Use the above drawing with chart to configure your ISI Transfomers INPUT voltage. The bottom two screw terminals are for output. By selecting your input voltage as closely as possible- then that makes the output voltage within the desired functional range of the SEM which is between 95V and 105V (the ac power of Japan) even though this microscope was made in South Korea - the mfg implemented 100V input to compete with the SEMs of that time period which ran at the 100V power requirement. Dont make the MISTAKE of thinking "Well I connected the 100V and O and got a buzz and arc so it must be a bad transformer" - NO dont jump to conclusions. Test it in the other settings shown in my above chart. Owners who have 100V - they dont need to use a transformer And the transformer is NOT an isolated type transformer HENCE it will make an arc and Buzz at that improper setting. But that does not mean it is a BAD transformer. Dont discard it. Note that there are 2 ways to select 220V input Note that when input voltages are "inbetween" then you must select either a one step higher or lower to best fit between the 95V and 105V operating voltage. The 100V -10V setting is a SPECIAL CASE SCENARIO wired into the transformer. I will explain it here gives output voltage of 1:1 ratio minus roughly 10% for example 119V AC input gives 107V AC output Dont make the mistake of thinking - well when I got the ISI transformer it was set to 100V -10V and gave me 107VAC and Ill just move the terminal over from the -10V to the O setting and therefore it will give me 97V WRONG. Instead use 140V -10 OR 100V +20 to more closely match your input. I have run into 2 sellers with this rare ISI transformer and they both came to the WRONG conclusion that the transformer is shorted and refused to sell it to me. What a waste of the transformer that I have been searching for and willing to pay for. I didnt realize or know before today (the current seller shows photos - so I figured out). At least I was able to figure out the mistake of both sellers and what they were doing in their incorrect testing procedure. SMART PEOPLE ARE TERRIBLE AT SIMPLE PROBLEMS. Smart people are often better at Calculus than Adding and Subtracting. My point is too many of these transformers are being wasted and probably entire SEMS for this very simple reason. Dont make that thinking error - the transformer is NOT shorted if it arcs and buzzes at 100V and O. DOY ... DUH ... SLAP YOUR FOREHEAD etc As local power (provided by the BIG ELECTRIC COMPANIES) has crept up over the years from 110V to 115V to 117V to 120V to 125V - the need to set the transformer differently from the original configuration - has been a real head scratcher and the cause of many WASTED SEM microscopes. Originally the ISI SEMS were installed by a professional installer very familiar with ISI SEMs. But the need to change the settings on the ISI transformer terminals has FOOLED SMART PEOPLE into thinking - well Ill just move the terminal over from the 100V -10 setting over to the 100V and O setting and BUZZZZ ARC then they think there is something wrong and time to buy a new one. Hence a wasted ISI SEM microscope.