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.

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

"David Farber" wrote in message
...
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.

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


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

Arfa Daily wrote:

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.

And the load not connected.

Graham

"Eeyore" wrote in message
...


Arfa Daily wrote:

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.

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

"Arfa Daily" wrote in message
...

"Eeyore" wrote in message
...


Arfa Daily wrote:

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.

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.

"Mark D. Zacharias" wrote in message
...

"Arfa Daily" wrote in message
...

"Eeyore" wrote in message
...


Arfa Daily wrote:

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.

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

"Arfa Daily" wrote in message
...

"Mark D. Zacharias" wrote in message
...

"Arfa Daily" wrote in message
...

"Eeyore" wrote in message
...


Arfa Daily wrote:

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.

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



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

"David Farber" wrote in message
...

"Arfa Daily" wrote in message
...

"Mark D. Zacharias" wrote in message
...

"Arfa Daily" wrote in message
...

"Eeyore" wrote in message
...


Arfa Daily wrote:

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.

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



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.



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

Arfa

"Meat Plow" wrote in message
...
On Tue, 13 Nov 2007 10:24:30 -0800, David Farber wrote:


"Arfa Daily" wrote in message
...

"Mark D. Zacharias" wrote in message
...

"Arfa Daily" wrote in message
...

"Eeyore" wrote in message
...


Arfa Daily wrote:

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.

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



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.

Circuit breakers?


They are circuit breakers.

mz

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.