strange scope readings

hi, this will probably turn out to be a stupid error by me but can anyone tell me why i get the following happen.
i am doing some transistor switching stuff and wanted to scope the line's,
the power supply in this case is a old ATX one from a computer, i disconnected my circuit and decided to see how lumpy the 12V and 5V lines on the ATX are as its old. first i measured with a good quality DMM and 12V line reads 11.889V (i figure thats ok) the 5V line reads 4.87V, now as there isnt a load on the supply i figured that was all pretty good. So i then measured using my cheaper not so good DMM and the measurements were pretty close! (well happy with that). Now come the odd bit
i tried scoping the lines i set probe to x1 and set scope to x1 i connected ground lead to calibration ground and probe tip to calibration tab (1V 1kH) and get a nice square wave at correct frequency and voltage.
i then attach probe ground to ATX ground and scope Tip to 12V and press auto set up. i get a amplitude reading of 63mV!! i have tried manualy setting the scope and get the same thing i have also tried another scope and get same reading, so what am i doing wrong???? why are the scopes reading wrong voltage and the DMM are correct??

Logan (frustrated)

hi Logan,
Do you have the scope set to AC input and not DC input.?
In Auto set up mode.?
E

yes i tried both but will doulble check

also tried good hp PSU and get correct results!! so maybe something to do with the GND on the atx????

SOLVED!!!!!!! YAY..

I finaly worked it out

dad has used both scopes without me knowing (must have been at school) on both scopes is a menu setting with the following on it ABS,POS,triangle symbol,MAG

dad had both set to triangle symbol. i set to ABS (i assume absolute??) and all is well! i assume that these setting relate to the datum line on the scopes? and when you use auto set up they dont change these settings? wich would explain why the hp psu read correct because the gnd is isolated and not tied to the scopes, where as the ATX and scope share the GND and the GND in this case was tied to the datum line if you see what i mean (not sure i do )