This display means: Nick chip running in uninitialised video memory, soo the ROM not started.
Reset not stuck, because if Dave chip dead then Reset stuck low -> Nick not running -> just black screen)
Most possible failures:
- dead Z80 (especcialy if it is Zilog type, I found many dead, all are Zilog, Made in Philippines, with a near data codes)
- partially dead Dave, which is not genarete ROM signal, can be substituted with few 74xxx ICS.
- onboard RAM, less chance because the ROM can make black screen at start even when onboard RAM faulty, just hang up later

Do you have a EPROM programmer? I wrote a short test ROM, which is try to test memory bytes, and make a beepig sounds of readed bits. Replace the original ROM to this, and if you got a beeping machine then know CPU and ROM access are working.

Ok, there are the test ROMs. These are don't use any RAM for the program running, no stack used, all variable in Z80 registers. (The original EXOS ROM use stack in unverified memory, if the onboard memory bad, system crashed before can report RAM error.)

The OLD version just beeping, the another are try to initialise screen, the character screen memory addressed to the tested memory.
Bytes tested with 00h,FFh,55h,AAh values, after byte write read back, and make beep sound from bits (low tone 0, high tone 1).
Connect speakers to the tape output (or use from monitor connector via SCART cable), for the case if internal speaker don't working)

If machine start beeping then good life sign

No, it is just about the random memory garbage at power on.