Thank you. It looks like it was combination of the sockets and the cap's causing the issue. Now on to that mouse port.

Yes!! That was it. I had a close look at the sockets and they appeared fine, but I still gave the pads a re-flow and added a bit more solder to some of them.
It is now booting properly from cold.

Thank you very much, very much appreciated.

The original TOS was 1.06 and soldered to the board. When I updated to 2.06 a fair few years ago, I first soldered in sockets just in case I wanted to use an earlier OS again. I did try the old 1.06 chips earlier - but, it made no difference. I will have a close look at my soldering on the sockets t...

... if I press the reset button, the ''Out of Range'' message momentarily disappears and then returns, which I take to mean that the reset is working.

What my initial problem was - that from a cold boot, there would be no activity upon the monitor besides the ''Out of Range'' message of the LED screen suggesting there was a signal going to the monitor otherwise I would have had a ''Check Signal Cable'' message. If I then pressed the reset button i...

... I should add that the actual jack on the modulator was missing when I first bought this machine although it has never seemed to be an issue for all these years.

I have tried re-seating all possible chips and cleaning RAM and the reset switch reads fine... I even dug out my old SM144 to be sure that it was not the VGA adapter. I am at a bit of a loss, although I did find a thread on Atari-Forum where someone had the same issue and it turned out to be the 74L...

... the signal to the monitor is resetting. Time to get the screwdriver out again.

Thank you both for your replies and advice.

When I switch on; the power light, Gotek and optical mouse are all active - but, there is no activity upon the monitor although the signal is there. This is the same issue I had before replacing the caps although pressing reset got it to boot.

I will have a look at that. But, after just replacing the cap's it was all working fine including the reset. Maybe, it needs a little rest period. When I first changed the TOS chips, some time ago, it was misbehaving for awhile after... I had never seen so many bombs,although it did settle down even...