine.

Is this a memory paging problem? Really strange.