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popcorn noise in CT 200


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#1 montreal

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Posted 08 August 2005 - 11:58 AM

Hi,

This is my first post.

My 15 year old CT400 (I thought it was a CT200) idles a bit warmer than it should because my fan's driving circuit has been misbehaving for about a year. Since I only run this amp at a few watts driving my efficient Tannoy Monitor Gold speakers, I have delayed diagnosing the thermister circuit that should be more frequently turning on the opto-triacs that drive the fan. When the fan does come on, it stays on only for 2 to 3 seconds.

Last week my left channel started generating some steady popcorn noise. This amp is on all the time and the noise started coming out at 4 AM in the morning waking me from my sleep.

It doesn't take more than a few dozen millivolts of amp noise to bring my speakers up to 80 decibels.

My service manual says that when the amp is generating a distorted signal (not my case), this is often due to the  signal-conditioning quad-opamps (U104) which precede the main power amp. This makes sense to me because if the main power amp was bad, the DC balance would probably shift enough to cause the IOC/ODEP error detection circuit to shutdown the amp.

It appears in the schematic that there are only 2 coupling capacitors present (C144 a 100 mfd. electrolytic, and C100  a 18 mfd. NP).

So I am asking if anyone feels that popcorn noise is more likely to be generated by an old electrolytic coupling cap or a slightly overheated quad op-amp chip?

Thanks for any comments.

CT400 schematic

#2 KWhitehead

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Posted 09 August 2005 - 04:47 PM

To eliminate the op amps and since it's only one channel, swap them with the opposite channel. If it follows then you found your problem, if it doesn't then you know to look elsewhere. If it's the ODEP circuit acting up you can remove Q100 and Q103 and that will eliminate ODEP from the signal path. Also check the header on the control board for cold solder joints. I have seen that header with crack solder connections cause various problems.

Your other posting on fan control, ODEP controls this by thermal senor imbedded into the heat sink located at S100/200 plus the PTC. You may also need to recalibrate the ODEP voltages if those are off that could cause the fan to operate a little different. There's 100 Ohm flame proof resistors that where used in the ODEP circuit that are green in color. Measure them for 100 Ohms +/- 10%, if out of tolerance that can cause the ODEP circuit kick in early. The fans run based on heat sink temperture coupled through the thermal senor. The fans can run at low speed even at idle.

Thanks

Kip Whitehad
Crown Technical Support

#3 montreal

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Posted 09 August 2005 - 08:23 PM

QUOTE(KWhitehead @ Aug 9 2005, 05:47 PM)
.............
.......... The fans run based on heat sink temperture coupled through the thermal sensor. The fans can run at low speed even at idle.


Thanks Kip for your detailed and very helpful post.

In an adjacent thread called:

CT800 HELP NEEDED

the poster MiamiU mentions that his amp has a fan that cycles on and off every few seconds, indicating a problem with hysteresis in the circuit (design?).

I have a similar problem with my fan only it is on for a few seconds and off for 10 to 20 minutes.

Would you agree that when one looks at the way this fan control circuit has been designed, it could be possible that when this amp is at idle in a room with a particular ambiant temperature, the heat sink temperature could be such that the steady voltage delivered by the ODEP to the voltage comparitor (U905c) could be at the threshold where U905c begins to toggle continuously? In otherwords, the voltage required to make U905c toggle on could be the same as the voltage required to toggle U905 off. If so, then that could explain why our fans can pulse on for a few seconds at a time while our amps are at idle for long periods of time.

#4 KWhitehead

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Posted 11 August 2005 - 11:09 AM

Sure. It's just a group of compartors and it sounds like the voltage at the input of the comparator is right at that threshold level to toggle. There may be a out of tolerance resistor on one of the comaprators inputs causing a higher voltage drop kicking in one of the 3 speeds of the fan. You can play around with the resistor values to change the threshold level as to when the fan will start but be careful in doing this. You don't want the fans to start to late and not be able to cool it down properly and cause heat damge to components over time. The design is for maximum long term performance and reliability. There's been install where they have changed the valuse so the fans run at a minimum low speed but I have not heard of anyone wanting to delay it out longer.

Thanks,

Kip Whitehead
Crown Technical Support

#5 montreal

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Posted 12 August 2005 - 11:58 AM

QUOTE(KWhitehead @ Aug 11 2005, 12:09 PM)
You don't want the fan to start too late and not be able to cool it down properly and cause heat damage to components over time. The design is for maximum long term performance and reliability. There have been installations where they have changed the values so the fans run at a minimum low speed but I have not heard of anyone wanting to delay it out longer.


Thanks Kip for your answer.

No I'm not looking to delaying the fan coming on. In fact it's reassuring to hear the fan kick in and stay on for a minute or so while the amp is idling. This tells me that everything is operational and ready for some heat generating periods.

What is spooky is when I never hear the fan come on at idle or at my usual low wattage. Also spooky is when the fan pulses on for a few seconds and sleeps for 10 minutes. In either of these cases, I am left with the incorrect impression that there is something wrong with the circuit and it might not perform when I might really need it.

In order to avoid the situation where the fan is going to pulse due to my room being at a particular temperature, I would sooner change the comparitor's resister value so that the fan comes on at a lower chassis temperature rather than a higher chassis temperature. This gives me the most reassuring feedback.

I just need to be careful not to set the resistor value such that the room air temperature is too warm for the fan to achieve its new goal and then shutdown. The fan could end up staying on all the time. It's a delicate balance that should take into consideration the warmest temperature that the room is likely to be at in summer. After all, the fan can never force the chassis to cool lower than the room temperature.

#6 montreal

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Posted 16 August 2005 - 07:57 PM

Happy ending,

After spending about 4 hours today slightly resetting the bias and OPED reference voltages, I still could not figure out why the fan wasn't coming on when I applied some heat to the heat sinks. It appeared to me that dual op-amp chip U904 was dead.

Fortunately I had a spare (TOSHIBA) dual op-amp chip around and for the first time in what I believe is years, the fan comes on at idle and stays on. It purrs.

Given that I have been running this amp without the benefit of a fan for quite some time now, I feel very gratetful that I have never needed to draw more than a dozen or so watts from it. Else, I would have surely cooked the components.

Perhaps the popcorn noise that I noticed (which went away last week on its own), was an indirect result of the chassis running at a higher temperature than would have been the case had the fan been working.

Remember if you have the complementary problem of the fan refusing to go off, it can be due to the opto-triac (U901/U902) on the control board, as was the case for me 6 years ago.

Thanks again to the team at Crown for helping me through this problem with their advice.

#7 montreal

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Posted 24 August 2005 - 01:30 PM

UPDATE: August 24, 2005

The replacement dual opamp chip U904 (LM 1458) that I installed turned out not to be equilavent to the original LM358, even though it had the same pin layout. When I installed the correct opamp, I now have 0.66 volts feeding the comparator instead of the 4.8 volts produced by the LM1458 opamp. The LM1458 probably couldn't sink as much current as the LM358. This 0.66 volts which is fed simultaneously to pins 5,7, and 9 of comparators U905a,b,c is much closer to what I originally started out with before I changed anything, so my original U904 may not have been dead after all.

Having 0.66 volts going to the comparators will result in the fan being turned completely off. And from what I can tell, it will take quite a bit of heat from the output transistors to get the fan to come on at its lowest speed.

It is a bit of a mystery to me how U904 operates. It seems to be acting as a current sink when the chassis is idling. For the fan to come on, U904 will have to turn off. That will only happen when the ODEP voltage coming from the main board falls enough below its starting point of 7.7 volts and thus cause U904 to switch off.

I used a hair dryer to heat up the output transistors and that wasn't enough to trigger the fan to come on. So it will take some serious power levels from to reach the threshold where the fan comes on.

I must admit I am happier with the fan staying completely off when the amp is idling or putting out the dozen or so watts required to bring my home cinema up to 100 decibels. Having the fan stay on 24/7 can only cause it to wear out faster, although hearing the fan gave me reassurance that the chassis was being protected even if it wasn't hot enough to require the fan to be on.

It remains to be seen if any pulsing of the fan will reoccur in the future. But if it does it means that this circuit can teeter-totter when the heatsink temperature is at some precise and stable value. If no pulsing occurs in the future, then that means that my original opamp U904 may have been defective all along.

#8 DGlass

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Posted 26 August 2005 - 05:25 PM

QUOTE(montreal @ Aug 24 2005, 01:30 PM)
UPDATE: August 24, 2005

The replacement dual opamp chip U904 (LM 1458) that I installed turned out not to be equilavent to the original LM358, even though it had the same pin layout. When I installed the correct opamp, I now have 0.66 volts feeding the comparator instead of the 4.8 volts produced by the LM1458 opamp. The LM1458 probably couldn't sink as much current as the LM358. This 0.66 volts which is fed simultaneously to pins 5,7, and 9 of comparators U905a,b,c is much closer to what I originally started out with before I changed anything, so my original U904 may not have been dead after all.

Having 0.66 volts going to the comparators will result in the fan being turned completely off. And from what I can tell, it will take quite a bit of heat from the output transistors to get the fan to come on at its lowest speed.

It is a bit of a mystery to me how U904 operates. It seems to be acting as a current sink when the chassis is idling. For the fan to come on, U904 will have to turn off. That will only happen when the ODEP voltage coming from the main board falls enough below its starting point of 7.7 volts and thus cause U904 to switch off.

I used a hair dryer to heat up the output transistors and that wasn't enough to trigger the fan to come on. So it will take some serious power levels from to reach the threshold where the fan comes on.

I must admit I am happier with the fan staying completely off when the amp is idling or putting out the dozen or so watts required to bring my home cinema up to 100 decibels. Having the fan stay on 24/7 can only cause it to wear out faster, although hearing the fan gave me reassurance that the chassis was being protected even if it wasn't hot enough to require the fan to be on.

It remains to be seen if any pulsing of the fan will reoccur in the future. But if it does it means that this circuit can teeter-totter when the heatsink temperature is at some precise and stable value. If no pulsing occurs in the future, then that means that my original opamp U904 may have been defective all along.
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Disclaimer note on the above custom Fan mod: Crown does not endorse or not endorse the Fan cooling mod. The Fan control circuit was designed to protect the amplifier by Crown Engineers. Performing this fan modification above could result in the premature failure of output devices and voiding of warranty if performed on amplifiers still covered under warranty.
smile.gif

#9 montreal

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Posted 27 August 2005 - 08:21 AM

QUOTE(DGlass @ Aug 26 2005, 06:25 PM)
Disclaimer note on the above custom Fan mod: Crown does not endorse or not endorse the Fan cooling mod. The Fan control circuit was designed to protect the amplifier by Crown Engineers. Performing this fan modification above could result in the premature failure of output devices and voiding of warranty if performed on amplifiers still covered under warranty.


Hi Mr. Glass,

Thanks for your comment.

It was never my intention to modify the cooling circuit.

Initially I failed to appreciate how the opamp U904 operates basically because I forgot that it was using a mono-polar power supply and the output was already biased off when the chassis is at idle or near so.

That sent me on a wild goose chase looking for a replacement U904. My substituted opamp  was a bad choice because it is not optomized to be bias off with a mono-polar power supply.

In the end, it appears that by my correcting the ODEP voltages using the trimmer resistors has resulted in the raising of the threshold heatsink temperature when the fan will first turn on. My fan no longer pulses. Pulsing can cause any amp owner to worry about the performance of the fan circuit.

As KIP points out in his above post: "The fans can run at low speed even at idle."

In my case, my fan rarely ran at idle when my amp was new. I suspect that my ODEP references voltages drifted over the last 13 years and this resulted in the fan's operate threshold moving close enough to my heatsink temperature so that my fan started to pulse due to the fan control circuit not having a different turn on and turn off temperature set point.


I have several schematics of the CT400 and CT400B and the values of the half dozen or so resistors that surround U904 and U905  have evolved. This indicates to me that Crown has changed the way the ODEP voltage affects the voltage that is fed to the comparators U905.

Knowing what the output voltage from U904 should be when the chassis is at idle holds the key to diagnosing any fan problems.

Would it be possible for you to post what you think this output voltage should be at pin 1 of U904a?

Thanks

#10 DGlass

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Posted 29 August 2005 - 09:41 AM

QUOTE(montreal @ Aug 27 2005, 08:21 AM)
QUOTE(DGlass @ Aug 26 2005, 06:25 PM)
Disclaimer note on the above custom Fan mod: Crown does not endorse or not endorse the Fan cooling mod. The Fan control circuit was designed to protect the amplifier by Crown Engineers. Performing this fan modification above could result in the premature failure of output devices and voiding of warranty if performed on amplifiers still covered under warranty.


Hi Mr. Glass,

Thanks for your comment.

It was never my intention to modify the cooling circuit.

Initially I failed to appreciate how the opamp U904 operates basically because I forgot that it was using a mono-polar power supply and the output was already biased off when the chassis is at idle or near so.

That sent me on a wild goose chase looking for a replacement U904. My substituted opamp  was a bad choice because it is not optomized to be bias off with a mono-polar power supply.

In the end, it appears that by my correcting the ODEP voltages using the trimmer resistors has resulted in the raising of the threshold heatsink temperature when the fan will first turn on. My fan no longer pulses. Pulsing can cause any amp owner to worry about the performance of the fan circuit.

As KIP points out in his above post: "The fans can run at low speed even at idle."

In my case, my fan rarely ran at idle when my amp was new. I suspect that my ODEP references voltages drifted over the last 13 years and this resulted in the fan's operate threshold moving close enough to my heatsink temperature so that my fan started to pulse due to the fan control circuit not having a different turn on and turn off temperature set point.


I have several schematics of the CT400 and CT400B and the values of the half dozen or so resistors that surround U904 and U905  have evolved. This indicates to me that Crown has changed the way the ODEP voltage affects the voltage that is fed to the comparators U905.

Knowing what the output voltage from U904 should be when the chassis is at idle holds the key to diagnosing any fan problems.

Would it be possible for you to post what you thing this output voltage should be at pin 1 of U904a?

Thanks
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No problem we just had to post a disclaimer that's all. I aked Kip to address your question on the U904 since he has already been working with you on it.

#11 montreal

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Posted 15 September 2005 - 10:06 AM

QUOTE(DGlass @ Aug 29 2005, 10:41 AM)
QUOTE(montreal @ Aug 27 2005, 08:21 AM)

........Would it be possible for you to post what you think this output voltage should be at pin 1 of U904a?........


........I asked Kip to address your question on the U904 since he has already been working with you on it........



Please tell Kip that his answer would be much appreciated.

Thanks

#12 KWhitehead

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Posted 19 September 2005 - 10:39 AM

As soon as time allows and I can get out of archive a older CT200 or 400 to measure I will get you that voltage. I just got back into the office from attending a trade show in London and know getting ready for 2 service schools. This is my priorities at this time so please be patient and if you need to please e-mail me direct for a reminder.

Thanks,

Kip Whitehead
Crown Technical Support
kwhitehead@crownintl.com

#13 montreal

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Posted 20 September 2005 - 10:51 AM

QUOTE(KWhitehead @ Sep 19 2005, 11:39 AM)
As soon as time allows and I can get out of archive a older CT200 or 400 to measure I will get you that voltage.


Thanks for your update and I hope you had a pleasant trip across the pond.