Hello,
i have here an A3640, rev. 3.1; it is not mine, anyway, after a short test that has given green screen, i have tried to clean some pads and resistors from leakage of acid, without any result. After surfing the valuable site of the user from Sweden 'Chukky', i have watched for damaged vias, without any success!
Anyway, the last night, maybe i was dreaming, maybe not, i have chose to desolder all the GALs and put a socket on everyone and, in the meantime, look for something strange under the surface. So, after desoldering of U400 and U401, and looked at the schematics, referring to the material on the Haynie archive, i have discovered this: some pads on this PLDs are not connected, despite the logic equations of the contents of this GALs considers some of this unconnected pads into the equations (so, in other words, they are considered sometimes positive and sometimes negative!). But a not connected pad (nor to GND, neither to VCC) is floating or? And this is not the best situations or? Today, with a fresh mind, i will give a new watch to all of this, but i am sure that what i have wrote here is true!
Cheers
Stefano
i have here an A3640, rev. 3.1; it is not mine, anyway, after a short test that has given green screen, i have tried to clean some pads and resistors from leakage of acid, without any result. After surfing the valuable site of the user from Sweden 'Chukky', i have watched for damaged vias, without any success!
Anyway, the last night, maybe i was dreaming, maybe not, i have chose to desolder all the GALs and put a socket on everyone and, in the meantime, look for something strange under the surface. So, after desoldering of U400 and U401, and looked at the schematics, referring to the material on the Haynie archive, i have discovered this: some pads on this PLDs are not connected, despite the logic equations of the contents of this GALs considers some of this unconnected pads into the equations (so, in other words, they are considered sometimes positive and sometimes negative!). But a not connected pad (nor to GND, neither to VCC) is floating or? And this is not the best situations or? Today, with a fresh mind, i will give a new watch to all of this, but i am sure that what i have wrote here is true!
Cheers
Stefano