Thanks both, appreciated. However, I have been re-checking my notes.... Doh! It seems I have made a mistake in the earlier assessment of a working reset circuit, also, what I said re: SVC being reset, was inaccurate. Those tests were actually made with the working GM811. Thank goodness I keep detailed notes.
So... revisiting the reset operation on the GM813, I can see now that the CPU pin 26 is permanently high, irrespective of either reset button being pressed.
Tracing the reset signal from the button shows IC23 pin 1 going low when the button is pressed and this being inverted on pin 3 to a high as expected. This signal can be seen on IC24 pin 2 also, however, there is no activity at all on IC24 pin 13,14,15 when the reset button is pressed. I quickly checked C23 (seemed like a good idea) and it read 10nf on my meter as expected.
IC24 is an 74LS221 and having read the data sheet twice I am not quite getting it. I imagine that IC is a means to give a nice reset transition from an otherwise possibly noisy button press, but you guys will know.
To prove my theory I reset the CPU manually with a jumper and the MFB welcome message appeared, e.g...
Code:
***** Gemini M-F-B 2 System *****
... not much else happened but this is progress.
Would you guys recommend that I replace IC24 (soldered in) or is there something else I should check first. I was thinking that I might just snip pin IC24 pin 13 to isolate IC29 and check again just in case IC29 was the issue, but would really appreciate your thoughts.