Razum Ekimov Andrejchin has born in Gabrovo.
He was a schoolboy in the Secondary School, created by Vasil Aprilov in Gabrovo, and a student in the Sofia University (1929 - 1933), where he become Professor Georgi Nadjakov's PhD student in physics (1940).
He investigated solid state physics and metrological problems mainly.
Razum Andrejchin was Assistant Professor (05.04.1937 - 03.05.1946), Associate Professor (25.02.1947), Research Fellow (01.03.1950), Professor (1961) and Researcher (1967).
Sofia University appointed him to assistant position in physics (05.04.1937 - 1941) and in astronomy (1941 - 31.05.1946).
Razum Andrejchin was Deputy-Director of the Institute of Physics at the Bulgarian Academy of Sciences (1952 - 1957).
Up to be pensioned off (1976) Razum Andrejchin worked in the Institute of Solid State Physics at the Bulgarian Academy of Sciences.
Razum Andrejchin lectured physics at the Russe High Technical School (1947 - 1949) and later on at the Sofia University (1950 - 1957).
Razum Andrejchin was chief of the Physics Department at the Agriculture Academy in Sofia, where he lectured special courses on “photo-electrical phenomena in semi-conductors” and “semi-conductor devices” (01.01.1962). He built agricultural physics laboratory.
He created Department of physics at the Technical University in Gabrovo and lectured there course "physics of semi-conductors" (1974 - 1978).
R. Andrejchin, Uber einen photovoltaischen Effekt in aufgedampften Bleisulfidschichten, Festkorperphys. und Phys. Leuchtstoffe, Berlin, 1958, s. 94-97 
R. Andrejchin, Uber den Kontaktpotential-Photo-EMK, Festkorperphysik, Berlin, 1961, s. 280-289 
R. Andrejchin, M. Nikiforova, A. Ivanov, U. Stanislavova, Photovoltaischer Quereffekt im monokristallinen System CdSxSe1-x, Physica Status Solidy, Berlin, v. 3, Nr. 8, K280-K283 
Ñòî ãîäèíè ÁÀÍ 1869 - 1969, ò. 3, Ñîôèÿ, 1972, ñ. 30-33