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NAD 2700 Amp, goes into protection >25% volume

rpeck

rpeck

New Member
Hey all, I’ve been searching the web and this site for some info on this, but I can’t find anything that seems to depict my exact problem. I have a NAD 2700THX Amp that goes into protection when I turn up the volume on my pre-amp past about 25%. It sounds fine, both channels play; all settings and connections are fine until I turn up the volume. I can also turn the volume back down after its hit protection and it comes right back and plays normal at lower levels.

Can anyone point me in the right direction on where to start troubleshooting this? I have the service manual, but I am at a loss about where and how to start testing things. I have opened it up; I don’t see any bad connections, popped caps, or anything that looks out of the ordinary. I have a second functional one of these amps as well; I have not tried swapping at part yet because again I’m not really sure where to start. My tools right now are pretty much limited to a multi-meter and soldering iron.

I appreciate any assistance, also if there are any guru’s in the N. California (Stockton, Sacramento) area that might be willing to assist I would be willing to pay.


Thank You,
 
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So, when I got home tonight I cracked open a liquid therapy (Beer) and decided to open up both of my amps, the broken one and the working one to compare and contrast a bit. A little background, I bought both amps used, including the non-working one in "non-working" condition, the current problem was described before the purchase, but the person I bought it from claimed to have no idea what the problem was. Both of the Amps are the exact same model NAD 2700THX, about 400 digits in the serial number separate the 2 amps.

Attached are come pics, where all 4 of the C300's caps are missing on the broken amp. I have not a clue what these caps do, and I am not sure if this was a design change, I am hoping from the pic's and the attached screenshot of the service manual some one can tell me if this could be related to the problem.

Thanks again.
 

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That board looks like it has gotten pretty hot.

Is there a bias adjustment? Maybe the bias is out of spec. How is the DC offset?
 
I'd say somebody's been messing with that and I'd say you've found at least part of the problem...hopefully it's the only one you need to find...

as for there being design changes,it's possible,but i would remove and check the back of that board...if someone's been in there with a soldering iron taking parts out it should be pretty obvious
 
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mhedges said:
That board looks like it has gotten pretty hot.

Is there a bias adjustment? Maybe the bias is out of spec. How is the DC offset?
Click to expand...

I have not checked that out yet, there is an adjustment though, I have done it with my working amp before. However the service manual says to check it with the speaker outputs are hooked to a 8-Ohm load, the closest I have is a speaker with a 6.7 Ohm load. How important is it to be exactly 8 Ohm, anyone know?
 
Rodzilla said:
I'd say somebody's been messing with that and I'd say you've found at least part of the problem...hopefully it's the only one you need to find...

as for there being design changes,it's possible,but i would remove and check the back of that board...if someone's been in there with a soldering iron taking parts out it should be pretty obvious
Click to expand...

Yea, I really think this is also my next step, it's not like I can make it any worse :) It's not working now .. right. I can see little metal leads when I look close coming up where those caps would be, almost as if they were clipped or cut out, seems to me that this may be an obvious sign that it's not a design change, or at least not a factory design change ;)
 
rpeck said:
Yea, I really think this is also my next step, it's not like I can make it any worse :) It's not working now .. right. I can see little metal leads when I look close coming up where those caps would be, almost as if they were clipped or cut out, seems to me that this may be an obvious sign that it's not a design change, or at least not a factory design change ;)
Click to expand...

right...so now the question is...why?or more importantly,did the person who did this actually know what they were doing and have some reason for doing so[a thought out attempt to bypass some other fault or purposely change the circuit]...or was it just random hacking for some unknown reason?...I'd almost hope it was the latter,in that case restoring the missing parts may be all that's needed to fix it..and that it works at all even with it's problems is a damn good sign!...if it's the former tho there may be changes or problems elsewhere that need to be found and properly fixed as well.

i would start with replacing missing parts and making it match the good amp and see what that gets you,it might fix it entirely and if not,it may at least bring to light whatever the real problem was that lead to this "mod"
 
If the working amplifier cuts out around 25%, the next question I have is what is it connected to? What sort of preamp? What speakers? anything else connected inline?How are your tone control set? (EQ?) Have you checked all the basics (speaker cables, etc)

Some of those NADs were continue to play into some pretty low impedance loads for a while before the protection comes on.
 
C302 missing should not cause it to go into protection early, just more distortion. Anything else missing? Can you post the entire schematic? I would be more concerned about the protection circuit and the over current and DC voltage sensing circuitry in the output stage.
 
Sorry for any confusion, the working amp is fine at all volume levels. As soon as I get home I will be happy to post more of the schematic.
 
So.........just to clarify, your working amplifier is ok, and your non-working amplifier is the one that cuts out at 25% volume?
 
One more update, I swapped out the "Input PCB" with all the attached wiring from the good amp to the bad amp, the bad amp did the exact same thing. So I guess that pretty much eliminates anything on that board being the issue.
 
century tek said:
You need to check the bias and/or DC offset.
Click to expand...

I checked it tonight, spec is 7.5, it was 9.5 on one side, 9.8 on the other, I adjusted it to spec, still going into protection. the speakers output test was right at spec. 0.0

one more revelation tonight, where those caps are missing, I figured out that newer-bigger ones were soldered in on the underside of the board.
 
century tek said:
The next thing I was going to mention was maybe you have a bad output or driver transistor.
Click to expand...

Any way you could help me locate that/those based on the service manual? I'm open for ideas. I just replaced the 2 Caps on the power board, no change. I'm new to all this.

One more thing worth the mention maybe, it doesn't matter if I hookup only the left or right side, either channel seems to make it fail.
 
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rpeck said:
Any way you could help me locate that/those based on the service manual? I'm open for ideas. I just replaced the 2 Caps on the power board, no change. I'm new to all this.

One more thing worth the mention maybe, it doesn't matter if I hookup only the left or right side, either channel seems to make it fail.
Click to expand...

That means the problem is in the protection circuit for sure. I have a bunch of stuff to do tonight, but I'll try to get back to you with something to look at. Basically I think you will just need to replace the caps in the protection circuit. I will have to look at the schematic to identify them.
 
I don't know if this will help you, but I have a 2200PE and it was serviced twice due to protection board issues. The first time it was serviced the tech replaced 2 caps in the protection circuit and adjusted the bias etc. The second time, a year or so later, it was going into protect mode at turn on after the relay kicked in the speaker outputs. I narrowed the problem down to another bad cap in the protection circuit. I decided to just replaced the half dozen or so caps in the protection circuit and it has worked great ever since.
 
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