faulty insulationresistance meter.

[kakha4]

have a faulty IR continuity tester (CABAC model T2732). On the 500 ohm setting(when doing initial testing) the needle deflects to 50 ohm only whereas it is meant to give a full scale deflection to 500 ohm whereas on the 3 ohm setting the needle give s a full scale deflection. any suggestions as to what could be causing this?.

[themadhippy]

Does the zero adjust do anything?

[kakha4]

its zeroed properly. tested on the 3 ohm scale. its just the 500 ohm setting when testing needle only deflects to 50 ohm range and not past it.confused??