Technics SA-K5L info

I got an old Technics amp with an interesting "direct drive" turntable from a friend, fully working, nothing special.
On the rear it got an "external unit" input and output that is linked out, what is that for?

If I remove the links, there are hardly any sound, not even on the headphones.
 

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Probably to insert an equalizer between the pre-out and the amp in. It was pretty common back then to allow the pre/amp pieces have stuff put between them, or so you could use only the preamp or the power amp. So, without link, amp gets no signal.