How to check a DMM?

[slidincharlie (Carlo P)]

My Fluke DMM shows a few hundred mVDC and between 1 and 2 VAC when I connect the black probe to ground and the red probe is in the air.
Is this normal? Can this affect readings during working operation?
Is it time for maintenance and general setup?

[Ray Ivers]

Carlo,

My 83 shows over 7VAC, and about 7mV DC, with the same connections.

My feeling is that the house AC-wiring conductors - being tightly bunded together - couple a fair amount of voltage to each other, especially in long cable runs.

Ray

[Tom Phillips]

I get similar readings with my test equipment. For AC the test lead is acting as an antenna. It picks up the AC field that's radiated by all the power distribution wiring. If you hooked an unshielded wire the same length as a test lead to the input of a guitar amp you would pick up lots of buzz. The meter shows that same signal as a voltage reading on the AC scale. You can do the same thing with an oscilloscope and you will see a dirty looking sine wave.

When you hook your meter to a circuit to make a measurement, the impedance of the circuit under test is usually much lower than the input impedance of the meter. Under those conditions the stray field do not affect the reading significantly. In most cases we can just assume that we are getting an adequately accurate reading of the circuit voltage. As the impedance of the circuit under test rises, improved test hookups, different procedures and better test equipment is required. This is usually not a factor for guitar amp servicing. One example where the meter can load down a signal under test is when you try to measure the signal level at an input grid pin. Again, the reading will be close enough to do amp service. But if you where trying to accurately characterize circuit performance, an error can be introduced just by hooking up the test equipment.

Anyway…I also say that your meter seems to be OK.

Regards,
Tom

[slidincharlie (Carlo P)]

Ray, Tom:
thanks for your comments.
I feel fine now!