Cannot see it to well as to which 5.0A fuse it is as there are two according to the circuit diagram.
They are in the audio output stage, just before the speaker selector switch switch A or B which then go on to the appropriate Left and Right channel speaker connection terminals on the amp.
They are designed to blow to protect the amp in case there is a s/c (or perhaps an earth on the wrong wire in the pair to the speaker) in the speakers or speaker cables. If they are blowing without any speakers connected you have a problem either in the speaker selection switch(s) or the speaker terminal pcb in the amp or in the wiring between the two..
This is based on viewing a circuit for a Fisher CA-270 which I hope is close enough to yours to be relevant as I cannot find one for a CA-270A.
Here is a link to the website. Scroll down to to find Fisher CA-270 and click to download. The schematic is at the end of the .pdf
Unplug the amp from the power and then use your DMM in ohmmeter mode to test the continuity of the speaker circuits back from the amp speaker terminals to the fuses (which should be left out at this stage).
Ensure that each wire has continuity and that there is no connection (as shown on the DMM) between the two wires or between the wires and earth (or ground). Normally the chassis of the amp will be earthed so that is one point where you can connect one lead of your Ohmmeter to to see if there is a connection between a speaker wire and earth. Do not get confused as there is an Earth terminal next to the L and R speaker terminals.
Here is an image of the relevant part of the circuit.
(Click on it to enlarge for better viewing)
Hopefully it is a start.