No front speaker sound

Surround R + L + Center are wired in but won't work Voltage for all relay drive seems OK.
What you should do is to exchange the front speaker relay with
those suspected such as SR and center. If sound comes out,
the problem must be relay. If still gets no sound, check all circled components. Remember to download service manual and read p.83.

Sounds like you may have a stuck relay. Unplug the unit from the outlet and leave unplugged for about 10 mins. Then plug it back in and the control board will reset itself. If you get the error again, the control board will have be replaced.

The fuses are on the power board at the bottom of the unit. Fuses 901, 902 and 903 are for the left, right and sub speakers. The service manual is here, remember to unplug it from the supply. Page 62 shows PWB C1. The speaker terminals was the answer to your original question.

http://diagramasde.com/diagramas/audio/Sharp%20CD-PC672.pdf

A and B is for two sets of front speakers. Which one you choose for surround sound depends on which terminals for front speakers included in a surround sound setup that you actually plugged your speakers into.

Terminal B is designated for the output of sound to a second "zone" ie; another room for example, if you have another set of front speakers connected to it.

Protet mode is a special diagnostic mode that starts when a fault is detected.

When in protection mode the unit is prevented from starting by protection circuit, so to prevent further damage.

Check the speakers and speakers wiring, a contact in speaker wires, or faulty speakers can send the unit in protection.

Try unplugging power cable for one hour or so. then connect cable back and see if you get rid of protect state. Sometimes accumulation of electrostatic charges inside circuit capacitors can kick in protect mode. In that case unplugging the unit from main for some times can get rid of electrostatic charges, and reset circuits by draining power completely.
If that was the problem, you will not get Protect when plugging back power cable.

If you do all the above and still get Protect, then there is a real fault.

In that case the unit may be disassembled and tested to find the fault.

If that is the case I suggest contacting the manufacturer at the number listed on the owners manual, or asking a quote on repair to a local technician.

gotta unplug it let it reset, let it sit for at least 3 minutes, then plug it back in...but you may need to clean it out remove the bulb grab a rag and a shop vac and clean out the bulb housing. then take off the back along with the left speaker and clean it out (the same way) basically the dust is keeping the heat in and it is causing an overload,.,,

What is the radio make and model?

Try a Master Reset on the unit. All of your settings will have to be re-entered but it may solve your problem.

For a Master Reset: unplug the unit. Depress the SPEAKER A and the SPEAKER B buttons on the front panel at the same time and hold them in. While still depressing those buttons, plug in the unit to the wall outlet THEN release the speaker selector buttons.

It definitely sounds like it took a surge. yhe Power Amop IC in those things are pretty sensitive. Any decent repair shop should be able to repair it for under $125.

If you can hear a hiss then the spkrs and amp are OK. It sounds like you're not on whichever source you're wanting to hear or it is on mute. Make sure ther is no function enabled that turns the sound off or over-rides it (like a tape monitor or AUX in...).