Yesterday evening I received the missing IC socket, soldered it to the board, plugged that 74HC245 IC, connected the keyboard, and quickly moved to test the board. For testing purposes I used this Diag ROM. During the initial power on no magic smoke escaped, and it all looked pretty good. My oscilloscope was showing the right CPU clock frequency, it appeared that CPU is fetching the instructions, and it was some kind of signal on the video output. So I decided to connect the board to the TV... Well, the board appeared to be dead: I didn't get any picture, and it didn't generate any sounds either. It turned to be a few simple mistakes - I plugged it into an Audio input instead of the Composite, and I forgot to install jumpers one connecting the speaker, another one connecting A14/PGM signal to the EPROM :-). That was soon corrected, and I proceeded with the testing.

This time Diag ROM started properly, and I've got the picture on the TV, and some beeps sounded through the speaker. While performing the upper RAM test Diag ROM reported errors at address $FFDC, I repeated the test, this time the error was elsewhere but still within the last 256 bytes of memory space (addresses $FFxx). I tried swapping out and replacing the SRAM chip, which didn't help. So I am still puzzled, perhaps there is something wrong with the address decode / chip select logic?! (I have to go back and check the schematic) Any ideas?

I did a few more tests and finally I replaced the Diag ROM with the ZX Spectrum ROM. It booted up happily. And I even ran a small BASIC program (oh boy, now I need to recall how to program this thing):

To be continued...