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Old 11th December 2012, 07:06 PM   #1
goviboo is offline goviboo  Trinidad and Tobago
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Default help with QSC PLX1602

hi,
my plx1602 stopped working during play. i have had the amp a couple of years now and it has worked very well since. i have the amp hooked up on one 18" celestion. it is hooked up to channel 2 and the inputs are paralleled, nothing connected to channel one. yesterday whilst palying the bass stopped, when i looked at the amp the power LED and the the two clip LEDs would come on, them turn off, no signal LEDS would come on. the relay would click on and off with the power and clip LEDS. i isolated the amp out of the rack no signal or speakers connected, powered it up and it did the same as before with the lights and relay coming on and off. i did notice however i didnt turn on the clip limiter on channel 2, it was turned on, channel one. the speaker and cabling checked good, and the speaker is playing on another amplifier. The amplifier keeps cycling on and off.
Iíve read through a couple of posts, and I have downloaded the SM. i do have some experience with repairing amplifiers, but not these with the switching supplies. i have the basic test equipment, fluke and osc. i did check the outputs and drivers on the board without removing them and they were around 15K. the amplifier is not staying on long enough to check for DC at the outputs, i just get a flash on the meter around 3v. any suggestions on what to check next?
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Old 11th December 2012, 08:22 PM   #2
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Could be dried up electrolytics, seen this on repair videos by maxxarcade on youtube.

If it was a power stage failure, you'd see a brief flash of the full rail voltage on the output rather than just a mere 3 volts, which would be the normal power on "thump"

These amplifiers don't fail "just like that" under light duty as they are designed to run near or even beyond clipping all day every day for years, most likely its a capacitor in the psu causing the voltage regulation loop to become unstable, causing the SMPS to cycle on/off constantly, eventually leading up to the inrush resistor burning out and/or the switching fets/igbts blowing out as well.

And if they do indeed blow a output stage, they dont just fizzle out like you describe, but rather go out with a loud bang with sparks flying due to the high voltages invloved, its not uncommon for PA amplifiers to have from around 50VDC for the lower power models up to around 160VDC per rail inside for the bigger models.

This means what you find inside is usually exploded transistors and charred emitter resistors or even charred circuit board areas with vaporized traces.
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Old 12th December 2012, 01:05 AM   #3
goviboo is offline goviboo  Trinidad and Tobago
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thanks for the response Tekko, i've already opened it up and everything looks visually ok, no charring or the regular goodies with shorted outputs. the outputs checked ok. i'll give the electrolytes in the PS section a check .
thanks!
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Old 12th December 2012, 01:36 AM   #4
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You could check the power supply volts out.
If it is Ok then you can rule out the SMPS.

The output transistors going will blow fuses or just put the output to one rail.
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