NAD 2240PE bias adjustment.

I don't have any specific knowledge of this particular amp's bias set up, but usually, if I find myself in that situation, I look for some way of measuring a comparison voltage with the channel that wasn't worked on, which is, as you say, usually the voltage across the emitter resistors, if it has them. I've also often found that there are bridged out test points, where you can break in with an ammeter to measure the idle current, which is rule of thumb typically 20mA. If all else fails, the temperature that the output transistors run at on the rebuilt channel, compared to those on the other channel, is usually a pretty good guide as to how evenly the two channels are running with respect to one another. This should be checked with the amp just idling. If the rebuilt channel gets hotter than the ok channel quite quickly, then the chances are that the bias is set too high. If the rebuilt channel is still stone cold after 15 mins, and the ok channel is just warm, then the bias is too low. As a final check, sometimes you can see the crossover distortion on a sine wave, when the bias is set wrong.

Arfa

And the load not connected.

Graham

I've never really thought about whether or not a load should be connected when making that assessment. I suppose it might depend on how the bias arrangement works, or if there are any other adjustments such as offset, but in theory, surely if it's a standard complimentary output stage, with the output DC coupled to the midpoint, and with the amp just idling with no input, there should be no current in the load ? If there's any kind of asymmetry, leading to a DC offset at the midpoint, then yes, I can see some value in not having a load connected, but also, I would suggest that it would indicate a further problem that needed resolving ?

Interested in your thinking here, Graham.

Arfa

Even a slight DC offset would affect the voltage measured across an emitter resistor, if a load is connected. I put a pencil to it and found that with a

50 millivolt offset (a bit high but certainly not uncommon) and a 6 ohm DC resistance representing the speaker, this would mean about 18 millivolts additional across a .22 ohm emitter resistor. Reason enough to adjust the offset first, I would say. Unfortunately, many amps have no offset adjustment as such. I will try to look up the 2240PE bias adjustment at work and e-mail Dave directly. I've seen some NAD's which require measurement across a 1 ohm resistor which is made available by desoldering two adjacent foil pads. Once the adjustment is done, the pads are resoldered.

Mark Z.

That's a good point, and I expect that was what Graham was thinking

Arfa

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Thanks for all the tips. The dc offset was about 1 mv. Not too bad. Now that Mark has e-mailed me the details about the desoldered joints, it will be easy. I should note that I did notice the solder bridge across the 1 ohm resistor and I thought that I had caused the solder bridge when replacing the transistors. Long story short, you don't want to run the amp under stress with the foil unbridged or else you will be fixing more problems.

By the way, is anyone familiar with the component on the power amp board that looks similar to a microswitch? It measures a direct short. Is it a resettable fuse? The location numbers are BK-1 and BK-2.

--
David Farber
David Farber's Service Center
L.A., CA

Yes. I've seen these in NADs before. They should self reset, but I have had them go totally open.

Arfa

Circuit breakers?

They are circuit breakers.

mz

Dave,

Set the rear switch for 8 ohms for these tests, then set it back to 4 ohms before giving it back to the customer.

First, make sure the center voltage is 0.00 +/- 30mV at the outputs with no load. Adjust with VR401/VR402 as needed. You should really be able to get this down to better than +/-10mV, but they do drift with temperature, so true 0 is not a huge issue.

From the NAD manual: (with some minor changes to wording by me)

1) Remove short across R471 and R472.

2) Connect a DC millivolt meter across R471 (R472) (1 ohm output transistor collector resistor.) and adjust VR403 (VR404) for 26-30mV reading on the meter. (No loads

3) leave power on for 5 minutes and recheck both offset and bias.

4) DON'T FORGET to re-short resistors 471 and 472.

5) Repeat step 4

6) Repeat step 5

In my opinion, I like to set the bias a bit on the low side on these, around 20-24mV. They still don't have a crossover notch, and run a bit cooler. DO NOT run the amp up on loads while R471/R472 are unshorted (in circuit) as the 1/4 watt resistors will burn open and cause you to repair the amp again.

Before you start, make sure all of the pot wipers are free. I've had a few spring apart while trying to force the locking agent, and that can be big trouble if the bias pot opens. Much better to have the pot spring apart with the amp turned off.

If you have any questions, e-mail me directly.

Regards, Tim Schwartz Bristol Electronics

Check the amp for symmetrical clipping and that the soft clipping circuit is working. You can do this visually on a scope with loads.