Hello,
I have a NAD C272 amp which died recently and I'm trying to test it to determine what needs to be replaced for it to work again but I'm a bit of electronics beginner so bear with me! I've read a lot about problems with this model, mostly with the main capacitors and transistor Q712. Most of these issues are discussed or mentioned in this thread:
https://www.diyaudio.com/forums/solid-state/329113-nad-c272-refresh-5.html
However, my unit has the upgraded 100v capacitors and also the same updates as MaxMartin lists in the thread regarding his unit:
"I have myself a C272 of the newest batch from 2009. They’ve made some changes regarding transistors. Q710, 711 is 2240 and q712 is a970. On left and right boards there is now also 1220 and 2690 (input stage) like down in the +68v regulator. The 2240 pair and the 970 goes very hot still.. "
---MaxMartin
Apparently, NAD updated the later units with these modifications in response to problems the 272/372 series were having.
Ok, with that preface out of the way, the problem I have is the unit simply stopped working. No noise or other problems; it just has no output. The unit will power up meaning that the LED comes on when the power switch is pressed and it still clicks as it always did but there is no sound. I assume this points to the power board. I've looked closely at all parts inside the unit and have found no obvious problems or evidence of high temps. Everything looks normal.
So I tested the main power supply on the power board and it checked out fine. +68.7v and -68.6v in the appropriate places. However, there are also 2 more sets of test poles, one labeled "CB716, M- Test" and the other "CB717, M+ Test." I could not get a reading from either of these. And what does the CB716 signify? Every test site has a unique CB number.
Here are some pics in case I'm not describing in appropriate ways:
Here is the overview with the power board towards the top and the large 4 capacitors.
By the way, my power board is labeled as Version 5.0 as I guess there are different versions that NAD used to combat the capacitor and transistor issues.
At the top left is the site where I measured +68v and -68v so the board is getting the correct power. However, the other test site M- in the photo measured 0 as well as the M+ site in the following photo. I assume these should be -68v and +68v respectively but hopefully someone out there can help me in measuring the test sites properly.
I also measured the 18v (CB723) test site but again, readings were 0. The other site I did not measure as I'm not sure what it is. What is a "P Test?"
Oh, and here is the factory service info for the 272:
View attachment hfe_nad_c272_service.pdf
Thanks in advance for any help you can give regarding this amp.
I have a NAD C272 amp which died recently and I'm trying to test it to determine what needs to be replaced for it to work again but I'm a bit of electronics beginner so bear with me! I've read a lot about problems with this model, mostly with the main capacitors and transistor Q712. Most of these issues are discussed or mentioned in this thread:
https://www.diyaudio.com/forums/solid-state/329113-nad-c272-refresh-5.html
However, my unit has the upgraded 100v capacitors and also the same updates as MaxMartin lists in the thread regarding his unit:
"I have myself a C272 of the newest batch from 2009. They’ve made some changes regarding transistors. Q710, 711 is 2240 and q712 is a970. On left and right boards there is now also 1220 and 2690 (input stage) like down in the +68v regulator. The 2240 pair and the 970 goes very hot still.. "
---MaxMartin
Apparently, NAD updated the later units with these modifications in response to problems the 272/372 series were having.
Ok, with that preface out of the way, the problem I have is the unit simply stopped working. No noise or other problems; it just has no output. The unit will power up meaning that the LED comes on when the power switch is pressed and it still clicks as it always did but there is no sound. I assume this points to the power board. I've looked closely at all parts inside the unit and have found no obvious problems or evidence of high temps. Everything looks normal.
So I tested the main power supply on the power board and it checked out fine. +68.7v and -68.6v in the appropriate places. However, there are also 2 more sets of test poles, one labeled "CB716, M- Test" and the other "CB717, M+ Test." I could not get a reading from either of these. And what does the CB716 signify? Every test site has a unique CB number.
Here are some pics in case I'm not describing in appropriate ways:
Here is the overview with the power board towards the top and the large 4 capacitors.
By the way, my power board is labeled as Version 5.0 as I guess there are different versions that NAD used to combat the capacitor and transistor issues.
At the top left is the site where I measured +68v and -68v so the board is getting the correct power. However, the other test site M- in the photo measured 0 as well as the M+ site in the following photo. I assume these should be -68v and +68v respectively but hopefully someone out there can help me in measuring the test sites properly.
I also measured the 18v (CB723) test site but again, readings were 0. The other site I did not measure as I'm not sure what it is. What is a "P Test?"
Oh, and here is the factory service info for the 272:
View attachment hfe_nad_c272_service.pdf
Thanks in advance for any help you can give regarding this amp.
Well, after the main +-68 power supply, the next check would be the +-18 v supplies at C744 and C745. Those or feed C742 C743 may have shorted, or feed cap C738 C739 or feed cap C765. If the load caps 744 or 745 shorted it may have taken out regulator IC 02 or 03.
There is also a 5 v supply at C709 with feed cap C708, either of those caps may have shorted or taken out IC 01.
Load caps C750 or C751 may have taken out +-18.
There are 3 electrolytic caps around the protection IC05 that may have sent it into protection mode.
Be sure to mark the board as to + terminal before replacing any electrolytic caps. I use a sharpie. I usually change 1 or 2 at a time then test to see if I made things better or worse. I make a high percentage of bad solder joint. If it gets worse after 1 or 2 parts, I know right where the problem is, what I just did.
I like to use caps with a service life over 3000 hours, so I don't have to do this job again in a couple of years. I can buy 10000 hours up in some of the smaller sizes. Lots of 500 hour caps on the shelves of TV parts stores. I like newark or digikey that has the service life spec in the selector table. Mouser makes you download the datasheet & read it on every cap in stock. Alliedelec only carries 1000 hour caps, I use them only for other parts. In Europe check RS or Reichelt.
I write a short date on top of new cap, last 2 of year usually.
BTW, if one cap is bad, I change them all. If you leave the old ones in they just fail one by one and the appliance is broken all the time. If you like it well enough to take the case off & put it back, it's worth refreshing to make reliable. If caps are new and you didn't make a bad joint, the only devices to go bad later are volume or tone pots, or oxidized connectors on low voltage connections. Doesn't look like a lot of those in this amp.
Basic safety for newbies: Don't use 2 hands to check voltages, >24 v from one hand to the other can stop your heart. Connect negative probe of DVM to analog ground with alligator clip lead with one hand, then use same hand to make tests.
Don't wear metal on hands wrists or neck, 1 v @ high current can burn your flesh to charcoal. Wear safety glasses, parts explode and solder splashes, especially desoldering.
Happy hunting.
There is also a 5 v supply at C709 with feed cap C708, either of those caps may have shorted or taken out IC 01.
Load caps C750 or C751 may have taken out +-18.
There are 3 electrolytic caps around the protection IC05 that may have sent it into protection mode.
Be sure to mark the board as to + terminal before replacing any electrolytic caps. I use a sharpie. I usually change 1 or 2 at a time then test to see if I made things better or worse. I make a high percentage of bad solder joint. If it gets worse after 1 or 2 parts, I know right where the problem is, what I just did.
I like to use caps with a service life over 3000 hours, so I don't have to do this job again in a couple of years. I can buy 10000 hours up in some of the smaller sizes. Lots of 500 hour caps on the shelves of TV parts stores. I like newark or digikey that has the service life spec in the selector table. Mouser makes you download the datasheet & read it on every cap in stock. Alliedelec only carries 1000 hour caps, I use them only for other parts. In Europe check RS or Reichelt.
I write a short date on top of new cap, last 2 of year usually.
BTW, if one cap is bad, I change them all. If you leave the old ones in they just fail one by one and the appliance is broken all the time. If you like it well enough to take the case off & put it back, it's worth refreshing to make reliable. If caps are new and you didn't make a bad joint, the only devices to go bad later are volume or tone pots, or oxidized connectors on low voltage connections. Doesn't look like a lot of those in this amp.
Basic safety for newbies: Don't use 2 hands to check voltages, >24 v from one hand to the other can stop your heart. Connect negative probe of DVM to analog ground with alligator clip lead with one hand, then use same hand to make tests.
Don't wear metal on hands wrists or neck, 1 v @ high current can burn your flesh to charcoal. Wear safety glasses, parts explode and solder splashes, especially desoldering.
Happy hunting.
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Many thanks for the possibilities and information above but how do I go about testing at this point? I've already tested +/-18v and found zero so that circuit is dead. I'm a complete beginner to the testing of capacitors, IC's, Transistors, diodes, et al so I either need instruction or maybe a beginner's guide to testing these parts, so if a thread on this forum exists that you can point me to, I'd appreciate it.
And, yes, I'm aware that I have no business being inside this amp but I want to learn. Thanks for the safety tips as I'm aware that there are dangers to poking around inside the amp and I'm still unsure of it all. (And speaking of two hands and such, I work in photography/film and once when working on a sound stage, I heard an electrician tell me all the wonders of his job and how he only uses his right hand with his right knee on the ground at all the times he's connecting cables as the goal is to not allow the current to travel through the heart which results in instant death. Yeah, scary stories in his field and not to be taken lightly, no pun intended!
Anyway, I found the IC's you mentioned; can I test them directly or are they easily damaged? Or do I test at some other point downstream of the IC? Is it likely that some IC's are dead? Will I be able to find replacements? I never hear of people replacing IC's in regard to this amp so not sure about the implications. You mention replacing caps and other parts and then checking if the unit works; this seems like a lot of trial and error. Can't the parts be tested before removing and being replaced?
Also, this amp also has +/-64v power; is this power circuit significant in my situation? I rarely hear it mentioned so just checking what that circuit does.
And some basic things I should know:
What is the ATO board?
What is the Insense or sensing board?
Well, if you use only one hand testing, and touch 300 v say, you get a tingle, not death. Whoops! Could have been a serious burn if you had a ring on. If you have a cut on your probe hand, wear rubber glove. Polyester is okay. Don't probe around the power switch or input to the transformer, 20 amps from the wall socket @ mains voltage is more serious, can cause melted copper to splash off your meter probe.
You already said +-64 were okay, and usually the small caps fail before the big rail ones. Electrolytic caps are the time fuses of electronics, parts they put in to make sure the appliance gets replaced at a time convenient to the vendor. Our goal is to not send everything to the dump the way the lords of commerce have designed the system.
There is no parts layout in the schematic you linked to, so I hope there are part names on the circuit boards. Electrolytic caps are labled for plus near one lead, or a minus in balls near the other, or they have NP after the voltage label, which means they are non-polar and go either way (rare). The board is not always labeled for polarity, which is why I suggest labeling the board before removal of an e-cap.
IC 1,2,3,5, have pinout drawings on page 5 the middle, which make them convenient place to test voltage. Voltage regulators are high failure parts especially if overloaded with leaky electrolytic caps. Be sure if reordering to check datasheet for package code the day you buy them. Manufacturers sold me a lot of half power ones which are fairly useless because they had a stupid package code that meant they were toys instead of 1 amp parts.
I wouldn't worry about output idle bias in the beginning on a dead unit. It is something you check & adjust at the end of your repairs, right before buttoning up. Bad idle adjustment pot can blow up the output transistors in your unit, but a totally dead unit, no +18, has more serious problems.
You can invest $50 in a bench supply , set up current meter of DVM in series with electrolytic caps, and test them after removal. As they cost about $.10 and are 90% sure one of them is leaky at age 20, I just replace the **** things, after proving the voltages they support are incorrect. The box from the distributor costs $8 shipping, that is the cost of a hundred small caps.
That other C272 thread natters on about fusible resistors somethere, that could have been installed to prevent stink when the electrolytic caps take them out. I don't see any 33 ohm resistors on here, but it is always possible somewhere between the bridge rectifier and the final filter cap like C734 C735. You probe around with the voltmeter to see if any voltages are missing and where , to find out what other damage has happened.
Note page 17 the protection IC has changed. Good, that TDA thing cannot be bought.
I don't know where the insense board is, but it will have S913 on it. Presumably this will be near the front or back panel so it can be reached, but if a Europe (220) North American switch (120 v) it could be inside.
I haven't found the test points on the schematic. Probably a couple of them are for testing the output transistor idle current, which is an important test at the end of work.
If the output transistors were bad, or had been bad, then they short & blow fuses, causing you to need a light bulb in series with the AC feed to slow down the current so you can test things. 60 W incandescent bulb usually. You may get away without having such a device on a product that blew the low voltage power first.
The ATO board has digital CMOS parts on it which do some sort of clock function. C807 or C809 on that could be leaky but the rest of it should be pretty reliable. If not you need a scope to find a problem with it. Your power supply problem can be debugged with a DVM only.
You already said +-64 were okay, and usually the small caps fail before the big rail ones. Electrolytic caps are the time fuses of electronics, parts they put in to make sure the appliance gets replaced at a time convenient to the vendor. Our goal is to not send everything to the dump the way the lords of commerce have designed the system.
There is no parts layout in the schematic you linked to, so I hope there are part names on the circuit boards. Electrolytic caps are labled for plus near one lead, or a minus in balls near the other, or they have NP after the voltage label, which means they are non-polar and go either way (rare). The board is not always labeled for polarity, which is why I suggest labeling the board before removal of an e-cap.
IC 1,2,3,5, have pinout drawings on page 5 the middle, which make them convenient place to test voltage. Voltage regulators are high failure parts especially if overloaded with leaky electrolytic caps. Be sure if reordering to check datasheet for package code the day you buy them. Manufacturers sold me a lot of half power ones which are fairly useless because they had a stupid package code that meant they were toys instead of 1 amp parts.
I wouldn't worry about output idle bias in the beginning on a dead unit. It is something you check & adjust at the end of your repairs, right before buttoning up. Bad idle adjustment pot can blow up the output transistors in your unit, but a totally dead unit, no +18, has more serious problems.
You can invest $50 in a bench supply , set up current meter of DVM in series with electrolytic caps, and test them after removal. As they cost about $.10 and are 90% sure one of them is leaky at age 20, I just replace the **** things, after proving the voltages they support are incorrect. The box from the distributor costs $8 shipping, that is the cost of a hundred small caps.
That other C272 thread natters on about fusible resistors somethere, that could have been installed to prevent stink when the electrolytic caps take them out. I don't see any 33 ohm resistors on here, but it is always possible somewhere between the bridge rectifier and the final filter cap like C734 C735. You probe around with the voltmeter to see if any voltages are missing and where , to find out what other damage has happened.
Note page 17 the protection IC has changed. Good, that TDA thing cannot be bought.
I don't know where the insense board is, but it will have S913 on it. Presumably this will be near the front or back panel so it can be reached, but if a Europe (220) North American switch (120 v) it could be inside.
I haven't found the test points on the schematic. Probably a couple of them are for testing the output transistor idle current, which is an important test at the end of work.
If the output transistors were bad, or had been bad, then they short & blow fuses, causing you to need a light bulb in series with the AC feed to slow down the current so you can test things. 60 W incandescent bulb usually. You may get away without having such a device on a product that blew the low voltage power first.
The ATO board has digital CMOS parts on it which do some sort of clock function. C807 or C809 on that could be leaky but the rest of it should be pretty reliable. If not you need a scope to find a problem with it. Your power supply problem can be debugged with a DVM only.
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First, thank you to Indianajo for all the time and help, I definitely appreciate it.
Ok, new information on voltage testing:
I have tested all of the voltage test sites on the power board and nearly all of them test as they should. I could have sworn earlier I was only seeing a working +/-68 voltage and all others were dead but whether I wasn't making contact or there was no voltage earlier, now there is. So in addition to the +/-68, I also have +/-18, +/-64 in the appropriate test sites. I don't know where to test for the 5v as there is no test site. And I also tested CB725 P TEST and found 1.38v but I'm not sure what this is supposed to be or what it is exactly.
Ok, new information on voltage testing:
I have tested all of the voltage test sites on the power board and nearly all of them test as they should. I could have sworn earlier I was only seeing a working +/-68 voltage and all others were dead but whether I wasn't making contact or there was no voltage earlier, now there is. So in addition to the +/-68, I also have +/-18, +/-64 in the appropriate test sites. I don't know where to test for the 5v as there is no test site. And I also tested CB725 P TEST and found 1.38v but I'm not sure what this is supposed to be or what it is exactly.
Ok, even further development and testing....
So after finding all the power supply tests were where they should be, I had the strange feeling that just maybe it had fixed itself and worked now. And do you know what? I was finally right for once in my life and the crazy thing works fine.
The only strange thing that happened in all of this is that once when I was testing the 18v and finding zero, I accidentally shorted the posts or something weird happened where there was a pop and a spark. I was worried I had damaged something but now I find the amp works so not sure if that did something or what might be going on here.
If you guys have any ideas, I'd love to hear them because I'm already fearing that whatever happened to cause the amp to go dead will only happen again.
Thanks, but for now, I have 150wpc of wonderful power again!
So after finding all the power supply tests were where they should be, I had the strange feeling that just maybe it had fixed itself and worked now. And do you know what? I was finally right for once in my life and the crazy thing works fine.
The only strange thing that happened in all of this is that once when I was testing the 18v and finding zero, I accidentally shorted the posts or something weird happened where there was a pop and a spark. I was worried I had damaged something but now I find the amp works so not sure if that did something or what might be going on here.
If you guys have any ideas, I'd love to hear them because I'm already fearing that whatever happened to cause the amp to go dead will only happen again.
Thanks, but for now, I have 150wpc of wonderful power again!
The instructions on what to do with CB725 are on page 3. Unfortunately you won't have a 4 ohm 150 w load, nor a 20 khz signal generator. So leave it alone. I don't even have a 20 khz signal generator.
I would check DC voltage on speaker jacks at idle silent. This is equivalent to TP2 & 5. Should be <30 mv they say. 200 mv in general. Adjust VR301 & 401 as they say. If greater than 200 mv something may be leaking somewhere and need replacing.
One thing that could have caused silence is oxidized connection on low voltage signals. With power off I would reseat 9 pin CZ61 from input board. I would spray out switches 705 A B C & D with contact cleaner from the lumberyard. CRC is one brand. Also the volume pot, probably RV1A & B. Warning contact cleaner is flammable, no smoking, open flame, pilot lights, turning electric switches on or off, within 10 m. Set up a fan previous to spray to dissipate the fumes. Test contact cleaner previous to use on cheap PVC plastic (not the amp) to make sure you don't have a brand that dissolves cheap plastic. Like a cheap toy.
Volume pots do wear out and have dropouts, so if you find this silence to be chronic you may have to change it. Usually one side or the other, not both at once.
Then before putting the case on I would check Idle bias current via procedure on page 3. Spraying out the idle bias pots could also be useful.
Happy listening to your working amp.
I would check DC voltage on speaker jacks at idle silent. This is equivalent to TP2 & 5. Should be <30 mv they say. 200 mv in general. Adjust VR301 & 401 as they say. If greater than 200 mv something may be leaking somewhere and need replacing.
One thing that could have caused silence is oxidized connection on low voltage signals. With power off I would reseat 9 pin CZ61 from input board. I would spray out switches 705 A B C & D with contact cleaner from the lumberyard. CRC is one brand. Also the volume pot, probably RV1A & B. Warning contact cleaner is flammable, no smoking, open flame, pilot lights, turning electric switches on or off, within 10 m. Set up a fan previous to spray to dissipate the fumes. Test contact cleaner previous to use on cheap PVC plastic (not the amp) to make sure you don't have a brand that dissolves cheap plastic. Like a cheap toy.
Volume pots do wear out and have dropouts, so if you find this silence to be chronic you may have to change it. Usually one side or the other, not both at once.
Then before putting the case on I would check Idle bias current via procedure on page 3. Spraying out the idle bias pots could also be useful.
Happy listening to your working amp.
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And the problem is back! The amp worked great for a few days and now it's going into protection mode. If I turn it off and then back on, it works for between 30 seconds and a couple of minutes and then goes off with the main power led turning red which I believe signifies protection. I also tried going into the variable inputs and playing around with the sensing switch but it still goes off. The last time it switched off I was increasing volume on the pre amp. it also switched off once when I jiggled the RCA input connectors. And once again when I had it connected to the variable inputs and slightly increased the input level. And at no time during all of this was I driving the amp hard; in fact I was listening to tv input at normal to probably lower than normal volumes.
One more thing which may or may not be important is that when it was working over the past few days, I would hear a strange sound or blip which sounded like when an early cell phone would ring near a radio/stereo and you would hear that strange electronic distortion-do you remember that? I'm referring to back 20 years or so. I don't think cell phones do this anymore or at least I haven't heard it in awhile.
I have not done any of the things Indianajo mentioned in the post directly above this; I was waiting to see if the amp would continue working. I will check the DC offset and report back.
Not sure if any of this gives any new information. I guess it does point to the protection circuit though.
One more thing which may or may not be important is that when it was working over the past few days, I would hear a strange sound or blip which sounded like when an early cell phone would ring near a radio/stereo and you would hear that strange electronic distortion-do you remember that? I'm referring to back 20 years or so. I don't think cell phones do this anymore or at least I haven't heard it in awhile.
I have not done any of the things Indianajo mentioned in the post directly above this; I was waiting to see if the amp would continue working. I will check the DC offset and report back.
Not sure if any of this gives any new information. I guess it does point to the protection circuit though.
If the DC out is >200 mv when protect LED is on, then protect circuit is acting correctly. Cause of DC out must be investigated & corrected.
If DC out is <30 mv when protect LED is on, Then I woulld look at age of C754 C755 C 756 C758 around IC5. D718 can also fail, but you can check its forward continuity with a DVM. Electrolytic capacitors you could check with a $120 peak ESR meter after removal, but I prefer to use the calender test on electrolytic caps. > 10 years old, circuit doesn't work, out they go.
Blip noise is not a recognized diagnostic. Clicking of protect relay is. Should click in after power up delay, then stay in, no more clicking.
If DC out is <30 mv when protect LED is on, Then I woulld look at age of C754 C755 C 756 C758 around IC5. D718 can also fail, but you can check its forward continuity with a DVM. Electrolytic capacitors you could check with a $120 peak ESR meter after removal, but I prefer to use the calender test on electrolytic caps. > 10 years old, circuit doesn't work, out they go.
Blip noise is not a recognized diagnostic. Clicking of protect relay is. Should click in after power up delay, then stay in, no more clicking.
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Thanks for the thorough answer.
I measured the DC offset and found .02v in both channels. I left the amp on for quite awhile measuring these numbers and it never went to protection mode.
Also, I measured via the speaker outputs and not on the amp boards. I would think this doesn't matter but I'm not sure.
Should I try measuring DC offset with some input from the preamp? And keep measuring as the amp goes into protection?
In alignment procedure, there is some voltage ranges. Measure the voltages, are they in the range or changing in seconds?
Like that; you adjusted the 30mv, when you start the amp. it may alternate to 100mv. if so, the amp. will be closed. Your adjustment must be less than 30ma.
This is the one of the way to go protection mode.
Like that; you adjusted the 30mv, when you start the amp. it may alternate to 100mv. if so, the amp. will be closed. Your adjustment must be less than 30ma.
This is the one of the way to go protection mode.
Ok, I also set the idling current as described in the service manual where it says to measure mv at TP1 & TP3 while adjusting VR302 for L and TP4 & TP6 while adjusting VR402 for R to 6-7.5mv. After adjusting, I reinstalled the amp and have been listening for a couple hours now with no problems.
The DC voltage went down slightly after this, down to around .015or so.
Not sure if this is affecting the protection circuit from blowing or even what the idling current even does but if it works, it works! Of course, I've been down this road before so we'll see....
The DC voltage went down slightly after this, down to around .015or so.
Not sure if this is affecting the protection circuit from blowing or even what the idling current even does but if it works, it works! Of course, I've been down this road before so we'll see....
It's been a few days now and the amp has been working well so thanks to Indianjo and others who helped me with this. However, one small problem I'm having now is that the sensing circuit isn't as sensitive as it used to be and when using the amp with TV and the volume is low, the amp will turn off. I don't see anyway to adjust the sensitivity so I'm not sure why the amp shuts off now with lower volumes when it never did in the past. Is there a way to adjust or solve this problem?
If not, I'm still happy that the amp appears to be working properly otherwise. I just like the sensing circuit since that means one less button to turn on and off.
Thanks!
If not, I'm still happy that the amp appears to be working properly otherwise. I just like the sensing circuit since that means one less button to turn on and off.
Thanks!
No, I'm referring to the input sensing circuit which senses input at the amp RCA connectors and then turns the amp on. It then switches the amp off after 5 minutes of no signal. However, my amp shuts off when the input signal is low, such as when I listen to low volume settings.
Here is the official NAD description:
"The SLEEP/WAKE, SENSE/DEFEAT switch logic controls the
standby/on-state of the amplifier via the presences or absence of audio
signal at the amplifier channel inputs. The SLEEP/WAKE,
SENSE/DEFEAT switch must be in the SLEEP/WAKE position in order to
use this logic. When the SLEEP/WAKE, SENSE/DEFEAT switch is in the
SENSE/DEFEAT position, this logic control is deactivated.
When the switch is in the SLEEP/WAKE position, the C 272 amplifier will
instantaneously turn on from a standby state, sensing any input signal
from any channel as seen by a lit green SENSE LED on the front panel of
the amplifier (approximately above 20mV RMS input). If all of the audio
signals are absent for approximately 5 minutes, the amplifier will switch
automatically to standby condition, with the green SENSE LED off, and
the amber LED over the front panel switch lit."
I think the sensing circuit is misreading the min 20mV input and for some reason not turning on the amp until a much higher input is sensed. Consequently, the amp will also shut off when the input is below this erroneously elevated level.
Hi Guys, just got my C272 running, thanks to you!. My amp was stuck in protect (red LED). R727 and R723 were mixed up. I would not have believed it if I didn't have an identical amp for comparison! Amazingly, this was not the problem as they are in the +68 side of the power supply. I was missing the -68 V side. I replaced Q712 (tested as 'dual diode') and Q709 (shorted). All seems to be good now, 180 WPC into 8 ohms. I will let it run overnight.
Thanks again!
Peter
Thanks again!
Peter