On Mon, 28 Jul 2008 12:33:17 +0100, "N Cook" wrote:


or perhaps Fluke 77 DVM problem.

Have removed all the rear securing bits, so partly veiw interior, and before
removing all the front panel knobs etc, probably on Wednesday. One of the
first things I do , as a matter of course, is measure the mains primary DC
resistance and note it down.

On autoranging of the DVM, cannot do so, as unstable display as though there
was a voltage on it.
No DC or AC V on the primary which would , if absentmindedly, place it in
ohms setting on an active circuit. Can measure 237R if I set the Fluke on
fixed range. And also measures 237R on another meter. Thinking it is a "bug"
with the autoranging if it hits about 237R, I added 1K in parall with the
Fluke 77 in autoranging mode and it still is unreadable.

What would be inside this Peavey with the what looks like conventional
linear mains transformer that would give this DVM problem?


==============
Measuring large inductances with an autoranging ohmmeter can be disconcerting.
The reason you're experiencing that phenonenon is because of the
range-determining circuitry in the meter.
Each ohms range injects a specific current into the test leads, measures the
voltage developed by that current, and then displays the result as a resistance.
When the meter switches range, the current undergoes a step change. The
inductance of the xfmr winding causes an abrupt voltage change, and the meter
sees that as an overrange or underrange condition, and switches range. This
causes another step change in the current. The meter senses overrange or
underrange condition. The range switches again... ad infinitum.

Best way to measure the resistance of large inductances is with an analog
ohmmeter or a manual ranging DMM.

Dave M

Never take a laxative and a sleeping pill at the same time!!