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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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NAD 2400 amp intermittant protect
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 |
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NAD 2400 amp intermittant protect
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NAD 2400 amp intermittant protect
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 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 |
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NAD 2400 amp intermittant protect
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" wrote in message ... 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 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 |
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NAD 2400 amp intermittant protect
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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