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Realistic STA-2250 receiver

I need some help with a Realistic STA-2250 receiver.
Everything works O.K., except the tuner preset memory is always cleared after power off.
I do not find a memory backup battery or capacitor.
I also do not find a schematic diagram of this receiver.
Any suggestions?
Kind regards, 968driver.

it looks like sta2100 but

it looks like sta2100 but better in the final amp section.