No problem
A possible interesting result then... suggesting that maybe the front end is compensating for an imbalance in the output stage. Hmmm, I have some ideas on that but you might not like it
It's a case of proving things one way or the other.See what those other readings show first.
No problem
A possible interesting result then... suggesting that maybe the front end is compensating for an imbalance in the output stage. Hmmm, I have some ideas on that but you might not like it
See what those other readings show first.
I'm intrigued
Ok for the other points..
left channel:
between R341/344 => -5.07V
between R332/334 => 4.97V
right channel
between R441/444 => -5.07V
between R434/432 => 4.92V
Those are all very close which in a way is good. You see the action of negative feedback that is wrapped around the whole amplifier circuit can correct for imbalance and that imbalance shows up as the voltage on R428 and on the current flowing through R428. The front end does what is needed to keep the output at zero volts and it can do that even if there is a minor issue in the output stage.
The earlier measurements I'm going to say suggest that they show that the front end is trying to pull the output back to zero volts when the natural DC condition of the output stage is keep wanting to create a small offset.
What you won't like is what I propose we do next
Working on the theory that only the output stage was affected I'm going to suggest this...
1/ Look at the markings of the original devices fitted for Q317 and Q318. Just see what the other channel has fitted. They were 2SD669A and 2SB649A on the diagram.
Are there any other letters after them such as B or C which are gain groups.
2/ I also wonder over Q313 and Q314 which I don't think you replaced and yet we know that one of the resistors connected to them got frazzled.
This is the bit you won't like... one way or another we have to prove or otherwise that the problem is simply one of imbalance and that when operating normally the stage is fine but when powering up the imbalance requires the front end to correct this. This 'difference' in current flows in the driver stages will heat the transistor junctions at different rates and so alter the balance of the stage... which is no problem in normal use. The gain of a transistor alters dramatically with temperature.
When switched off the imbalance makes its presence felt again by suddenly needing a correction and that correction appears as a noise at power off.
So lets look at swapping Q317 and Q318 for different devices. Do you see Q709 and Q712 in the power supply?
Do these 2SD/2SB devices have identical markings to those used in the good channel?
We could swap these for the Q317/318.
That puts the replacement devices back as far as possible to original spec.
If that shows no improvement then we look to replacing Q313 and Q314 which are a bit of an unknown condition wise. It's very unusual for a transistor to partly survive an overload, usually they fail short and thats it but it is possible one of the pair got 'impaired' in some way due to the overload that we know happened (because that 100 ohm went up).
You could swap those with the other channel as a test.
The earlier measurements I'm going to say suggest that they show that the front end is trying to pull the output back to zero volts when the natural DC condition of the output stage is keep wanting to create a small offset.
What you won't like is what I propose we do next
Working on the theory that only the output stage was affected I'm going to suggest this...
1/ Look at the markings of the original devices fitted for Q317 and Q318. Just see what the other channel has fitted. They were 2SD669A and 2SB649A on the diagram.
Are there any other letters after them such as B or C which are gain groups.
2/ I also wonder over Q313 and Q314 which I don't think you replaced and yet we know that one of the resistors connected to them got frazzled.
This is the bit you won't like... one way or another we have to prove or otherwise that the problem is simply one of imbalance and that when operating normally the stage is fine but when powering up the imbalance requires the front end to correct this. This 'difference' in current flows in the driver stages will heat the transistor junctions at different rates and so alter the balance of the stage... which is no problem in normal use. The gain of a transistor alters dramatically with temperature.
When switched off the imbalance makes its presence felt again by suddenly needing a correction and that correction appears as a noise at power off.
So lets look at swapping Q317 and Q318 for different devices. Do you see Q709 and Q712 in the power supply?
Do these 2SD/2SB devices have identical markings to those used in the good channel?
We could swap these for the Q317/318.
That puts the replacement devices back as far as possible to original spec.
If that shows no improvement then we look to replacing Q313 and Q314 which are a bit of an unknown condition wise. It's very unusual for a transistor to partly survive an overload, usually they fail short and thats it but it is possible one of the pair got 'impaired' in some way due to the overload that we know happened (because that 100 ohm went up).
You could swap those with the other channel as a test.
Really hard to see but...
On the D669A which is printed on the middle - underneath that to the right it has 'WC' - and under that it has 'O' on the left and 'C1' on the right.
On the B649A which is printed on the middle - underneath that to the right it has 'C' - and under that it has 'O' on the left and 'G1' on the right.
Not identical.
Q712 B649A is printed on the middle - underneath that it has 'C' - and underneath that it has '2' on the left and 'D1' on the right.
Q709 D669A is printed on the middle - underneath that it has 'WC' - and underneath that it has '2' on the left and 'C1' on the right.
Working on that now - fiddly swapping these bits
...Will try this if there is no difference
Update!
After swapping those transistors from the power board...
left channel
power on...max => 0.008V ... min => -0.032V
power off...max => 0.216V ... min => -0.036V
right channel
power on...max => 0.012V ... min => -0.088V
power off...max => 0.004V ... min => -0.160V
This is looking promising...time to check with speakers....
After swapping those transistors from the power board...
left channel
power on...max => 0.008V ... min => -0.032V
power off...max => 0.216V ... min => -0.036V
right channel
power on...max => 0.012V ... min => -0.088V
power off...max => 0.004V ... min => -0.160V
This is looking promising...time to check with speakers....
...doh!
I forgot to put the jumpers back in from pre => main (too many parts for me to remember so late at night after taking so much apart)
once more audio sprang forth
no more pops on power on/off!
You're a genius Mooly! Thank you so much!
So those fairchild transisitor replacements were not right for the job - they can only be used in place of those in the power supply?
I forgot to put the jumpers back in from pre => main
(too many parts for me to remember so late at night after taking so much apart)once more audio sprang forth
no more pops on power on/off!
You're a genius Mooly! Thank you so much!
So those fairchild transisitor replacements were not right for the job - they can only be used in place of those in the power supply?
Fantastic LOL I've surprised myself with that one and have you seen how many views the thread has attracted in such a short time
Two ways of looking at it... those Fairchild devices should be absolutely suitable and yet it seems they are not... at least here.
We could turn that around and say that the amp design is a bit to specific on device characteristics/matching of devices etc...
or,
We did have another option available as well. We knew the amp seemed OK once it was on for a few seconds and so we could have looked at increasing the speaker delay time a little by increasing the cap used to set the delay. That is C784 I think.
We could also have looked at decreasing the cap used to detect the presence of AC in an attempt to get the relay to drop out even faster at power off (C783).
NAD will have tried to make the delay as short as possible because no one likes to wait for the amp to 'warm up' and they will also make the drop out fast but not that fast that a few missing cycles cause the relay to drop unexpectedly such as might occur in areas with a really poor mains supply that is subject to 'brownouts'.
So ultimately nothing really wrong, just an unfortunate set of circumstances where we got caught in the maze of substituting old obsolete devices.
Brownout (electricity) - Wikipedia)
So now you have to set the bias correctly, remember to finalise that on full mains and do not be tempted to set it to high. Check when the amp has reached normal temperature after having been on a good while.
LOL I've surprised myself with that one and have you seen how many views the thread has attracted in such a short time Two ways of looking at it... those Fairchild devices should be absolutely suitable and yet it seems they are not... at least here.
We could turn that around and say that the amp design is a bit to specific on device characteristics/matching of devices etc...
or,
We did have another option available as well. We knew the amp seemed OK once it was on for a few seconds and so we could have looked at increasing the speaker delay time a little by increasing the cap used to set the delay. That is C784 I think.
We could also have looked at decreasing the cap used to detect the presence of AC in an attempt to get the relay to drop out even faster at power off (C783).
NAD will have tried to make the delay as short as possible because no one likes to wait for the amp to 'warm up' and they will also make the drop out fast but not that fast that a few missing cycles cause the relay to drop unexpectedly such as might occur in areas with a really poor mains supply that is subject to 'brownouts'.
So ultimately nothing really wrong, just an unfortunate set of circumstances where we got caught in the maze of substituting old obsolete devices.
Brownout (electricity) - Wikipedia)
So now you have to set the bias correctly, remember to finalise that on full mains and do not be tempted to set it to high. Check when the amp has reached normal temperature after having been on a good while.
Fantastic
Wow - I hadn't checked the number of views before now, that's astonishing! I'm amazed at how you managed to diagnose all this from a distance. I hope that this thread can help somebody else in the future. I think there are still quite a few of these amps in the wild, they were once fairly popular being pretty decent value for money.
Interesting, you even had some other strategies lined up!
Yes, will get that done today - tune the offset as close to zero and get the bias around 5mV
Thank you so much for all the help Mooly
- you have been extraordinarily kind and generous with your time and expertise. You saved me and taught me a lot in the process. Please let me send you a token of appreciation - pm me your paypal or something.
Thanks for the kind words... if you really want to give (and its very kind of you) then please donate to a charity or perhaps support the forum (donations, top line of the page)
But thanks, its appreciated
Successful fault finding as you can probably tell by now is really all about gathering as much evidence as possible and to try and reason through what may or may not have happened.
When you set the DC offset remember that small amounts don't cause a problem. We used to reckon on anything less than 100mv was OK years ago.
If you calculate how much current 0.1 volts would generate in an 8 ohm speaker its nothing (12.5 milliamps)
So you can 'fiddle' this to your advantage if needed. Why not set the offset to be zero at the instant the relay closes, that way you get a truly silent switch on.
And set the bias with the speakers disconnected. That avoids any current flowing in the speaker (from a DC offset) affecting the result.
But thanks, its appreciated
Successful fault finding as you can probably tell by now is really all about gathering as much evidence as possible and to try and reason through what may or may not have happened.
When you set the DC offset remember that small amounts don't cause a problem. We used to reckon on anything less than 100mv was OK years ago.
If you calculate how much current 0.1 volts would generate in an 8 ohm speaker its nothing (12.5 milliamps)
So you can 'fiddle' this to your advantage if needed. Why not set the offset to be zero at the instant the relay closes, that way you get a truly silent switch on.
And set the bias with the speakers disconnected. That avoids any current flowing in the speaker (from a DC offset) affecting the result.
Thanks for the kind words... if you really want to give (and its very kind of you) then please donate to a charity or perhaps support the forum (donations, top line of the page)
But thanks, its appreciated
Successful fault finding as you can probably tell by now is really all about gathering as much evidence as possible and to try and reason through what may or may not have happened.
When you set the DC offset remember that small amounts don't cause a problem. We used to reckon on anything less than 100mv was OK years ago.
If you calculate how much current 0.1 volts would generate in an 8 ohm speaker its nothing (12.5 milliamps)
So you can 'fiddle' this to your advantage if needed. Why not set the offset to be zero at the instant the relay closes, that way you get a truly silent switch on.
And set the bias with the speakers disconnected. That avoids any current flowing in the speaker (from a DC offset) affecting the result.
Very well, I shall do as you say on all counts
Thank heavens for people like you!
Have a great weekend!
Thanks
And you...
(and just to wrap this one up I would say it's worth looking over all the work done, make sure everything is fitted correctly, no bits of wire or bits of test links left tagged on the board anywhere... it's so easy to forget all that has been done and overlook something)
And then enjoy
And you...
(and just to wrap this one up I would say it's worth looking over all the work done, make sure everything is fitted correctly, no bits of wire or bits of test links left tagged on the board anywhere... it's so easy to forget all that has been done and overlook something)
And then enjoy
Mooly - when Houston had a problem they should have called you ! legend !
Really fabulous (and entertaining!) so thank you both Jam and Moly for this and for the happy ending
Legend is right! Mind-boggling how he managed to diagnose it all from a distance. It's not just that he could either - it's that he would. What a gem!
Completely agree - we are so fortunate to have a Mooly amongst us.
Thanks guys
So kind of you to say. I have to admit that a part of me is actually sad that the amp is now fixed! It was a really fun and rewarding experience to be tutored by you. I learned so much, from the use and construction of a dim bulb tester to the basic steps to take when fault finding. Some of the insights you made are clearly things that require a much higher level of circuit analysis to be able to understand but it was a delight to witness a master do his thing.
Hmm, I'm almost looking forward to the next time I mess something up - but for your sakes I will try my best not to
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I have a NAD C370 from year 2000 that finally that gave up. Retired as I am now I have time to re-use the good old am and found this great thread to follow! Reading it over several times and made many improvements stepwise the amp still don't work.
I would be very happy for advice how to proceed. At this stage I can set the DC and bias on each channel separate to the correcta value. However as soon as I connect the other side flat cabel CZ302/402 the DC jumps up to around 500mV on both. After some seconds the relay disconnect the speakers. If I leave out the cable for one channel it will power up correctly.
I have cahnged all e-caps and resolder most of the joints.
I would be very happy for advice how to proceed. At this stage I can set the DC and bias on each channel separate to the correcta value. However as soon as I connect the other side flat cabel CZ302/402 the DC jumps up to around 500mV on both. After some seconds the relay disconnect the speakers. If I leave out the cable for one channel it will power up correctly.
I have cahnged all e-caps and resolder most of the joints.
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