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?

Good idea to test the boards without ROMS, they didn't act the same, so I suspected the clock parts (crystal and 2 x 27pf caps) so I changed them out. This made no change to my problem, so I decided to change the same parts on the board where IC20 was being selected.

Unfortunately I now have both boards doing the same thing, failing on the IC20 test.

I couldn't find any shorts on A12.

The behaviour while in test is quite different now. When IC20 should being tested on IC15 pins 13 and 14 are held low and pin 15 pulses - which makes pin12 low. to make pin 11 low and select IC20 pin 14 on IC15 would need to be high. Looks like more digging required.

For the clock I have seen in the repair guides that it should measure about 3v but I don't think I am getting that from gnd to pin38 and 39, maybe I should be measuring from pin 38 to 39, I'll recheck that tomorrow.

Ah just remembered I also changed the CPUs and I better check if I have 6802 or 6808 and how the boards are set up for the use of the 6810 memory. Another thing to do tomorrow, it's getting late here.

Thanks that really helps to understand what I am seeing on my scope. I am getting a square wave on pin37. each square is approx 500ns long, which make the time from the signal going high to the next time it goes high is 1000ns. by my calculation this is close to the expected 0.895mhz.

I am back to 1 board where chip 20 is working and the other where chip 20 is not. I didn't change anything so I probably did something dumb like check the same board twice.

Thanks for the great test Roms, they have helped me get a number of boards working again.

For my current problem, I have checked the jumpers they are the same on both boards and I also checked against the documentation that they are set correctly.

Address lines and Data lines are all showing continuity between chips, I also checked that the buffer chips are working correctly.

To check the symptoms, I removed IC20 on both boards. On the board that works the test loops as expected and I see pulses at A11 on the CPU. On the board that does not work when it gets to the point of testing IC20 A11 is held low. It is very puzzling.