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CT800 Help Needed


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#1 MiamiU

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Posted 12 July 2005 - 10:37 AM

I have 18 CT800 amps at my arena. We have noticed that a few of them seem to be running hot. We purchased all need fans for the amps. We replaced 2 of the fans and powered up the system to test them.... Here is what we noticed, One of my amps, with an old fan, the fan runs all the time. One of the other amps with a new fan just cycles, runs for a few seconds, stops for a few seconds and starts again.

What is the normal operation of the fans? Run all the time or cycle?

#2 KGring

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Posted 13 July 2005 - 09:19 AM

The circuit controlling these fans is variable speed . Heat changes voltages which change the speed of the fan. The bias voltages in the amplifier can generate enough heat to occaisionally turn the fan on. This is why you may see some of the fans cycle on and off with no load or signal.



QUOTE(MiamiU @ Jul 12 2005, 10:37 AM)
I have 18 CT800 amps at my arena. We have noticed that a few of them seem to be running hot. We purchased all need fans for the amps. We replaced 2 of the fans and powered up the system to test them.... Here is what we noticed, One of my amps, with an old fan, the fan runs all the time. One of the other amps with a new fan just cycles, runs for a few seconds, stops for a few seconds and starts again.

What is the normal operation of the fans? Run all the time or cycle?
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Kevin B. Gring
Crown Audio
kgring@crownintl.com

#3 montreal

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Posted 08 August 2005 - 12:57 PM

QUOTE(MiamiU @ Jul 12 2005, 11:37 AM)
I have 18 CT800 amps at my arena. We have noticed that a few of them seem to be running hot. We purchased all needed fans for the amps. We replaced 2 of the fans and powered up the system to test them.... Here is what we noticed, One of my amps, with an old fan, the fan runs all the time. One of the other amps with a new fan just cycles, runs for a few seconds, stops for a few seconds and starts again.

What is the normal operation of the fans? Run all the time or cycle?
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I just posted a problem about my CT 200 (actually it's a CT400) concerning popcorn noise. But I also mentioned that I have had fan problems (somewhat like yours).

What I found was that when I operated my amp in my apartment in the summer at very low wattages (under 10 watts), my fan would come on for a minute at a time every half hour or so, even while my amp was idling with no music since many hours.

After about 8 years of ownership, the fan stayed on all the time until I shutdown the amp, which reset something. Eventually I swapped out the opto-triac (U901) and the amp worked as it had when new. The opto type chips in my factory are known to deteriorate over time.

Then I moved to a house and put the amp in a cooler basement and I hardly remember the fan ever coming on. Then I noticed that the fan would come on for a few seconds only,  at ten? minute intervals.

I again changed all the opto-triac drivers to be safe and this did not improve the situation. My conclusion was that the circuit that monitors the two temperature sensors (thermistors) was bad or both thermistors themselves were bad.

It is also possible that the idle temperature creeps up slowly to the threshold that would cause the thermistor voltage to slightly exceed the reference voltage and trigger the fan to come on. As the fan comes on, it draws a miniscule amount of power which slightly upsets the delicate thermistor reference voltage enough to cause the fan driver to shut down. If this was truly happening, it would mean to me that the temperature of an idling chassis (or one that is hardly working) is slightly below the threshold required to trigger the fan. The ambient room temperature might play a critical role in creating this delicate situation.

To prove this theory, I would have to drive a dummy load with a high output signal (say the equivalent of 50 watts rms – too high for my loudspeakers) so that the chassis would really begin to heat up. If the fan then came on and stayed on for several minutes, then I would be convinced that both our amps are OK even if they pulse the fan from time to time. The pulsing may just be an annoying situation.

Or there may be something defective in the thermistor circuit.

Stay tuned.

#4 montreal

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Posted 09 August 2005 - 10:03 AM

I want to continue with my previous post by talking in more detail about our fan control circuit and the annoying pulsing that is taking place.

Recently I was working on a prototype board using OPTO-TRIACS to switch off an air conditioner ventilation fan 20 minutes after the compressor shuts down. I was feeding the OPTO-TRIAC with a DC control voltage derived from a timer circuit that took 20 minutes to slowly discharge a capacitor. What I discovered was that as the control voltage drifted slowly downward over time, it eventually approached the threshold that turned the OPTO-TRIAC off. But as it passed through the threshold, the fan would pulse a couple of times before permanently shutting off. In other words, the voltage required to turn off the OPTO-TRIAC was the same as the voltage required to turn it on. When the control voltage was precisely at the critical point, the OPTO-TRIAC could not make up its mind if it wanted to switch off or switch back on.

I inserted an electromechanical relay upstream of the OPTO-TRIAC. The relay’s coil has a radically different cut-in voltage versus cut-out voltage.
Problem solved.

Which brings me back to the fan circuit of the CT400. Here we have the same OPTO-COUPLER design and we are using a voltage comparator(s) to drive the OPTO-TRIAC. What is not clear to me is if the voltage comparator has a different cut-in voltage versus its cut-out voltage. If these voltages are the same, then the fan circuit is going to pulse whenever the amp is idling and the chassis temperature remains at the threshold that causes the voltage generated by the thermistor to be exactly at the point where the voltage comparator begins to switch on (or off).

The only way to avoid annoying pulsing would be to have a voltage comparator circuit where you need one voltage to switch on and another voltage to switch off, with a gap between these two values to buy some tranquility. That would require adding some sort of Flip-Flop which sets and resets at different temperature of the chassis.

In real world situations where these amps are working hard, the fans would come on and stay on until the chassis  would eventually cool down. Our problem is that our chassis don’t cool down to a temperature significantly below the threshold. Room temperature surely plays a role here.