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Troubleshooting Work Book
Adder Diagnostic Troubleshooting #1
One IC pin has a cold solder joint and is effectively isolated from the PC board. Observe what is working and is not working to determine which one and only one of the IC pins listed below could be the problem.
Note: Though this problem could occur a several nodes only one of the above is correct.
U2-10 has a cold solder joint.
Another close choice might appear to be U4-5, but that would still permit signal at U5-5 to vary which it cannot.