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Good morning, Joe. I kind of hoped you would have given the Radio Shack part number -- they've got manuals for many of the meters they sell or sold online.
There are two basic things they're trying to tell you here in A and B
-- the signal amplitude at which the meter is made to work, and the amplitude at which damage will occur.
I read what you've copied as saying:
"This meter was made to give accurate readings of AC signals between
150mV and 20V if the signal is 1MHz or less, and between 300mV and 20V if the signal is over 1MHz to the limit of the meter."Input protection circuitry kicks in above 20V. But it only works to
250V, so be careful, you might damage the meter. If you try to measure signals above 20V, we can't guarantee the meter accuracy.""If you try to measure voltages over 250VAC you will damage the meter for sure."
Item C is warning separately that it's a very bad idea to turn the switch while you've got the probes on line voltage, and will damage the meter.
You know what line frequency is anyway -- just don't bother measuring frequency over 20VAC. Line voltage isn't a very good way of checking meter accuracy in measuring frequency, anyway (although everybody seems to want to do it). 50/60 Hz is right at the bottom of the measurement range.
And you shouldn't ever switch a meter switch while measuring anything with any meter, just as a matter of principle. Remove the probes first.
Good luck Chris