need help with 20-30 year old equipment!

https://preview.redd.it/eo3lf4sw9dbe1.jpg?width=1152&format=pjpg&auto=webp&s=bc2994ca2765185619b6e69b2a76a897362dce25

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https://preview.redd.it/94sa81sw9dbe1.jpg?width=1152&format=pjpg&auto=webp&s=98da508b2b5d722e55cb91c47adf4e6dcb998f2f

https://preview.redd.it/9wiu81sw9dbe1.jpg?width=1152&format=pjpg&auto=webp&s=400d35ba5b69023b80663d51374dbcb5bad376eb

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Hey! This is my fathers old equipment. It is really good, but recently i have noticed that only left right front speakers are working. There is 2 front speakers, 1 center channel, 2 rear speakers and one sub. I am now testing different cables in different speakers, and ALL the speakers work, but only if i plug them in the front channels. The sub is NOT working, and we cant figure out why. We were thinking that if the sub was broken, none of the channels would be working?

after testing i figured out that all the speakers are working, but seems like they only emit sound from the output that says FRONT. i have tested the same cables and speakers from the ones that says surround and center. No sound.

Any tips here? any solution? and if the sub is broken, is it possible to get a new one that will work with the same speakers?