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Troubleshooting Work Book

Adder Troubleshooting #3

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 is the problem.

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U4- Pin 11 is floating (not contacting PC board) due to a cold  solder joint.

Rational: When no switches are depressed the MSB is high. Indicating either U4-11 or U4 -12 is high.  When the next to the MSB stage generates a Carry, the MSB is forced low indicating that U4 -12 passed the signal.  We also note that the carry did not cause a Carry Bit OUT to be generated by  U5-C indicating that U5-8 and thus U4-10 are low. U4-11 is always high. Thus it must be floating.