A family member had a NAD 3120 go quiet on one channel a while ago. With competent/honest repair shops not easy to find, I was chosen to volunteer to have a go. A telephonic quiz revealed that they had made sure that the one channel was indeed dead by swopping around sources etc. They also swopped the speakers around.
This is actually a bad idea if your amp has developed a DC offset, but it was too late to tell them that. I know from painful experience that doing this leaves you with two blown speakers to fix as well as an amp with one dead channel. Don't ask... :sulky:
I had a hunch that the output transistors were gone. Since the required 2N3055 and 2N2955 are cheap as chips, I asked them to pick up some at Electronics 123 on the way to my place. They duly delivered the amp plus spares to me. I put the amp in my study and proceeded to forget about it. Too many other things to do, you see. I did get hold of the service manual on the interwebs.
After some time passed, the folks started asking questions so I finally cleared my desk and took a look. First off, the smoke test. Switch it on and look for smoke coming out. There weren't any so I grabbed my trust Fluke and checked the DC offset on the speaker terminals. At 200mV it was a bit excessive but not too bad I guess.
I then took off the cover and was met by a very thick layer of dust, so I grabbed the vacuum cleaner and sucked up the worst of it. Not having the proper equipment to get rid of the dust properly I left the rest as it was.
The power supply voltages were OK at about plus and minus 32V. The voltages indicated on the schematic were correct, if allowances were made for the DC offset. I switched off the amp and did a few checks with the Fluke's diode test but did not pick up anything wrong.
Next I hooked up a signal generator and oscilloscope, planning to do some signal tracing. Not that it helps much in a negative feedback circuit, but it is a start. Lo and behold, both channels showed up a nice undistorted output waveform. So I hooked up a speaker and listened to the test tone. Finally I hooked up a CD player and music played on both channels.
I still don't know what was wrong, if anything. My family was quite adamant that one channel was quite dead. Besides cleaning out the worst of the dust I did nothing. I did push two resistors sitting next to each other slightly away from each other in order to do a voltage measurement but that was it.
Anyway with the amp working everybody was happy. I probably should have adjusted that DC offset away, but then again those old pots have a way of going open circuit if you molest them. Replacing them would mean fixing something that is not really broken. It was getting late anyway. And I was feeling lazy. :whistler:
I feel a bit ashamed about not really going to town, taking the amp apart and really cleaning up everything. Maybe even doing a recap. The thing is more than two decades old, after all. But the amp is playing music again and the owners are happy.
Any thoughts on what went wrong/right?
This is actually a bad idea if your amp has developed a DC offset, but it was too late to tell them that. I know from painful experience that doing this leaves you with two blown speakers to fix as well as an amp with one dead channel. Don't ask... :sulky:
I had a hunch that the output transistors were gone. Since the required 2N3055 and 2N2955 are cheap as chips, I asked them to pick up some at Electronics 123 on the way to my place. They duly delivered the amp plus spares to me. I put the amp in my study and proceeded to forget about it. Too many other things to do, you see. I did get hold of the service manual on the interwebs.
After some time passed, the folks started asking questions so I finally cleared my desk and took a look. First off, the smoke test. Switch it on and look for smoke coming out. There weren't any so I grabbed my trust Fluke and checked the DC offset on the speaker terminals. At 200mV it was a bit excessive but not too bad I guess.
I then took off the cover and was met by a very thick layer of dust, so I grabbed the vacuum cleaner and sucked up the worst of it. Not having the proper equipment to get rid of the dust properly I left the rest as it was.
The power supply voltages were OK at about plus and minus 32V. The voltages indicated on the schematic were correct, if allowances were made for the DC offset. I switched off the amp and did a few checks with the Fluke's diode test but did not pick up anything wrong.
Next I hooked up a signal generator and oscilloscope, planning to do some signal tracing. Not that it helps much in a negative feedback circuit, but it is a start. Lo and behold, both channels showed up a nice undistorted output waveform. So I hooked up a speaker and listened to the test tone. Finally I hooked up a CD player and music played on both channels.
I still don't know what was wrong, if anything. My family was quite adamant that one channel was quite dead. Besides cleaning out the worst of the dust I did nothing. I did push two resistors sitting next to each other slightly away from each other in order to do a voltage measurement but that was it.
Anyway with the amp working everybody was happy. I probably should have adjusted that DC offset away, but then again those old pots have a way of going open circuit if you molest them. Replacing them would mean fixing something that is not really broken. It was getting late anyway. And I was feeling lazy. :whistler:
I feel a bit ashamed about not really going to town, taking the amp apart and really cleaning up everything. Maybe even doing a recap. The thing is more than two decades old, after all. But the amp is playing music again and the owners are happy.
Any thoughts on what went wrong/right?
