I have a NAD T742 that is has an issue when it heats up. When the amp heats up enough, distortion rather than audio will output on all speaker in any of the DSP modes.
This does not happen with 2ch stereo mode. I traced the problem to the AC3 board, specifically what is happening is the components responsible for the 5V supply to the dsp are heating up and causing the problem.
I found a NAD service bulletin (for the T761 but still relevant) that talked about changing a 85 degree cap to a 105 degree cap because at higher temps the lower-rated cap wasn't filtering the voltage ripple.
Here's the Problem:
The 85 degree cap was swapped for a 105 degree cap by a NAD service technician. I don't know if it completely fixed the problem but the amp was trouble free for about 2 months. It just recently started making the noise again.
So I'd thought I try to fix it myself this time a save a few bucks (paid $109 last time)
I power cycled the amp to have it stop making the distortion noise, it was clean souding when the measurements were taken
I checked the voltage on the components:
- Capacitor C107
- 5V 7805 Regulator U123
- Diode Bridge Rectifier RB152
Results
---------
Vcap = 8.12 - 8.18 V (it's a 16V cap so I don't think it's broken)
Vrectifier (DC)= 8.7V (soon as I touched it to measure the voltage the distortion noise started, I also noticed it was very hot to the touch)
Vinput_regulator = 7.75V
Voutput_regulator = 4.99V (regulator appears to be functioning fine)
So I believe the culprit is the bridge rectifier. The trouble is I'm not sure if something further up the daisy chain could be causing the problem or whether it is only the rectifier's fault (an easy fix if this is the case)
Any ideas or insights?
I wanted to post the entire t742 service manual but it is too big (5mb) I'll post the relevant sections instead
This does not happen with 2ch stereo mode. I traced the problem to the AC3 board, specifically what is happening is the components responsible for the 5V supply to the dsp are heating up and causing the problem.
I found a NAD service bulletin (for the T761 but still relevant) that talked about changing a 85 degree cap to a 105 degree cap because at higher temps the lower-rated cap wasn't filtering the voltage ripple.
Here's the Problem:
The 85 degree cap was swapped for a 105 degree cap by a NAD service technician. I don't know if it completely fixed the problem but the amp was trouble free for about 2 months. It just recently started making the noise again.
So I'd thought I try to fix it myself this time a save a few bucks (paid $109 last time)
I power cycled the amp to have it stop making the distortion noise, it was clean souding when the measurements were taken
I checked the voltage on the components:
- Capacitor C107
- 5V 7805 Regulator U123
- Diode Bridge Rectifier RB152
Results
---------
Vcap = 8.12 - 8.18 V (it's a 16V cap so I don't think it's broken)
Vrectifier (DC)= 8.7V (soon as I touched it to measure the voltage the distortion noise started, I also noticed it was very hot to the touch)
Vinput_regulator = 7.75V
Voutput_regulator = 4.99V (regulator appears to be functioning fine)
So I believe the culprit is the bridge rectifier. The trouble is I'm not sure if something further up the daisy chain could be causing the problem or whether it is only the rectifier's fault (an easy fix if this is the case)
Any ideas or insights?
I wanted to post the entire t742 service manual but it is too big (5mb) I'll post the relevant sections instead
are you sure that the soldering is solid on that filter cap? That 8.something volts may be lower than expected. It might be cause the filter looks open, due to poor soldering, etc. It also may be that the cap is mechanically intermittent, where a little physical movement is causing it to re-connect. Hmmm, the rectifier is getting hot? It could be leaky, causing the filter to fail prematurely. Good time for an o'scope.
mrshow4u,
The joints look good, I believe the .8 voltage difference is being cause by the rectifier. I checked everything the moment the distortion happened and only the rectifier voltage change which is likely the reason why there is a voltage difference on the filter cap.
Looking at the spec sheet for the rectifier it says it's forward voltage should be 1.0V and I'm getting 8.7V so I definately think the rectifier is hooped just not sure if anything before the rectifier is causing this or if the rectifier is the only component at fault here.
If you look at the AC3Board__1 & 2 photos you'll see that the input of the rectifier should be 8V
The joints look good, I believe the .8 voltage difference is being cause by the rectifier. I checked everything the moment the distortion happened and only the rectifier voltage change which is likely the reason why there is a voltage difference on the filter cap.
Looking at the spec sheet for the rectifier it says it's forward voltage should be 1.0V and I'm getting 8.7V so I definately think the rectifier is hooped just not sure if anything before the rectifier is causing this or if the rectifier is the only component at fault here.
If you look at the AC3Board__1 & 2 photos you'll see that the input of the rectifier should be 8V
I didn't mean anything about o.8 volts. I mean't to say that 8 volts or so read by a DC voltmeter might indicate that there is substantial ripple. Possibly due to an open filter (cap). Can you post the schematic of that power supply? Is there any indication of what AC voltage is feeding the rectifier? The rectified and filtered AC voltage should show up as about 15% higher DC voltage on the filter. I'm assuming this cap is pre 5 volt regulator that feeds the Dolby Digital processing IC. Be careful how your going about measuring the Forward Voltage (Vf) of the bridge. If you're reading 8 volts "Vf", it's most likely you're measuring the rectified voltage and not Vf. Try setting your voltmeter to AC volts and measure across the filter cap when it's in it's failure mode. You shouldn't get more than a volt (I'm guessing) af AC voltage. If you are measuring more than that, I think the cap is open
there are only a few dollars worth of components were talking about.. mabey it would be worth just replaceing the three of them and hopeing that fixes it? lol.. not the most scientific aproach i know but your right, it does sound like the regulator is dodgy to me, and then i wouldnt really want to trust the filter cap either.. so i'd replace them both if it was my amp.. and i'd probably replace the regulator aswell because they are cheap and i have loads of them lying around.. This is probably not the right way to do it though, you should wait and see what other people suggest, mabey im missing something..
Owen
edit: i mean it sounds like the rectifier is dodgy.. not regulator.. teaches me for posting so late at night..lol
Owen
edit: i mean it sounds like the rectifier is dodgy.. not regulator.. teaches me for posting so late at night..lol
mrshow4u,
I checked the input voltage on the rectifier it's supposed to be 8V according to the schematic (you'll see 2 pins labelled AC8V) those are connected to the AC inputs of the rectifier.
Since they don't specify the voltage orientation on the input side depending how I connect the lead from my volt meter (on AC setting) I get either 7.8 or 8.5 V (seems reasonable to me)
I did what you suggested and tested the DC side of the rectifier with the volt meter on AC setting. The voltage measured 1V for about 2 seconds then it shot up to 16.3 volts and remained there. You mentioned in your post that if the voltage isn't 1v the cap is open (hooped?) so most likely both the rectifier and the cap are broken?
DoomPixie,
I know they are only a few dollars in materials but I want this problem to go away forever and not be bothered with it anymore
I checked the input voltage on the rectifier it's supposed to be 8V according to the schematic (you'll see 2 pins labelled AC8V) those are connected to the AC inputs of the rectifier.
Since they don't specify the voltage orientation on the input side depending how I connect the lead from my volt meter (on AC setting) I get either 7.8 or 8.5 V (seems reasonable to me)
I did what you suggested and tested the DC side of the rectifier with the volt meter on AC setting. The voltage measured 1V for about 2 seconds then it shot up to 16.3 volts and remained there. You mentioned in your post that if the voltage isn't 1v the cap is open (hooped?) so most likely both the rectifier and the cap are broken?
DoomPixie,
I know they are only a few dollars in materials but I want this problem to go away forever and not be bothered with it anymore
there are two pins for AC in on the rectifier.. being AC you should get the same reading no matter which way you hook your multimeter up to it, you should be measuring accross these two pins and not from ground.
I reacon that the bridge rectifier is at fault.. and possibly the filter cap aswell..
when you measured 16VAC on the output of the rectifier what was the AC voltage on its inputs? seems a little suspicious that theres only 8V AC going in but you get 16V AC out.. :S
I reacon that the bridge rectifier is at fault.. and possibly the filter cap aswell..
when you measured 16VAC on the output of the rectifier what was the AC voltage on its inputs? seems a little suspicious that theres only 8V AC going in but you get 16V AC out.. :S
Yeah, the cap sounds "hooped". That 16 VAC should not be there. Now there's a caveat. I've replaced lot's of those round packaged bridges. Mostly for going open. But if the one you have has gone leaky on any of the diodes, it might be applying a reverse voltage on that filter cap. That would definitely reduce the life of that cap. You could remove the rectifier and perform a diode check (forward and backwards) on each of the four diodes. I'd expect 0.6 ~0.7 volts Vf (forward voltage) and open reverse biased. If you get any reading measuring reverse voltage, toss the rectifier.
I don't think a larger capacitance would do very much here. I would get a high temp cap as NAD suggests and I'd get a low ESR cap. This might help keep the caps heating a little lower. It sounds like this supply feeds a digital circuit. The currents drawn by digital circuits have a large high frequency component. Try to find a cap that is designed to filter digital circuits. Low ESR is one spec you'll wan't. High temp is another. Also try to mount the cap away from temperature generating stuff like the regulator. But try to keep the leads pretty short. You don't want them to "look" too inductive to maintain good high frequency filtering performance.
Hey Dis, These aren't ideal (they aren't through hole), but I think they'd work well for your application: http://www.panasonic.com/industrial/components/pdf/ABA0000CE101.pdf
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