@DigitalKeeper Thanks for the advice. I just checked the indicated VIAs. All pins and components connected to each VIA appears to connect properly and within the specs I've seen with my other trace checks. At this time I don't believe the VIAs are bad (well, to rephrase I don't think they are the cause of my present symptoms. They quite likely will need a little love to remove the last of the corrosion
).
@solarmon That is a very good point. Checking the various power rails from the DC input port, I find the following:
5V (VCC)
To Ground: 100.7 Ohms
To -12V: No connection
To +12V: No Connection
-12V
To Ground: No Connection
+12V: No Connection
+12V
To Ground: No Connection
Interesting, is that reading between the 5V line and Ground normal?
Also, I absolutely agree with your statement about the DiagRom serial. This was the starting motivation for me checking the various traces on the board. I've traced everything back from the serial port to the custom chips and memory once already. I had found a couple breaks on U12 (which I had replaced with a socket and new IC), but bodging those pins hadn't resolved my issue. I have also already replaced U39 and U38 in case one of theme were to blame. The lack of a smoking gun has been a challenge with getting DiagRom serial to work.