In message <199803150238.UAA05175@homebase.citilink.com>, Dan Larson writes >Is it normal for switchers from old PC's to have a hot chassis? >I can't figure out why the ground shorting against the case of >the power supply would do this. This wouldn't be very good if >a ground loop like this existed inside the chassis of the PC from >which the supply was removed. Perhaps it was a hot ground from the >switching supply disaggreeing with the ground of the scope.... the >scope has a transformer based supply and the primary ciruit is >totally isolated from the chassis. The chassis in a PC power supply should be connected to earth, as (usually) is the ground connection on a scope. However it's common practice to disconnect the earth on a scope for safty reasons in the TV service trade. Many older TV's have a live chassis (or half live), touching the chassis and the scope, as you tend to when you are clipping the earth lead on, results in a severe electric shock - probably from hand to hand, directly across the heart!. Also assuming you don't touch the chassis as you fit the earth lead, there is an almighty BANG!!, and parts of the TV disappear!. For this reason it's common practice to remove the earth conection to scopes, I usually do it by leaving the earth lead dangling outside the plug, in this way it's very visable and obvious. One small problem is you still get 'small' shocks due to current passing through the RF bypass components, there are usually capacitors from chassis to both live and neutral - this results in the scope floating at half mains potential, but with only a tiny current capacity. -- Nigel. /--------------------------------------------------------------\ | Nigel Goodwin | Internet : nigelg@lpilsley.demon.co.uk | | Lower Pilsley | Web Page : http://www.lpilsley.demon.co.uk | | Chesterfield | | | England | | \--------------------------------------------------------------/