Had an interesting tech support problem last week. The customer was measuring wrong voltages all over his HobbyCNC board. The 5 Volt test point measured 4.85 Volts. The test point should be between 4.95 and 5.05 Volts (5 Volts +/- 1%). The output of the LM317HV should be 31.6 Volts, but was measuring out at 30.6 Volts.
All the resistor values were proper, the power supply was 36 Volts. None of the regulators was getting hot.
The customer and I arrived at the solution independently.

The 5 Volt line was reading 3.2% low. Interestingly, the 31.6 volt line was also about 3.2% low also.

The culprit was a bad voltmeter! Customer purchased a new voltmeter and all voltages were dead-on accurate.

Sometimes the problem isn’t where you think it is. We take our test and measurement equipment for granted, and once-in-a-while it lets us down.

On an only somewhat related topic, I have, and I occasionally use, one of the “Free with any purchase” Harbor Freight voltmeters. It’s fun to watch Dave Jones of the EEV Blog do a test and teardown. Accuracy is actually pretty impressive. Safety, however, is another issue.