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Dead NAD 106 preamp

P

Peer

New Member
Hello,

First post here - thanks for reading! :)

I've bought a used NAD 106 preamp. I know it's nice, but not this one - is hums all the time when connect to amp or active speakers. Sound like 50hz hum from the psu. Everything looks very nice on the pcb and no visual errors to be seen, the pcb looks well soldered. I have the schematics for it and can read it, but I cannot figure out where this loud hum should be found. I hums extra loud when I touch the chassis also. There are no sources connected in this test scenario.

Any suggestions how to fix this?

The amp is also sold as NAD 1000s
Schematics: https://www.hifiengine.com/hfe_downloads/index.php?nad/nad_106_1000s_service_en.pdf
 
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Welcome to AK!

The power supply transistors and/or dropper resistors can become unsoldered from the board, even though visual inspection might not show it. Main capacitors are probably old and tired by now, as will all the rest of them. You could start with fixing the power supply, and see if the hum goes away. But I would advise recapping the whole thing anyway, though this should be after you've fixed it.

Lee.
 
leesonic said:
Welcome to AK!

The power supply transistors and/or dropper resistors can become unsoldered from the board, even though visual inspection might not show it. Main capacitors are probably old and tired by now, as will all the rest of them. You could start with fixing the power supply, and see if the hum goes away. But I would advise recapping the whole thing anyway, though this should be after you've fixed it.

Lee.
Click to expand...

Thanks a lot Lee, I will try that tomorrow. I will try to re-solder and test, then eventually replace caps and test. Later on Nichicon Muse in the audio-section :)
 
Seems to be a lot of postings on this site regarding problematic NAD gear. One brand I'd stay away from. Come to think of it, I have a NAD receiver somewhere in the DOA pile. Hopefully, you have a relatively easy fix there, but if it looks like it may turn into a $$pit, I'd junk it & look for something else. Just my opinion, and you know the saying....
 
leesonic said:
The power supply transistors and/or dropper resistors can become unsoldered from the board, even though visual inspection
Click to expand...
Not just Nads.. quality of the orig. solder(ing) aged, transistors etc can heat and cool. We see many pcb photos of dark spots e.g. around resistors not blown to blown.
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Catmanboo said:
One brand I'd stay away from
Click to expand...
One mans trash is another mans treasure.. You're a subscriber put it up for sale or trade "as is" here on Barter Town.
I'll pay for the shipping if ya want to give it away.
 
Catmanboo said:
Seems to be a lot of postings on this site regarding problematic NAD gear. One brand I'd stay away from. Come to think of it, I have a NAD receiver somewhere in the DOA pile. Hopefully, you have a relatively easy fix there, but if it looks like it may turn into a $$pit, I'd junk it & look for something else. Just my opinion, and you know the saying....
Click to expand...

Seems like a bit off-tipic...
 
Binkman said:
Not just Nads.. quality of the orig. solder(ing) aged, transistors etc can heat and cool. We see many pcb photos of dark spots e.g. around resistors not blown to blown.
-------

One mans trash is another mans treasure.. You're a subscriber put it up for sale or trade "as is" here on Barter Town.
I'll pay for the shipping if ya want to give it away.
Click to expand...

Agree.
 
Now, everything is re-soldered... There was some bad solder-points on the pcb in the psu-section, they got fixed. Weirdly, the relay was removed - as far as I can see it's the relay to cut the signal from pre-out, when headphones is connected. It's replaced, and working. The 2 regulators in the psu was replaced with some weird ones, they are replaced with correct ones. Now I do have a stable supply for the signal-section, measured to correct voltages. But I still get quite some hum...

What's also very special, is that the sound from pre-out is in reverse phase, like an analog "karaoke-filter" - meaning I can only hear instruments recorded in stereo, not the ones in mono/center. The sound is generally just wrong, but when I turn the Balance to L or R, the audio sounds normal.

The sound is perfect on the headphone-plug in the front (the unit has it's own headphone amplifier-section incl. regulated psu).

Status: I get less noise now. I get extremely loud hum when the unit is powered off, and connected to mains. The hum dies out when I disconnect mains. Also, no matter what, I get quite some noise when touching the case.

Every other parts of my test-setup is 100% ok. Tested against a NAD 114 which sound perfect on the test setup, no hum what so ever.

The replacing of all caps is postponed for now, until I can find the error. But the error might be bad caps in the psu yes? The mono-issue is causing me headache, I cannot understand it!
 
This hum may be a ground loop issue.
Are the mains from the pre-amp connected to the same as the amp (same household outlet or power strip)?
 
Using a DVM can you measure if there is a potential difference from the chassis to the main power?
What about if you measure between the power amp chassis to the pre-amp chassis?

On the schematic I see C501, switch noise filter, is it possible someone changed this and put in a cap from line power to the chassis?
 
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