NAD 2400 amp intermittant protect

Guest
Hi. I have a NAD 2400 amp. It intermittantly drops into protect, may take a
minute or a day. Reset power, works fine, again maybe quick or slow. I have
checked over components for shorts, out of values, etc. I did replace one
shorted gen purpose small diode.

When I got the amp, right end side of heat sink ran near cold, while left
end side ran at a health feeling warmth. I looked at bias adjusts, which
were way off from each other. Experimentally, I set left to match right,
would kick off very quickly, so I set each the same, ran them both CCW an
eigth of a turn at a time, going longer between kickoffs on average, and
both sides run a healthy feeling temp. Both are at about one third setting
from full left on the pot.

As guessed I have no print for this, it was donated to me as a playtoy. I
have checked L&R side components for values, etc, with nothing obviously way
out of whack.

Now, the sound is perfect when running, could not be clearer, so audible
distortion isn't a problem.

Any ideas?

Cheers, Danny
 
djnoble35209@charter.net wrote:

Hi. I have a NAD 2400 amp. It intermittantly drops into protect, may take a
minute or a day. Reset power, works fine, again maybe quick or slow. I have
checked over components for shorts, out of values, etc. I did replace one
shorted gen purpose small diode.

When I got the amp, right end side of heat sink ran near cold, while left
end side ran at a health feeling warmth. I looked at bias adjusts, which
were way off from each other. Experimentally, I set left to match right,
would kick off very quickly, so I set each the same, ran them both CCW an
eigth of a turn at a time, going longer between kickoffs on average, and
both sides run a healthy feeling temp. Both are at about one third setting
from full left on the pot.

As guessed I have no print for this, it was donated to me as a playtoy. I
have checked L&R side components for values, etc, with nothing obviously way
out of whack.

Now, the sound is perfect when running, could not be clearer, so audible
distortion isn't a problem.
Click to expand...
Did those bias adjustments look like they'd ever been tampered with? It just
strikes me as odd that one would be that far off, for no apparent reason.
Or, do any of the parts look like they've been replace? Different-looking
soldering, etc.?

When I want to check bias in a solid-state amp the first thing I try to do is
locate the typical small-value emitter resistors. These are 1 ohm in the
Sony receiver I have here, typically a fraction of an ohm in anything with a
nontrivial output power. Put your meter across those, and the use of a
little math will tell you what the idling current is for upper and lower
transistors in either channel. You can draw your own conclusions from
there...
 
Danny,

The correct procedure for setting up (from the service manual) a NAD
2400 is:

1) Set unit to 8 ohm operation

2) confirm that DC output at speaker terminals is less than 100mV with
no load. If it is higher, then the amp needs to be repaired, and the DC
offset protection may be tripping.

3) Measure between TP 401 and 402 (TP451 and 452 for right channel) and
adjust R418 (R468 for right channel) for a reading of 14 +/-1 mV. The
amp should be cold when you do this, and then check again after 5
minutes. If anything I'd go a bit on the low side for cooler running,
but you should be able to get them to spec. The ambient temperature of
your room will have some effect on it too.

Note that the test points are specific leads of the dual emitter
resistors in each channel and should be marked on the circuit board.

4) set back to 4 ohm operation

The heat sink should not be uncomfortable to touch at any time.

Also, as I recall the speaker relays sometimes go bad in these amps.
When a channel cuts out, gently tap on the relays, and if that get the
channel back on, then replace them. You should be able to get sealed
contact replacements for around $10 each. I suggest sealed contacts, as
they are less likely to fail again. Make sure to get the correct
voltage ratings and at least the same current rating.

Regards,
Tim Schwartz
Bristol Electronics


djnoble35209@charter.net wrote:
Hi. I have a NAD 2400 amp. It intermittantly drops into protect, may take a
minute or a day. Reset power, works fine, again maybe quick or slow. I have
checked over components for shorts, out of values, etc. I did replace one
shorted gen purpose small diode.

When I got the amp, right end side of heat sink ran near cold, while left
end side ran at a health feeling warmth. I looked at bias adjusts, which
were way off from each other. Experimentally, I set left to match right,
would kick off very quickly, so I set each the same, ran them both CCW an
eigth of a turn at a time, going longer between kickoffs on average, and
both sides run a healthy feeling temp. Both are at about one third setting
from full left on the pot.

As guessed I have no print for this, it was donated to me as a playtoy. I
have checked L&R side components for values, etc, with nothing obviously way
out of whack.

Now, the sound is perfect when running, could not be clearer, so audible
distortion isn't a problem.

Any ideas?

Cheers, Danny
Click to expand...
 
Tim,

The PSX-2405 original relays are no longer available. Do you know of a
drop-in replacement? The 2405 has a rather odd physical pinout.


Mark Z.



"Tim Schwartz" <toschwartz@worldnet.att.net> wrote in message
news:3FAE3771.8D718BB@worldnet.att.net...
Danny,

The correct procedure for setting up (from the service manual) a NAD
2400 is:

1) Set unit to 8 ohm operation

2) confirm that DC output at speaker terminals is less than 100mV with
no load. If it is higher, then the amp needs to be repaired, and the DC
offset protection may be tripping.

3) Measure between TP 401 and 402 (TP451 and 452 for right channel) and
adjust R418 (R468 for right channel) for a reading of 14 +/-1 mV. The
amp should be cold when you do this, and then check again after 5
minutes. If anything I'd go a bit on the low side for cooler running,
but you should be able to get them to spec. The ambient temperature of
your room will have some effect on it too.

Note that the test points are specific leads of the dual emitter
resistors in each channel and should be marked on the circuit board.

4) set back to 4 ohm operation

The heat sink should not be uncomfortable to touch at any time.

Also, as I recall the speaker relays sometimes go bad in these amps.
When a channel cuts out, gently tap on the relays, and if that get the
channel back on, then replace them. You should be able to get sealed
contact replacements for around $10 each. I suggest sealed contacts, as
they are less likely to fail again. Make sure to get the correct
voltage ratings and at least the same current rating.

Regards,
Tim Schwartz
Bristol Electronics


djnoble35209@charter.net wrote:

Hi. I have a NAD 2400 amp. It intermittantly drops into protect, may
take a
minute or a day. Reset power, works fine, again maybe quick or slow. I
have
checked over components for shorts, out of values, etc. I did replace
one
shorted gen purpose small diode.

When I got the amp, right end side of heat sink ran near cold, while
left
end side ran at a health feeling warmth. I looked at bias adjusts, which
were way off from each other. Experimentally, I set left to match right,
would kick off very quickly, so I set each the same, ran them both CCW
an
eigth of a turn at a time, going longer between kickoffs on average, and
both sides run a healthy feeling temp. Both are at about one third
setting
from full left on the pot.

As guessed I have no print for this, it was donated to me as a playtoy.
I
have checked L&R side components for values, etc, with nothing obviously
way
out of whack.

Now, the sound is perfect when running, could not be clearer, so audible
distortion isn't a problem.

Any ideas?

Cheers, Danny
Click to expand...
 
Hi. Tim sent me the setup for the amp, for which I thanked him earlier by
email. The right channel is showing 1.2 vdc at shutdown, while left
maintains 0 volts dc at output, so I have an aim point to fix it with. I
have set the biases to spec, and will start fresh in the morning.

As for your question about tampering -- yes, another shop had it. They
threw it away when they closed down. Poor thing looked perfect on the
outside, so I had to salvage it. I call it a donation. (Not a good idea to
leave a customers name and phone number ticket on a tossed high-dollar item
like they did, but I digress) Anyway, the biases had been messed with, and
some soldering done.

I think I fix the dc problem in the right channel and it will be right as
rain.

Again, thanks for the great input.

Cheers, Danny
 
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