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PS18/28/35 Troubleshooting Guide

Theory of Operation

slow decay of the voltage at the two energy storage capacitors C110, C111. The power down

circuit gives the audio power amplifiers enough time to mute before the rail collapses, preventing

the speakers from making a pop noise.

Figure 13. Power down circuit

1.7 Automatic Voltage Selector

An automatic voltage selector is built-in for the dual version of the power supply. Figure 14 shows

the automatic voltage selector, which consists of the automatic voltage selector IC U401 and the

triac Q401. Jumper JP1 is not installed for the dual version. If the AC line voltage is below 170V

rms, IC U401 sends a train of pulses to triac Q401, turning it ON. The rectifier becomes a voltage

doubler. If the AC line voltage is above 170V rms, IC U401 sends a low signal to triac Q401,

turning it OFF completely. The rectifier becomes a full wave bridge rectifier.

For the US and Europe version, all parts for the automatic voltage selector with reference designa-

tion 4XX except SMD parts are left empty. For the US version, jumper JP1 is installed and the

choke L103 is not used. For the Europe version, the choke L103 is installed and jumper JP1 is not

used. At either 120V US line or 240V European line, a DC voltage at about 340V is obtained at the

rectifier output.

Figure 14. Automatic voltage selector