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DIY For Audio
Help me "flick a relay" (QSC USA 1300 Amplifier)
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<blockquote data-quote="ludo" data-source="post: 62931" data-attributes="member: 691"><p>If the output stage is OK, which you seem to be certain of:</p><p></p><p>First check if the PTCs have about the same resistance on both channels.</p><p></p><p>Fire the thing up and mind you have 180VDC on a couple of milifarad at your fingertips. (Not good for dental implants and such.)</p><p></p><p>Then check if you get 36V across the zener (Z6) and capacitor E5. You should also see about half that voltage over E4. The voltage on E4 is in large part what makes it tick. If there is 36V on E4, R19 has gone open. If there is no voltage over E4 then either E4 is leaking, or R20 has gone open, or Q20 has switched on. </p><p></p><p>Q20 is the DC detector. It switches on via the small diode bridge from DC stored on E6. When all is well in terms of no DC on the output, the voltage from Q20's base to emitter should be well below 0.6V and it should stay off.</p><p></p><p>If Q20 switches on, it switches Q19 (and LD3) off, which switches Q21 off, and then the relay is off. Then the red part of the bicolour LED is on, via the feedback resistors and R51 (not on your picture.)</p><p></p><p>These Romans are crazy.</p><p></p><p>Overtemp works like so: If the PTC gets too hot its resistance goes up until it has more than 7.5V across it, switching Z5 on. This switches Q19 off, which switches Q21 off, and the relay. So if the PTC is flakey or Z5 is a dud the relay will stay off.</p><p></p><p>Lots of possibilities.</p><p></p><p>HTH</p></blockquote><p></p>
[QUOTE="ludo, post: 62931, member: 691"] If the output stage is OK, which you seem to be certain of: First check if the PTCs have about the same resistance on both channels. Fire the thing up and mind you have 180VDC on a couple of milifarad at your fingertips. (Not good for dental implants and such.) Then check if you get 36V across the zener (Z6) and capacitor E5. You should also see about half that voltage over E4. The voltage on E4 is in large part what makes it tick. If there is 36V on E4, R19 has gone open. If there is no voltage over E4 then either E4 is leaking, or R20 has gone open, or Q20 has switched on. Q20 is the DC detector. It switches on via the small diode bridge from DC stored on E6. When all is well in terms of no DC on the output, the voltage from Q20's base to emitter should be well below 0.6V and it should stay off. If Q20 switches on, it switches Q19 (and LD3) off, which switches Q21 off, and then the relay is off. Then the red part of the bicolour LED is on, via the feedback resistors and R51 (not on your picture.) These Romans are crazy. Overtemp works like so: If the PTC gets too hot its resistance goes up until it has more than 7.5V across it, switching Z5 on. This switches Q19 off, which switches Q21 off, and the relay. So if the PTC is flakey or Z5 is a dud the relay will stay off. Lots of possibilities. HTH [/QUOTE]
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DIY For Audio
Help me "flick a relay" (QSC USA 1300 Amplifier)
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