NAD C 275BEE Idle Current Adjustment

turbodan

Member
Hi All,
Has anyone here ever adjusted the idle current on a NAD C 275BEE? I have repaired a few other unrelated circuit issues in this unit, and once I got it up and running, noticed a small amount of distortion on the right channel. It only occured during low volume passages, which led to believe something was off with the idling current.

The service manual states that potentiometer RV102 should be adjusted to 6.6mV between test points 1 and 2. When i measure the value, it's 0.9mV, and adjusting the potentiometer doesn't seem to have any affect. The odd thing is, the left channel, which sounds ok, is at 0.2mV, and that potentiometer doesn't seem to have any affect either.

Can you think of any reason i wouldn't be able to perform this adjustment on both channels? Or maybe there is something I am missing, haven't worked on a NAD before. I've attached a schematic, and highlighted the relevant pot in green.

Thanks!


Schematic.png
 
Greetings;
Referring to page 5 of the SM I assume you understand the pin test points and identified on main boards. (not the DC offset adjustment. ) I believe you have to set the idle from a cold start and rather quickly as procedure then says allow warm up then recheck. I did see that the procedure test pins TP1 and 2 on pg 5 didn't actually match up to main board component side view TP labels. Hope you were turning the correct VR pot(s) hence no reading on your meter (that should be set to mv scale)
 
Thanks Binkman,
TP1 and TP2 are labelled on the actual amp boards, so I am pretty sure that I am measuring the correct points. I just spent some time working on it, focusing on the right channel for now. I noticed that when I start up the amp, the voltage slowly increases to the correct 6.6mV value over the course of 30 seconds or so, then quickly drops off to the 0.9mV value. Makes me wonder if I could have some cracked solder joints or a failing transistor in the bias circuit.
 
Your first test of left channel @0.2mv seems comparable and after some reading about the circuity features, i.e. 1 watt stand-by power consumption etc., I think this is normal. I would assume that under load the circuits are then powered up so to speak.. not referring to 'power mode stand-by' but actual performance under load.

So perhaps this minor distortion heard is perhaps after some continuous use?
 
Ill try and listen immediately upon turning it on and let you know what happens as far as distortion. I did try cooling the board off with some compressed air to see if there was any affect, and i did notice some "blips" where it started reading a value. My test speaker also started to slightly oscillate when this happened. I think ill need to get some freeze spray, as the components are densely packed, and my air nozzle isn't very narrow.

The amp consumes about 60W at idle, and <1W off, so I think the <1W spec is definitely with the unit off.

Thanks for the help!
 
Okay... 'stand by mode' enabled consumes 1 watt. Idle mode is on. (sorry for confusion). I believe after reading at idle this amp operates like a class A amp but much less power consumption, then when power is excited then the good stuff happens.

Look at the power this puppy can produce... 150w continuous and regulated . Your assumptions for distortion spraying cool spray only negate things as operating temps should be normal. However, You might go back and check on the DC offset settings. I'm not saying things are not erroneous because I'd be a fool to assume all is normal but other factors come into play like perhaps one of your speakers is amiss or have you swapped out. I'm also not negating that factory didn't consider adequate cooling and I didn't see a cooling fan in the SM either for a power amp. with all that surface mounted close knit the outputs maybe cooled but those compact components are subject to heat too.

Might try spraying cool spray not on transistors but the few electrolytic caps... might even contact mfg if still has service techs available.
bink
 
If you measure voltages as low as you describe across the collector resistors of the output transistors with the amplifier switched on with no signal, you would be hearing intermodulation distortion. This occurs where the bias at the base of the output transistor is so low that it is able to completely switch off as the signal waveform passes the zero crossover point transition. This causes a small, but audible, step in the output waveform which occurs every time the waveform passes zero (in a positive or negative direction). The output transistors are biased into class A as low levels to prevent them from ever turning off completely.
The bias voltage (mV) measured as volt drop (measured across the collector resistors in this case) are in fact a measurement of minimum collector current in the output devices. You can measure across each transistor collector resistor independently (the test points are measuring across one of them) to get an idea if the problem is global or just limited to a single device. It could be a resistor with an incorrect value.
If the problem is global you probably need to look at the bias circuit (the gain through the transistor alongside the bias adjustment pot).
It may also pay to measure the output DC offset (across the speaker terminals) with no signal present and see if this is adjustable. Although not directly connected, it may give pointers to the source of the issue.
Also, 60watts dissipation with the amplifier at idle (not in standby, but fully on with no input signal) seems a very high figure. Is this thing heating up your lounge? Although some NAD amps have driver transistors that run quite hot. If they have individual heatsinks, this is quite normal.
 
Thanks everyone, I'll take another look at it tomorrow and let you know what I find.

Powertech - The driver transistors do have heatsinks, and there are a couple of resistors that seem to be kicking out some serious heat. Conveniently, there are couple of small transistors (TO-92 size) without heatsinks right in the heat path :yikes:. Ill post a couple of pics tomorrow.
 
Ok, here is a pic of the board in question. It's mounted to a heatsink as large as the board for the power transistors. The four large blue resistors on the left are the ones that dissipate a lot of heat.

YidfnRy.jpg


I did find a few more things wrong with it today. First, I noticed that the +80V and -80V rails were at about + and -92V. However, these rails don't appear to be regulated according to the schematic so I think this is ok.

I also noted that several resistors are open, notably R197 and R153 (which feeds the bias circuit). These happen to be in circuits fed by an unregulated rail. Makes me wonder if the amp was the victim of a power surge...

I'll go through and replace any out of spec resistors, and hope that none of the transistors were damaged. Will post a follow up when I make some progress.
 
With regard to the main unregulated supply rails, the +/-80V values could well be the voltage rating when the amp is loaded. It's not unusual to see higher voltages with no load. This is down to transformer regulation.
 
Hi All,
Just wanted to provide a quick update on status of this project, as it is now complete! The issue was caused by a couple of burned out resistors in the bias circuitry of the power amplifiers. The same resistors were actually failed on both boards. All of the failed components were located directly above the 4 blue power resistors, so I bet they failed due to ambient heat exposure. One quick order to DigiKey and I had everything up and running. Ended up replacing the potentiometers as well just make sure everything is in good shape.

The amp is working great now and is distortion free, thanks for all your help!

Dan
 
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