The IO-20 is a specialist scope. It has only two time base ranges. The measurement cable has a pickup coil for the trigger signal coming from the spark plug on the first cylinder, a crocodile clip to connect to the primary winding of the ignition coil and a clip to clamp around the secondary winding of the ignition coil, for a capacitive pick-up of the signal of the secondary.
This instrument belongs to a friend who collects old cars. He has about 15 cars, dating from the fifties until the seventies. He desperately needed an instrument to check his ignition and was happy to find this one. There was one problem however: it did not work. My friend was careful enough to first read the manual to find out this scope was designed for 120 V mains, but when he tried it using a step-down transformer, it did not work. He suspected that the previous owner had tried to use it on the 230 V mains here in Holland, because a European mains plug had already been fitted.
When we opened the case and switched it on, some of the valves remained dark, and the others glowed only faintly. Of course, it you use a piece of valve equipment on a doubled mains voltage, the valves' heaters may burn out very soon. The mains fuse cannot always prevent that.
When I measured the heater voltage, I got only 3.2 V instead of 6.3, and the HV supply winding that should give 90 V delivered only 45 V. So it was obvious: this was a European version with a 230 V mains transformer.
We connected it directly to mains and now the tubes all glowed. The heater voltage was OK and so was the supply voltage. Connecting a test signal gave a good image. I corrected the focus and astigmatism settings and closed the device. As you can see on the top photograph, it worked well with my friend's 1971 Volvo. You can see the IO-20's inside below.