I need help with an old NAD319 amp 
. I have attached an excerpt of the schematic which is referenced heavily in this post. Can attach the full service manual if needed - i wasn't sure if that was allowed.
So - R202, one of the fusible resistors was blown. After checking the board over and testing for any obvious shorts/failed components, I decided to temporarily replace it with a 10ohm non-fusing resistor, so that i could try to find what was heating up and drawing too much current through it. The culprit appeared to be C211, which was heating up and swelling up gradually once the unit was powered. The cap tested fine out of circuit.
Anyways, I replaced it with a new cap, and unfortunately this cap does exactly the same thing! I have checked all the diodes and little ceramic caps around c211 and all seem fine. Also checked the ceramic caps around the bridge rectifier and no issues there. I've also checked the 6 4700uf caps on the positive side of C211.
I believe I have tested every possible component which could solely cause a lot of current to pass through C211. The curious thing is - if the problem is elsewhere in the circuit, I would expect either C210 or C213 to be heating up too, which they are not.
I have scanned the board many times and can't see any solder bridges or anything else which might cause this to happen.
Important notes...
1. The test point +67v over to the far left of the picture below is measuring well over +100v (i say well over because it keeps rising and i'm only leaving the amp on for a few seconds at a time).
2. The test point -67v over to the far left of the picture is measuring about -16v. This indicates to me that whatever is drawing too much power away from that line and through C211 is not a total short.
3. If i test the 4 pins of the bridge rectifier, I get perfect voltage readings, meaning the problem definitely starts after that.
4. The mains fuse was not blown and isn't blowing, none of the other fusing resistors are blowing other than R202.
. I have attached an excerpt of the schematic which is referenced heavily in this post. Can attach the full service manual if needed - i wasn't sure if that was allowed.So - R202, one of the fusible resistors was blown. After checking the board over and testing for any obvious shorts/failed components, I decided to temporarily replace it with a 10ohm non-fusing resistor, so that i could try to find what was heating up and drawing too much current through it. The culprit appeared to be C211, which was heating up and swelling up gradually once the unit was powered. The cap tested fine out of circuit.
Anyways, I replaced it with a new cap, and unfortunately this cap does exactly the same thing! I have checked all the diodes and little ceramic caps around c211 and all seem fine. Also checked the ceramic caps around the bridge rectifier and no issues there. I've also checked the 6 4700uf caps on the positive side of C211.
I believe I have tested every possible component which could solely cause a lot of current to pass through C211. The curious thing is - if the problem is elsewhere in the circuit, I would expect either C210 or C213 to be heating up too, which they are not.
I have scanned the board many times and can't see any solder bridges or anything else which might cause this to happen.
Important notes...
1. The test point +67v over to the far left of the picture below is measuring well over +100v (i say well over because it keeps rising and i'm only leaving the amp on for a few seconds at a time).
2. The test point -67v over to the far left of the picture is measuring about -16v. This indicates to me that whatever is drawing too much power away from that line and through C211 is not a total short.
3. If i test the 4 pins of the bridge rectifier, I get perfect voltage readings, meaning the problem definitely starts after that.
4. The mains fuse was not blown and isn't blowing, none of the other fusing resistors are blowing other than R202.
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All of those test fine out of circuit, but I'm happy to replace them to confirm.
Both of those test fine out of circuit - replace anyways?
Yes, how are you testing these parts?
I would replace C206 207 210 211 212 213 and related diodes.
Check R201, 202, 203, 204.
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"1. The test point +67v over to the far left of the picture below is measuring well over +100v"
Where do you have the negative voltmeter lead connected? To get 100 volts on the 67 volt buss would require;
1. CT of transformer open - open black wire AC0, use your ohmmeter to confirm transformer center tap is intact and properly connect to the system ground (0V, SPOR, SPOL, OL, OR)
2. Failure in the +67 volt regulator
Note that the -67 volt line is actually regulated via a pair of zener diodes (D208, D209) and the +67 volt line is regulated as a mirror of the - 67 v.
For test purposes, lift one end of R203 and R204. Then measure the voltages across C210 and C211.
Where do you have the negative voltmeter lead connected? To get 100 volts on the 67 volt buss would require;
1. CT of transformer open - open black wire AC0, use your ohmmeter to confirm transformer center tap is intact and properly connect to the system ground (0V, SPOR, SPOL, OL, OR)
2. Failure in the +67 volt regulator
Note that the -67 volt line is actually regulated via a pair of zener diodes (D208, D209) and the +67 volt line is regulated as a mirror of the - 67 v.
For test purposes, lift one end of R203 and R204. Then measure the voltages across C210 and C211.
Yes, the minus supply provides the reference, but the +67 is configured as inverted tracking -- if the -67v goes towards zero, the +67 will follow towards zero. So -16 volts on one side and +100 volts on the other shouldn't happen.
It cannot - I'm testing for shorts first using the Fluke
I have the negative connected to the chassis. I can confirm that the center tap is connected to the points you listed.
It seems that the general consensus from you all is to replace a lot of components in the area - I will replace everything i've been instructed to replace and report back. Once i've done that, I'll run the tests mentioned by Ylli, if the problems are still there.
Thank you ALL so much for the suggestions! This forum is amazing
Sad to report that I have replaced all requested components and no change in symptoms - I'm going to do as Ylli suggested now and lift R203/204, and then test volts on C210/211
EDIT: I've also noticed that -VR & -VL are dead, no -60v on them. But +60v is present at VL+ & VR+
EDIT: I've also noticed that -VR & -VL are dead, no -60v on them. But +60v is present at VL+ & VR+
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Then you need to start over, and check the diode bridge carefully first.
This part, or a connection to it, would be the cause.
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I think something on -vL or -vR must be shorted, which could explain lots of current flowing through C211. I'm going to try de soldering all the wires which connect the PSU to the left and right channel boards.
The fuse would blow if that were the case. An open circuit is more likely,
either in the bridge rectifier itself, or in connections to it.
The basic problem is that there is no negative supply.
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Well, the fusing resistor did blow - the mains one didn't but i suppose if it was a partial short with some resistance, it could be enough to blow the resistor but not enough to blow the mains fuse?
That may have happened due to the abnormally high voltage across it, from the lack of a negative supply.
There is no negative high current voltage supply present, which causes the rest of the symptoms.
This is termed a cascading failure.
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