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We present observations of the C-band 110–111 (4.8 GHz) and Ku-band 211–212 (14.5 GHz) K-doublet lines of H2CO and the C-band 110–111 (4.6 GHz) line of H213CO toward a large sample of Galactic molecular clouds, through the Shanghai Tianma 65 m radio telescope (TMRT). Our sample with 112 sources includes strong H2CO sources from the TMRT molecular line survey at C-band and other known H2CO sources. All three lines are detected toward 38 objects (43 radial velocity components) yielding a detection rate of 34%. Complementary observations of their continuum emission at both C- and Ku-bands were performed. Combining spectral line parameters and continuum data, we calculate the column densities, the optical depths and the isotope ratio H212CO/H213CO for each source. To evaluate photon trapping caused by sometimes significant opacities in the main isotopologue’s rotational mm-wave lines connecting our measured K-doublets, and to obtain 12C/13C abundance ratios, we used the RADEX non-LTE model accounting for radiative transfer effects. This implied the use of the new collision rates from Wiesenfeld & Faure. Also implementing distance values from trigonometric parallax measurements for our sources, we obtain a linear fit of 12C/13C = (5.08 ± 1.10)DGC + (11.86 ± 6.60), with a correlation coefficient of 0.58. DGC refers to Galactocentric distances. Our 12C/13C ratios agree very well with the ones deduced from CN and C18O but are lower than those previously reported on the basis of H2CO, tending to suggest that the bulk of the H2CO in our sources was formed on dust grain mantles and not in the gas phase. [ABSTRACT FROM AUTHOR]