A HENTRMPA900QF constant-voltage power amplifier regardless of whether the volume is large or small, uncertain when broadcasting was severed. Power off and then boot, sometimes can return to normal, sometimes not working. This machine used for the school, with the PC to complete unattended Bell, calisthenics, music playback capabilities.A5Gservice manual download site
A5Gservice manual download site
Back to test amplifiers, confirmed that there is no signal output from the protection relay is not closing. Check the relay drive circuit problem-free, in the trial and found a pattern: when the relays when there is signal output and 4 Red LEDs light; when the relay is not timely, with only two LEDs brightness only normal, the other two serious lack of brightness, in order to ascertain the cause mapping schematic diagram is shown in Figure 1.A5Gservice manual download site
A5Gservice manual download site
Control schematics and found two dimming LEDs are L205, L206, R216 power supply for them (4.7k), C206 (47 μg F/16V) normal, remove separate measurements is also normal L205, L206, strung in a 4.7 kΩ resistor connected to the voltage regulator 12V power supply testing, normal brightness,A5Gservice manual download site
A5Gservice manual download site
L205, L206 itself properly and restoring L205, L206. L205, L206 brightness caused another darker reason may be that their power supply voltage drops, or 12V Zener diodes DW202 bad performance, filter capacitor C209 (470 μg F/16V) severe leakage, or current-limiting resistor 8225 (4.3k ω/5W resistance of cement) poor contact. C207 (470 μg F/16V) and C209, void DW202 (1W/12V) and DW201 (1W/12V) Exchange, void R225 (4.3k) and R244 (4.3k) ExchangeA5Gservice manual download site
A5Gservice manual download site
L203, L204, dimming, relay does not snap, just R225 internal connection is bad, leading to differential circuit DC operating point disorder, after the level of abnormal DC operating point, the relay is protected do not snap, no signal is output. With three 1.5k ω resistor in series after substitution R225 long-term tests, amplifier is working correctly, troubleshooting.A5Gservice manual download site