I've been working on this board for a while and it is now booting and with Marco's test ROM in IC 17 socket I am getting all the correct signals on the 2 PIAs.
I have then installed the Marco Test Roms for IC14, 20 and 26. The Marco test for the sockets fails after testing IC14 - which means that there is a problem with IC20 or its selection.
I have another System 7 board where I have compared all signals on IC20 and also IC15 (74LS139 decoder), this is what I get.
The select for IC20 is at pin 20 this connects to pin 11 of IC15. On the good board during the Marco test when IC20 is being tested my logic probe shows 1 quick pulse which shows that IC20 is being selected and the seven segment LED shows 20 which confirms IC20 is working.
On my problem board there is no quick pulse at pin11 of IC15 and of course the IC20 test fails because it is not selected.
On the problem board I have pulsing at pins 13, 14 and 15. Pins 14 and 15 seem to pulse at the same pace on both the good board and on the problem board. On the problem board pin 13 seems to pulse much quicker than on the good board.
Strange thing to me is that pin 13 is just address line A12 which comes straight from the 6808 CPU with just the buffer in between. The pulsing pace is the same on both sides of the buffer.
So I think I need to figure out why A12 is pulsing much faster on the problem board.
Does anybody have any ideas as to what could cause this?
Or any other idea why IC20 is not being selected?