I have an Optimus STA 825. One channel or the other cuts out randomly and simultaneously the overload clicks in and out. It clears up if I pound on it; but returns later. I resoldered all the connections on the printed circuit board but to no avail.

Hi gordon_d_woo,

The receiver has two special protection circuits:

1. The thermal overload protection circuit automatically turns off the receiver's amplifier if it overheats.

2. The overdrive protection circuit automatically turns off the amplifier if too much power is being drawn from it.

If a protection circuit turns off the amplifier, the PROTECTION indicator comes on.

If this happens, press POWER to turn off the receiver's power. Check for proper ventilation and proper speaker connections, and be sure the speakers have an impedance of 8 to 16 Ohms.

The protection circuits usually reset in only a few minutes, so you can turn on and use the receiver again. The circuits seldom take longer than 30 minutes to reset.

If the receiver stops working and the PROTECTION indicator is not on, please replace the Fuse.

Hope it helps.

Thank you for using FixYa.

This is the basic schematic of the final amplifier in your unit
http://www.diy-electronic-projects.com/p38-2N3055-Power-Amplifier
the only difference being that your unit uses 2SB775 as finals.
In it the overload protection is done by measuring the current absorbed by each final transistor - this is dependent of the load given by the speaker, the amplitude of the input signal and the temperature of the bipolar transistor.
If the problem gets fixed by banging on the case then either you have a bad connection either at the speaker jacks or in the connections on the board - one of the copper tracks might have fractured and to find it you must inspect the entire board with a magnifying glass - or one of the final transistors no longer makes a good contact to the radiator - find it, take it off completely, clean the old silicon past ans add new one and refit it properly.