NEED HELP: NAD T973 Power Amplifier

Jasper Dog

New Member
HI, I'm new here. This is my first post, so any advice or response is greatly appreciated. I am needing some direction on an NAD T 973 powere amplifier that isn't functioning correctly. It is a seven channel amp thats basically seven monoblocks sharing a power supply as I understand it. This one had a big surge or something hit it. One channel fried, two channels worked fine. The other four were and still are screwy. I took it to a reputable local repair shop. The fixed issues with the power supply. The amp card that fried we have set to the side for now. The other four are working, but they don't really have any high end sound at all. Just lower mid and low range. The technician has switched out these amp cards from one slot to the next and determined that the problem is in the amp cards themselves and as the two that are good work in any slot and the bad ones don't play good in any slot. We just both find it very strange that all four amp cards would be doing the exact same thing. We are really at a loss. he has tested them every way he can think of and says that all the voltages everywhere in the amp seem correct. He repaired some damage to the powere supply to get all the slots functioning correctly. He has performed a frequency sweep and says that the bad amp cards pass high frequencies during those albeit at about 60 percent of the volume of the cards that are totally good. He has adjusted the dc offset and bias to factory specs. So, we have no idea at this point how to proceed further. ANY advice would be greatly appreciated.
 
Greetings and welcome to AK.

so you need to rebuild refurbish the better boards or all for spares. Can you post the cards pcb numbers? might get an answer for a parts list.
 
I will get the card numbers as soon as I can. Trying to isolate problem on boards to avoid a complete rebuild as it seems cost prohibitive. I was hoping I would find someone with a simil
 
............................... He has performed a frequency sweep and says that the bad amp cards pass high frequencies during those albeit at about 60 percent of the volume of the cards that are totally good. He has adjusted the dc offset and bias to factory specs. So, we have no idea at this point how to proceed further. ANY advice would be greatly appreciated.
I don't quite understand what you mean, this is very important piece of info, can you explain in more detail how is the frequency sweep comparison between the good and the bad boards?

What do you mean about 60% of the good board? You mean the sweep of the bad board show the amplitude goes down to 60% of the amplitude of a good board at high frequency? At what frequency it starts to go down?

Where did you feed the frequency? Right on the card or at some input?
 
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Assuming you mean the bad card roll off at high frequency, ask your tech to check the different point I labeled A, B, C, D and E. I want to see which point you start to see the high frequency roll off.

NAD T973.JPG

For this kind of power amp, you should see the amplitude remains flat from low frequency to 20KHz, find out which point you see the roll off to limit the problem to a smaller area so I can concentrate on that section.
 
Thank you very much Alan. I will try to update soon. The tech was checking boards directly not at input. He said frequently sweep showed good but said the bad boards had 60 percent of volume vs good boards during sweep. Since then he has gotten out the oscilloscope and the bad boards definitely show distortion in the sign wave. That was last I heard. I will try to see the tech on Monday
 
Now this is a totally different story. I thought you said you lost highs only. Ha ha, you need scope to work on this from the get go.

If the tech still working on it, you shouldn't have to worry for him. If he still run into problem, tell him to post here and we'll go over it. Shouldn't be that hard to fix.
 
Hello Alan, this is Chris. I am the guy attempting to repair this amplifier. Thank you for the test points. R101 seems to be underneath an add-on board "1973 Poweramp Addon V11.0" so it will be inaccessible with the scope. However, I should be able to probe R107 and R144. I also wanted to bring to your attention that the two amps boards that are working perfectly are a different version than the other boards. The two boards that are working do not have the add-on board, and appear to be a different design. I tried removing the two newer boards and only having the older boards in the unit, I still get the same results. The newer boards work perfectly, however the older boards are at about 60% of the volume of the newer ones, and the scope shows heavy distortion on a 1Khz sine wave. I did not check the amplitude on a frequency sweep yet with the scope but I did it by ear as one of the first initial tests and it seemed like the bad boards were reproducing the highs at the same level as the lows but I will verify with the scope hopefully sometime today. When play music, the bad boards have a noticeable high end deficiency. thank you!
 
Interesting! I scoped a good amp and the amplitude remained the same from 20Hz to 20Khz. I scoped one of the bad amps and the amplitude would increase and decrease proportionally to the frequency. As I went lower in frequency, the amplitude would increase. As I increased the frequency the amplitude would get smaller. There wasn't an apparent cutoff of frequency.

The scaling of amplitude happens also on R107 and R144
 
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Hi Chris

I am busy with the eclipse and I have to go out, I'll take a look at the schematic and get back to you with more question this afternoon.

Alan
 
Interesting! I scoped a good amp and the amplitude remained the same from 20Hz to 20Khz. I scoped one of the bad amps and the amplitude would increase and decrease proportionally to the frequency. As I went lower in frequency, the amplitude would increase. As I increased the frequency the amplitude would get smaller. There wasn't an apparent cutoff of frequency.
Can you scope the few points I showed in post #6 to see at what point you starting to see the amplitude starts to go down with frequency?

This will rule out the part before that still give you flat response from 20 to 20KHz.
 
Hello Alan, this is Chris. I am the guy attempting to repair this amplifier. Thank you for the test points. R101 seems to be underneath an add-on board "1973 Poweramp Addon V11.0" so it will be inaccessible with the scope. However, I should be able to probe R107 and R144. I also wanted to bring to your attention that the two amps boards that are working perfectly are a different version than the other boards. The two boards that are working do not have the add-on board, and appear to be a different design. I tried removing the two newer boards and only having the older boards in the unit, I still get the same results. The newer boards work perfectly, however the older boards are at about 60% of the volume of the newer ones, and the scope shows heavy distortion on a 1Khz sine wave. I did not check the amplitude on a frequency sweep yet with the scope but I did it by ear as one of the first initial tests and it seemed like the bad boards were reproducing the highs at the same level as the lows but I will verify with the scope hopefully sometime today. When play music, the bad boards have a noticeable high end deficiency. thank you!

How about the boards that are burned? If they are the same as the good boards, we can fix them also.

To make it simple, just probe point C either at R144 or the base of Q105, or Q110. This is the input to the main amp.

Q101, 102 and 103 forms a small amp that amplifiers and buffer the signal slightly before driving the main amp ( base of Q105 and Q110). I want to see if you see the amplitude decreases with frequency at the input of the main amp. If so, the problem is with Q101, 102 and 103. If not, then we continue to go down the signal path.

R107 and C112 forms a low pass network to prevent RF like AM and FM radio from entering into the amplifier. Normally, it should not affect the amplitude from 20 to 20KHz. But if something goes wrong, you never know.


In post #6, I want to look at the frequency sweep at the output of the main amp. Point D in post 6 is "OUTPUT" in the schematic. This is before the inductor L101 and relay RL101. I want to rule out the inductor and relay.



BTW, this is the schematic I use:https://www.hifiengine.com/manual_library/nad/t973.shtml
 
I read you checked all the main voltages, did you check pin 3 of IC101 and IC102 to make sure you have +12V and -12V respectively? These power Q101, 102 and 103, also they power the input IPS stage of the main amp.
 
I read you checked all the main voltages, did you check pin 3 of IC101 and IC102 to make sure you have +12V and -12V respectively? These power Q101, 102 and 103, also they power the input IPS stage of the main amp.
Hey Alan it's Mike I'm curious if you do repair? I could mail you these boards if so. The time Chris is putting in is really not cost effective for him I know. I think the ad on boards mine seem to have are making this job very time consuming on his end
 
I'll check R144 today. Some of the test points you outlines in the schematic are on the back side of the boards between the PCB and the heatsink.
 
I went to test R144 and realized I already did yesterday as I noted in post #10. Q101, Q102, Q103 is labeled underneath the add-on board but its just empty traces. The components on the add-on board are not labeled. All of the amps with issues have these add-on boards and the amps that do work, do not have them. Thank you again for your help.
 

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Hey Alan it's Mike I'm curious if you do repair? I could mail you these boards if so. The time Chris is putting in is really not cost effective for him I know. I think the ad on boards mine seem to have are making this job very time consuming on his end
No, I just join in for the fun of it.
 
I went to test R144 and realized I already did yesterday as I noted in post #10. Q101, Q102, Q103 is labeled underneath the add-on board but its just empty traces. The components on the add-on board are not labeled. All of the amps with issues have these add-on boards and the amps that do work, do not have them. Thank you again for your help.
Sorry I did not see that.

That's really good news. The problem is not on the main amp at all!!!

NAD T973.JPG


Next to check is probing the point "A" in this markup. If "A" have flat frequency response from 20 to 20KHz, the problem is confined in this small area only. Then it should be easy to troubleshoot.

1) R107 and C112 forms a low pass network to block AM and FM radio signal from getting in. From schematic, it should not roll off until 234KHz. Check to make sure C112 is really 1000pF and R107 really measure 680ohm.

2) Q101 to 103 forms a little opamp. Gain from "A" to "B" should be R102/R101= 2K/1.5K=1.3333. Check if you get the correct gain from "A" to "B". This will verify the opamp is behaving correctly.

3) If you see "A" is normal and "B" have problem, the problem is on the opamp, check the voltage labeled. R105 creates the current for the differential pair Q101 and 102, assume Q101 and 102 matched and split the current. Current through R103 is 1mA and you should read -11V at the collector of Q102 ( same as base of Q103 and R103 as shown.

4) Verify "B" is 0V.


If you see see amplitude going down with frequency at the "A", the problem is not in this amplify board.
 
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