David Farber wrote:
This NAD 2240PE had nine bad transistors replaced on one channel and I was
wondering if anyone had knowledge of the bias test points. This set does not
seem to use the normal 5 watt emitter resistors to check the bias current.
It's operating fine as is but I'd like to see if it's within spec.

Thanks for your reply.

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.