That is a unit from the late 90s. It is an old prologic receiver pre HDMI and prior to the discrete multi channel codecs. It is no longer compatible with current TVs.
So it now has a fault that you will not be able to fix. The unit eve working is of about zero commercial value. It is not worth the cost of repair at all, as it is terminally out of date.
The truth is that it needs replacement with an up to date unit, and needs a one way trip to the recycling center. I am actually surprised it has worked this long. There are very few receivers from that era, working or in use at this time. In any event the problem is most likely on the processing boards, and parts for those boards would not be available.