Greenlee DC clamp meter, need schematic or cal procedure

AFAIK, this is the nature of the beast, residual magnetism and environmental magnetism are the cause, and since a DC clamp-on doesn't see a changing (A/C) field, you need to zero it each use.

My Amprobe is exactly the same (the error is different, not quite as much) but it must be zeroed each time.

Yeah But... It has 0.1A resolution. I can waive it around with no change in residual reading. The 7.5A error term is virtually constant. Should be able to zero that out. Yes, you have to zero it for precise measurements...but I'd still like to be able to whip it out and be within an amp. Need the cal procedure.

Was it actually 0 when new?

I can see the 7.5 being a model identifier at power on.

John

NO idea, didn't buy it new.

Where do you see that? On mine, it's the amps reading before you press the zero button.

On Thu, 23 Jul 2009 01:26:37 -0700, spamme0 put finger to keyboard and composed:

I also suspect that "7.5" could be a model identifier. The instruction manual for models CM-700 and CM-750 suggests that the two models differ only in that the former displays average values whereas the latter displays RMS. Maybe the CM-700 identifies itself as "7.0". Why don't you try powering on the meter with a 7.5A current flowing through the clamp?

Instruction Manual For Greenlee Model CM-700 & CM-750 Digital Clamp-on Meters:

http://65.36.183.19/greenlee/im/im1464rev01.pdf

- Franc Zabkar

Thanks for trying to help, but you gotta quit trying to make something significant out of the coincidence between the power-on reading and the model number. In fact, the number is really 7.8 amps, I just picked a round number that I could remember to indicate the problem.

Yes, the power-on reading is whatever is running thru the clamp plus 7.8 amps.

Please, no more model number display theories. It's COINCIDENCE. It's NOT the model number. It's OFFSET in the reading.

I don't need help interpreting the symptom. I need help FIXING the problem

What I need is a calibration procedure...or schematic. Thanks, mike

On Mon, 27 Jul 2009 08:20:41 -0700, spamme0 put finger to keyboard and composed:

Why not tweak the pots and see what happens? In fact you wouldn't need to disturb the pots -- just solder a resistor, or an additional pot, or a resistor wheel, between the wiper and one end. You may still need to repair the meter if the offset is due to a bad component.

- Franc Zabkar

Been there, done that, no joy.