Hello. Newbie here. I just got a NAD 3155 with some problems.
The amp is entering in protecting mode and i dont know why, also only one of the inputs works.
I got all the capacitors new but for the moment i only replaced half. I tested for the dc voltage into the test points like the service manual say and i get a variable voltage in the left channel quite high. If i make the both potentiometers of the left channel to the minimum, i get a lecture of around 0.50/o.55mv and is getting in and out of protection mode.
After i replace the rest of the caps, what should i look for?
The amp is entering in protecting mode and i dont know why, also only one of the inputs works.
I got all the capacitors new but for the moment i only replaced half. I tested for the dc voltage into the test points like the service manual say and i get a variable voltage in the left channel quite high. If i make the both potentiometers of the left channel to the minimum, i get a lecture of around 0.50/o.55mv and is getting in and out of protection mode.
After i replace the rest of the caps, what should i look for?
Welcome to the forum!
I would suggest a slightly adjustment procedure.
With power off, use ohmmeter to confirm bias current pots R721 and R771 are set to maximum resistance. This will preset the bias pot for minimum bias current, the safest initial setting.
Apply power and use R708 and R758 to set amp outputs to 0V. I suggest measuring the output voltage at L701 and L751. That way you will be monitoring the amp DC voltage even if protection mode may be blocking path to the speaker terminals.
Then adjust bias pots for prescribed readings.
Since test points aren't noted on the schematic--- they are noted below:
Left channel:
TP701 is at emitter of Q709
TP702 is at emitter of Q711
R721 Bias adjust
R708 Output offset adjust
Right channel:
TP751 is at emitter of Q759
TP752 is at emitter of Q761
R771 Bias adjust
R758 Output offset adjust
Give us details if you have trouble. Please be very descriptive.
Good luck!
I would suggest a slightly adjustment procedure.
With power off, use ohmmeter to confirm bias current pots R721 and R771 are set to maximum resistance. This will preset the bias pot for minimum bias current, the safest initial setting.
Apply power and use R708 and R758 to set amp outputs to 0V. I suggest measuring the output voltage at L701 and L751. That way you will be monitoring the amp DC voltage even if protection mode may be blocking path to the speaker terminals.
Then adjust bias pots for prescribed readings.
Since test points aren't noted on the schematic--- they are noted below:
Left channel:
TP701 is at emitter of Q709
TP702 is at emitter of Q711
R721 Bias adjust
R708 Output offset adjust
Right channel:
TP751 is at emitter of Q759
TP752 is at emitter of Q761
R771 Bias adjust
R758 Output offset adjust
Give us details if you have trouble. Please be very descriptive.
Good luck!
Nad had the bad habbit of running components very hot and close to their limits. On my 3150, which is lower power but otherwise similar in design and age, I had several resistors going open or way out of spec or intermittent. The heat also wreaks havoc on pcb tracks and solder joints. Especially around the voltage regulators but also in the input stages of the power amp. Check everything visually for a start.
Thanks a lot for this. I changed most of the capacitors still only 3 of the old ones because for the moment i was not able to find a replacement (C906, C905 and C802), i will probably get them next week.
As expected, the problem still there.
I adjusted R721 and R771 to maximum resistance. Then i turned on the thing and measured output voltage from TP701 to TP702. At the begining i got a result of 0.6vv and it started to go down on his own till 0.48v and the amp is getting in and out of protection. So i guess the relay is working properly.
On the right channel i was able to get it to 0.0v.
I guess the problem is only on the left channel. The big heatsink is getting extremely hot so probably some of the transistors of the left channel are shorted? I ordered a replacement for all of them, they are comming next week.
0.48V over 2x0.33R is over 700 mA idle current, that is way too high and the reason the heatsink gets hot. Check the vbe multiplier.
What is the DC offset on the output?
Just blindly replacing parts is not very likely to give the desired results. Test components one by one. When you replace parts do not order them from ebay or other similar sources, most of what is sold there is fake and will only complicate the fault finding process further. Order only from reputable sources such as Mouser, RS etc.
What is the DC offset on the output?
Just blindly replacing parts is not very likely to give the desired results. Test components one by one. When you replace parts do not order them from ebay or other similar sources, most of what is sold there is fake and will only complicate the fault finding process further. Order only from reputable sources such as Mouser, RS etc.
Thanks for the responses. Today I desoldered the transistors of the left part of the heatsink to ckeck for continuity, and they seems to be ok in that regard. Then i proceed to apply new thermal paste on the heatsink and when i solder them back to check for dc offset on the output and the vbe voltage, after i turned the amp on, in like 10 seconds the fuse pop out. So now i have to get a new one, but now im scared that if i make a new one in, the amp will blow up completly.
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Did your check of left-channel devices include Q705? I'm suspicious of a problem somewhere in the Vbe multiplier, so you might confirm its junctions appear to be intact.
Given slow to blow fuse, I'd check again for shorts in the output devices and of course short to ground. Assuming nothing new found, I'd install a short from emitter to collector across Q705 and confirm that the installed short does make continuity between base Q706 to base Q707. I'd hope that any damage was already done, replace fuse, and try again. If there's no excessive current, that's further suspicion of the Vbe circuit. With Q705 still shorted, try adjusting offset--- I think it should behave in the absence of other problems.
Good luck!
Given slow to blow fuse, I'd check again for shorts in the output devices and of course short to ground. Assuming nothing new found, I'd install a short from emitter to collector across Q705 and confirm that the installed short does make continuity between base Q706 to base Q707. I'd hope that any damage was already done, replace fuse, and try again. If there's no excessive current, that's further suspicion of the Vbe circuit. With Q705 still shorted, try adjusting offset--- I think it should behave in the absence of other problems.
Good luck!
@huggygood
I've seen you give lots of good advice, so I much appreciate your kind words.
Best,
Steve
I've seen you give lots of good advice, so I much appreciate your kind words.
Best,
Steve