Start by downloadiservice manual from elektrotanya and uploading it here.
I guess it has a motor driven magnetic drum inside, plus a couple recording heads touching it.
Being a mechanical device, and old to boot, it must at least be dirty, needing oiling, etc.
It must be able to be disassembled or open, just be careful.
*Hope* itīs not riveted together making it a sealed non repairableunit, but in any case it still can be carefully opened .
I faintly remember an *OLD* echo unit where the "tape" was thin magnetic wire wound around an aluminum cylinder and tape heads brushing against it, might be similar.