The Far East Broadcasting Company (FEBC) has had a station on Jeju-do since 1973. It first came on the air in 1973 as HLDA on 1570 kHz. In 1979, it changed its frequency to 1566 kHz and a few months later changed its callsign to HLAZ.
HLAZ may be the most widely heard medium wave station from South Korea. They could be heard during the daytime all year round in Seoul, a distance of about 300 miles. They are a reliable signal on the West Coast of North America and in Europe in the winter.
I heard HLDA in 1976 when I lived in Daegu and sent a reception report. They responded with the QSL on the top of the gallery below. Subsequently, I sent two reception reports to them when I lived in Utah. According to Jack Lentz in a 1982 letter, my reception in Utah was the furthest east reception of HLAZ as of that time.
Also please take the time to listen to the audio clips to hear what they sound like in Seoul. And scroll down to see all the QSLs I received from them. At the very bottom is a photo of me standing near their transmitter site in Jeju-do