Those early Grundig R_R recorders, were great - I remember as a lad, being very envious of a lad of a similar age (his father was a rich surgeon) who had a Grundig - he played me a recording of the radio Luxemburg top 20 - this must have been around 1959 - living in Symonds Green, a village near Stevenage - now overrun by new homes.
His name was Julian Richardson.
My first lowly tape recorder, was a secondhand "Gramdeck" which converted a record player into a tape recorder - though not very well
I expect your TK20, needs some Capacitors & perhaps valves replaced, Dave.