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Old 4th April 2005, 02:47 AM   #1
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Default QSC 1400 Schematics

Anyone Have any QSC 1400 schematics???

I have a dead one sitting on my floor...


Zc
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Old 4th April 2005, 08:39 AM   #2
mcp is offline mcp  United States
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You can download it from QSC website. Look under "Series One" at: http://www.qscaudio.com/support/tech...schematics.htm

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Old 4th April 2005, 11:27 AM   #3
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Thanks!

I found them....Now i need to figure out how these strange amps work!

Looks like the driver transistors for one channel are blown and the other channel works until you get close to clip, then the top of the positive waveform clips off at an angle, very strange, never seen an amp behave like that.


Zc
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Old 4th April 2005, 01:07 PM   #4
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I've worked with these amps for the last 20 or so years, basically since their release. Best thing to do is get the rebuild kit from QSC. Not only will the kit repair it but it will also upgrade it at the same time. They are pretty good amps overall for cinema use.

Mark
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Old 5th April 2005, 03:06 AM   #5
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I do not understand how these amps work! crazy things!

the output is taken from the middle of the filter banks and the output transistors are wired to load the transformer/rect. down with AC. seems crazy to me.

QSC has these schematics posted on there website free to download.
http://www.qscaudio.com/support/tech...schematics.htm
click on series one model 1400

I was hoping to fix this as inexpensivly as possible as i am doing this gratis for a friend.

Zc
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Old 5th April 2005, 08:17 AM   #6
Enzo is offline Enzo  United States
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Grounding the emitters may seem counterintuitive at first, but look at it this way. Forget the idea of ground for a minute. Normally you have power transistors conducting to the power rails, and they then connect together and go into the speaker. The other side of the speaker goes to the common of the power supply. (This is the point usually grounded)

The circuit here is really the same thing, they just connect the system to ground on the amp side of the speaker instead of the power supply side. If you had no chassis and built it on a wooden plank, you would have the same circuit - power rails through transistors through the speaker and back to the power supply common. Now to stuff it in a chassis, really you could ground any point you like.

Imagine a fan to cool yourself on a hot day. You could bolt the motor to the frame and the fan blade to the shaft, or you could bolt the shaft to the frame and connect the fan blade to the motor body (Oh pretend there are no wires, or it runs on batteries or sometihng) Either way, it spins.

One advantage of grounded emitter design is that a shorted output device does not put DC onthe speaker. Grounded emitter amps are also called flying rail amps.

Look for shorted output transistors, check the driver TO220s. and make sure the driver emiter resistors are not open. Check all the ballast resistors while you are at it, one per power transistor.

In troubled amps it is common to find the +/-15v supplies to the op amps way out of whack. WHile it is possible a bad zener happened, it is probably an artifact of the other trouble, so look to the power semis first before worrying about the LV rails.

Just a hunch, but on some of those - and there are two versions of the 1400 if I recall - there is a hum null or offset null. The pot likes to open and it can throw the rails way off. Remember the supply is not referenced to ground, so if the output transistor shorts, it just hauls the rails way offset. So instead of +/- 50 you get +20/-80 or something like that. Measured to ground. Measure across the bridge it will be 100v - or whatever - all the time regardless.
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Old 5th April 2005, 11:18 AM   #7
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I yanked all the output devices out and they all test fine! so i powered it up without the outputs as i have done hundreds of times and promptly smoked a driver resistor as i was coming up with the variac. and found a bad driver transistor.

So i yanked both drivers and brought it up very very slowley and just enough to get some juice in it and yes i find the power supply to be off. if it was up all the way it would probably be 80/20 like you mentioned....Need to get some parts and i will sork on it some more. i will look for some open pots...


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