U1253A的精度取决于背光位置吗

在与U1253A有不同的问题（并且没有得到回复）之后，我注意到了仪表的一些问题。
第一个是当我在测量带有空选项的电容器的情况下，即使范围本身确实根据上限值改变，范围图标也不会改变。
这是次要的，并没有真正影响测量，所以我可以让这一个去。
然而，仪表有一个更严重的问题：我用它来测量PS的输出纹波顺序你知道这个问题的原因是什么以及如何修复它？
当所有改变后的测量之间的这种误差是背光设置似乎是一个主要问题。



     以下为原文

  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.