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Old 8th January 2014, 07:54 AM   #1
rovers is offline rovers  Norway
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Default QSC GX5 Fan modification

I just bought a GX5 to test for home use, but the noice from the fan is really bugging me.

So, I looked around and found some are replacing the original fan with this one: Invalid Request

When I opened up the GX5 to look at the fan and how to remove it, I did not quite see how. The fan has no screws, it looks like it is slided into four plastic pins, and then kept in place by three locks in the plastic bracket. Picture of the inside here:
Test: QSC Endstufe GX5 - Seite 2 von 4 - AMAZONA.de

How to remove the fan?
Do I have to take out the whole plastic bracket, in case how?
Or do I have to open the plastic locks in the bracket and forcefully slide the fan out?


Rovers
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Old 9th January 2014, 02:16 PM   #2
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I think Enzo must be on vacation, you are stuck with me. Nobody but Enzo really experienced watches PA systems often.
Your purchase link didn't come up and I don't think you can buy any fan easily that is as long lasting as the one installed in your QSC, which is a premium brand product. What you need is a fan control circuit that only allows the fan to run fast if you are really playing loud. See the drawing below. The Peavey PV-1.3k has the fan circuit on the left, which allows the fan to run slowly when cold, and goes to full speed, 120 VAC, when the heat sink is hot. Unfortunately I don't know where to buy a thermostat like that except Peavey, they were common as dirt in the nineties but are going away. Microwave ovens used to have them. Newark, Mouser, Digikey, don't as far as I can tell. If you are going to buy from Peavey, just buy the whole fan control board if it will fit in your amp, it is about 3"x3" and has spade lugs for the 120 vac and neutral in and out. I use machine screws with elastic stop nuts (to not fall off) and 1/4" air tubing as standoffs when I mount stuff like this. The Peavey circuit is for a 120 VAC fan. It won't work if your fan is a DC driven fan.
If you can't buy a snap action thermostat, look at the circuit on the right for a 120 VAC fan. You put the 15k NTC thermistor on the heat sink, and it decreases resistance as it heats up. At some point the gate of the FET increases to the trip point and it switches on, putting ~50 ma to the gate of the triac. This runs the fan full blast. The fan resistor is picked from the peavey value, may be different for your fan. I'd buy a number of 47 ohm 10 w resistors (or 5 w) and stack them up for R3 until I got the cold fan speed I wanted. Similarly with R4, if I didn't have a junk box full of resistors I'd buy various 1/2 w 1000 ohm resistors and stack them up until I got the turn on temperature I wanted. You can test the temperature with a temp probe and a hair dryer on the naked NTC resistor.
If your fan is DC, blow off the triac and run the DC straight through the T1 to the fan, using it to bypass the fan dropping resistor R3. Similarly the power supply can be simplified, sucking power from the fan drive circuit instead of the AC line. Having a regulated ~8v DC source for the FET gate is important, you want that voltage to be stable. But you don't need a 5W 1n5344 regulator for just the fet without the triac, as the triac needs 50 ma to turn on. The fet will turn on with microamps, and any tiny wattage zener would do. Increase R2 the zener regulator resistor appropriately for a smaller wattage zener. Or any voltage regulator, you want 6 to 9 volts on the FET gate circuit to swing the fet gate from <1.5 v cold to >4 v hot.
I build stuff like this on NEMA LE laminate, 1/8" or 1/16", that I drill holes in. You can also buy expensive ($10) perf board from electronic supply houses and buy official standoffs instead of cutting them out of tubing. However, use elastic stop nuts, not regular one, as you don't want hardware to come loose and rattle around in an appliance with 120 VAC in it.
Read the safety sticky thread on tube amp forum about working around high voltage if working the AC circuit. Use a clip lead on the meter negative connected to neutral to measure power on, and use one hand at a time, 120 VAC electricity running across your heart can stop it. Make sure the big blade is really neutral, ie at ground potential, a lot of houses, wall and workbench sockets are wired upside down. No jewelry on hands or neck, 2 vdc can burn a finger to charcoal through a ring. Don't work alone. Don't work distracted by a phone or another person. Etc, read the thread.
Have fun.
Attached Images
File Type: jpg QSC-FANCONTROLEXAM1.1.JPG (79.6 KB, 94 views)
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Last edited by indianajo; 9th January 2014 at 02:32 PM.
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Old 9th January 2014, 04:13 PM   #3
rovers is offline rovers  Norway
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Thanks for the reply

The GX5 uses a 24V DC fan and runs at full speed all the time.
The fan is a
SUPERRED 8025 CHA8024EBN-K(E)
24V, 0,24A 80x80x25mm.

I do not know the CFM or noise but it is loud.

I have a fan lined up to replace it, just not quite sure how to remove the original.
I also bought a voltage regulator, but the space is very limited in the GX5 chassis, so going for the replacement fix first.

Any other hints/tips would be greatly appreciated.
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Old 9th January 2014, 09:36 PM   #4
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if it's a used amp and the fan is going full out i'd start by opening it up and checking it for dust contamination.
these amps do have a temp sense control over fan speed; the sensor is mounted on underside of the heat sink on the bad air flow side of the board. i'm not privy to any mod's or reported failures.keep in mind that these amps where intended for commercial applications and usually live in racks.(or separate rooms)
in your application you could consider putting it in something stylish(rack style) that still allows good ventilation but stops the fan noise from propogating into your listening area.
if your certain the fan and control circuit is bad the schematics are readily available and i think if i can't help many more that frequent here can!
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Old 14th January 2014, 07:54 AM   #5
rovers is offline rovers  Norway
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Sorry for the late follow up.

No, the amp is brand new.
I have seen a lot of PA-amps that have been modded for home use with a fan that is practically quiet compared to the stock fan. The only problem is how to remove the stock one, as the attachment looks a bit tricky.

Separate room is not an option.


Rovers
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Old 15th January 2014, 08:12 AM   #6
dangus is offline dangus  Canada
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Temperature-controlled fans are reasonably common in PCs (used for case, CPU, and power supply fans). Search ebay for pc fan temperature control and you'll find various things. Or google it plus diy. Perhaps you can salvage something from a junk PC or power supply; sometimes the temperature control is a separate board.

The Peavey amps that I own have two-speed fans that are virtually silent under normal conditions (as in not a hot day in summer, not playing as loud as possible). If you want to see how they did that, a service manual with schematics is available:
PEAVEY PV8.5C Service Manual free download, schematics, eeprom, repair info for electronics
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Old 15th January 2014, 12:32 PM   #7
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if i recall it's a plastic rivet assembly that holds the fan in.
pushing out on the center pin from the inside or a careful pry on the head of the fastener from the outside should release it.
can you upload a picture?
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Old 15th January 2014, 04:30 PM   #8
rovers is offline rovers  Norway
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Quote:
Originally Posted by turk 182 View Post
if i recall it's a plastic rivet assembly that holds the fan in.
pushing out on the center pin from the inside or a careful pry on the head of the fastener from the outside should release it.
can you upload a picture?
If you look at this link: http://www.amazona.de/wp-content/upl...Innenleben.jpg you are able to see the fan.

Do you mean to take out the black pins in the corners of the fan?
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Old 15th January 2014, 11:22 PM   #9
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yes it does appear that their split pins.
you may have to use some small needle nose pliers to squeeze the two halves together and push out but they're fairly easy to remove.

Last edited by turk 182; 15th January 2014 at 11:28 PM.
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Old 16th January 2014, 05:51 AM   #10
rovers is offline rovers  Norway
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Thank you for the advice :-)

That would explain why I had no luck when I tried the first time.
Will give it a go as soon as my new fan arrives in the mail.
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