DC voltage off on scope measurement ?

Thread Starter

DarthVolta

Joined Jan 27, 2015
402
I'm looking at -12.010VDC on a good DMM, and I just noticed on my Tektronix 2430A on 2V/div, it looks more like 11.75V, the trace is about 0.2-0.25V wide, that's normal, and it's not parallax. The trace is not as low as it should be to line up with the lines. However, using the math func. for voltage, it thinks it's about 11.96V The on-screen upper and lower voltage cursor's match with where GND is, and the CH1 trace. So both look noticeably off.

Yup just checked some other ranges, like on 500mV/div, placing the voltage cursor's across 6 blocks reads as 3.055, and the 3V from my PSU looks like it's 100-125m down below where it should be. And placing the voltage cursor there reads 2.960V


So what's that sound like this time ? And CH2 does exactly the same thing, same problem.

All the self-test's are still good. I don't use it a lot in a month, so IDK if it's been drifting for a while, but there's no doubt today.
 

nsaspook

Joined Aug 27, 2009
7,192
It's a testament of old technology that scope is still that accurate after all these years. Be very happy you still have a good self-test as the recalibration procedure is tricky unless you have a precision voltage and timing calibration standard.
 

Thread Starter

DarthVolta

Joined Jan 27, 2015
402
I don't remember it being off that much visually, Ive replaced 2 chips on it already. Someday I need to pull it apart for a tune up
 

nsaspook

Joined Aug 27, 2009
7,192
Ok but does this sound like something that some pot inside adjusts ?
No, there is no adjustment pot inside the scope for voltage calibration. There is a jumper that allows for extended diagnostic calibration using exact voltages from a power supply. You must complete the entire procedure for the voltage calibration to be valid. In clear terms, Don't Mess With It unless you want to risk seeing this.
 

crutschow

Joined Mar 14, 2008
24,750
Unless you need the 'scope for accurate voltage measurement (and there are few situations where you do), I won't worry about it.
 

nsaspook

Joined Aug 27, 2009
7,192
I just checked my recently in-house recalibrated TEK2432 with a much newer scope for P-P voltage readout.

To the OP, your old scope voltage image and readouts are what everyone sees. Don't worry, be happy.
 
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