Re: [CAD_CAM_EDM_DRO] Need help with O-scope

At 10:06 AM 4/14/2007, you wrote:

>I am having a problem with a mill set up with Gecko drives. There seems
>to be a noise problem that has cropped up on the Z axis and I would like
>to look at it with a scope. I have the use of a scope but have not used
>one for several years and have forgotten a few things. The part I worry
>about is; do I have to use an isolation transformer to keep from popping
>an IC do to different ground potentials? The scope is a B K Precision
>model 1570A with API 1X and 10X probes. Any help appreciated. Ed.

Most 'scopes had isolation transformers built in. BK Precision
SHOULD, but may not. It couldn't hurt.

A simple test: Plug both in and power both up but DO NOT CONNECT
THEM YET. Put a multimeter between the probe ground clip and the
ground on the motor. Measure first AC Volts, then DC volts. If the
numbers are small, switch to AC current, then DC current. If those
are small, there is no problem.

Another test, DO NOT CONNECT THE GROUND LEAD and clip the 10X probe
at 100 V/div to the machine ground. See any voltage? How much? If
less than 10 Volts shows up, put a 1 K ohm 1 Watt resistor between
the tip and ground lead (note the lead is still not connected to the
machine.) Does it get warm? How much did the voltage change? If
the voltage goes down to 10%, the source impedance is about 9 times
the 1K resistor, 1% would be 99 times, 50% would be 1K. Source
impedances of 10K or more should not break anything, voltages of less
than a volt should not break anything. If you have a DC offset on
ground, use a high voltage capacitor between the ground clip and the
machine ground so you can see the AC part at least. 5 uF 370 Volt
motor run capacitors are cheaper than any scope repairs I have ever seen.

David G. LeVine
Nashua, NH 03060