Ive spotted the above amp for sale very cheap. It has a fault which is that the red clip lights stay on permanently preventing the amp from working. Is this likely to be easy to fix?
It depends on your luck..usually qsc amps are easy to fix, but the gx5 is a bit complicated.You need to download the schematic .Start with the power transistors first.
Should be a fairly straightforward repair but if the output stage is damaged, you will need deep pockets to pay for the approved spares.
Deep pockets? For 2 or 3 pair of Fairchild C5200/A1943 per channel? There’s hardly anything in these amps. The real problem is what blown output transistors can take with them. Circuitry is mostly SMD and rather delicate and fixing collaterall damage can be a challenge.
+1.
Clip lights on all the time IIRC means either protection is activated, or there's DC on the outputs.
Either way, it's likely to be something serious, and who knows how many things were taken out along the way.
Chris
I have the Gx7 schematic. Two of them failed one time on the same channel.
I think one of our DJ's hooked the speakons form one to another instead of connecting them to the speaker. They were meant to connect to two subs and two tops driven by a Rane crossover.
I have replaced the power transistors and the emitter resistors. After powering they got on smoke. As those boards are all filled with SMT's I had to replace the whole boards. Only the toroids were kept since the boards came complete with inputs, pots and leds. I've reused the input boards as balanced to unbalanced converters. Repair cost was in the 2*300€ range.
I think one of our DJ's hooked the speakons form one to another instead of connecting them to the speaker. They were meant to connect to two subs and two tops driven by a Rane crossover.
I have replaced the power transistors and the emitter resistors. After powering they got on smoke. As those boards are all filled with SMT's I had to replace the whole boards. Only the toroids were kept since the boards came complete with inputs, pots and leds. I've reused the input boards as balanced to unbalanced converters. Repair cost was in the 2*300€ range.
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Looking into the protection circuitry it looks too smart to have a huge failure.Try to set VR1 and VR2 pot at lower gain and restart. The whole output current goes through q61-, q62 for CHb and q29, q30 for CHa.
It has 4 active protections on each channel and a crowbar which can trigger all of them at once by blowing any of the output fuses.
I'd check the +-15v, +5v rails too.
It has 4 active protections on each channel and a crowbar which can trigger all of them at once by blowing any of the output fuses.
I'd check the +-15v, +5v rails too.
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Its up for £50. From the posts above it definitely sounds like its way beyond me tbh. I would be concerned that a whole laod of it has been trashed beyond the hopeful quick fix
Well, if you fancy a project you could keep the PSU and heatsink, and install your own amplifier. There are a lot of kits out there.
That's hoping, of course, that the transformer hasn't gone, too.
Chris
That's hoping, of course, that the transformer hasn't gone, too.
Chris
I think it's worth the price .Only the transformer is more than 200 pounds...QSC amplifiers are very good and also very interesting to study.They have a very unique topology which is the safest possible among any other topology ever produced for high power amplification to use with a speaker.No matter what happens to the amp it will never damage the speaker.
That’s not true of all the QSC designs. The class H versions are DC coupled because you can’t do rail switching and make the virtual ground work like on their vintage models. This one is just a stripped down RMX2450. It is DC coupled and has a crowbar and fuse for DC protection. The amplifier is the usual transnova topology with bipolars instead of Lat-FETs and does have the usual inherent overload protections built into the design. But under cost pressure they’ve stripped it down to the minimum they can get away with and have it still drive a pair of speakers. Run 4 ohms off of it and you’re running it within an inch of its life - like running 2 ohms used to be on their old stuff.
The transformer is only a 600 VA unit. That size core only costs $65 from Antek. Would still be useful but not worth any $200 (or pounds). The heat sink is about the size of the one on my old vintage 100 MHz Athlon CPU. The caps may be ok, but I can buy those surplus any day of the week for $3 each. If it turns out the board isn’t reworkable that may be where you end up with your inverstment.
I do have a GX5. It works just fine but never sees any hard service. I use it mostly for testing speakers. The hardest t ever gets run at a gig is driving a pair of Selenium D3300’s above 1.2 kHz. Those drivers are only good for 75 watts average power and are 8 ohms.
Indeed i didn't see the transformer on the available schematic , I just assumed it's the same with all the others.Sorry about that!
I looked again at the power transistor data and i can't understand why they kept this insane topology for such a low power...I thought it should be a monster with +-125v rails...shame on me!
I looked again at the power transistor data and i can't understand why they kept this insane topology for such a low power...I thought it should be a monster with +-125v rails...shame on me!
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They don’t ever come out and say the amp cant put out 500 watts per channel. It can’t, and neither can any other 500 watt per channel amp. What they do claim is that it can put out as much peak power as amps rated at 500 watts per channel. Which is probably true.
But when I put a 500 watt per channel amp into service, I expect to be able to put out 150 to 200 watts of average power per channel without it quitting or otherwise getting upset. Do that with this thing and in a week you’ll have another burnt up amplifier. You can get that much out of an RMX2450 (at 4 ohms, not necessarily at 2) without issues.
But when I put a 500 watt per channel amp into service, I expect to be able to put out 150 to 200 watts of average power per channel without it quitting or otherwise getting upset. Do that with this thing and in a week you’ll have another burnt up amplifier. You can get that much out of an RMX2450 (at 4 ohms, not necessarily at 2) without issues.
Due to that huge voltage i suppose they are able to deliver huge instantaneous power though....yet they have 2x 80w transistor in parallel where i would have expected at least 4 x 250w transistors in parallel for stereo operation only.
Now i get the fact that this is a real joke.I never expected this from QSC.I fixed a few USA1310 amps and they were a great value for the money.
Now i get the fact that this is a real joke.I never expected this from QSC.I fixed a few USA1310 amps and they were a great value for the money.
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