The last time the experiment was tried was 9 July 1962. Google Starfish Prime.
A 1.4mt warhead was exploded 250 miles above Johnston Island. Here's how it looked from Honolulu, 900 miles away:
The resulting EMP knocked out 300 street lights, set off numerous burglar alarms, and damaged a microwave link.
Back then, everything was discrete components. Most radios used vacuum tubes. Transistor radios were new and were classified by how many transistors they had (these days, I carry around a few billion in my pocket). Ted Hoff wouldn't invent the microprocessor until about 1969.
There has never been a proper atmospheric test using modern electronics. The only reason Starfish Prime took place was that the Soviets had reneged on an earlier ban on above-ground tests.