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Accuracy problem with U1253A depends on backlight position +

Question asked by toli on Oct 27, 2011
Latest reply on Dec 19, 2011 by toli
After having a different issue with the U1253A (and getting no reply), I've noticed a few more problems with the meter. The first is the fact when I'm measuring capacitors with the null option on, the range icon doesn't change even though the range itself does change according to the cap value. This is minor and not really affecting the measurement so I can let this one go. However, there's a much more serious issue with the meter:

I'm using it to measure output ripple of PS's in the order of <1mV. I'm using a fully charged battery, and the meter is acting up. For example, with the last PS I've measured, the meter reads about 0.6mVAC when the back light is at its max or medium position, and about 0.4-0.5mV (I can't remember exactly what it was, as the measurment was taken a couple of weeks ago) when the back light is at the low setting. This is a very high error, about 30% between the 2 measurements! I don't have a good DMM at hand to compare to so I can't even tell which one is correct. I've used a keithley 2010 at one of the labs at work and it reads about 0.4mV, but this is using different probes, and at different time and place so the two measurements cannot be directly compared. Especially since my U1253A wasn't calibrated in over 2 years.

Do you know what is the cause of this problem and how it can be fixed? Such an error between measurements when all that is changed is the back light setting seems like a major issue.