For testing purposes its wiser to test on a current limited isolation transformer, not just a variac. Also, considering the age of it and that there was a transistor failure, it would probably be ideal to recap the unit and ensure the power supply and filter caps are good. Since it seems its a piece of gear from the late 80’s or early 90’s all the electrolytics are likely due for replacement just from ESR. With a high current failure, I’d replace them all just for insurance.
Nice work ! How do you know output transistors blown? I have power amp which does not blow fuse, even no signal connected, if turn on amp it making loud unbearable screeching sound from speakers , Does it mean output stage is shot ?
Thanks for this video, subscribed. I found this video while searching for amp repair info. I have an NAD 902 obtained in a craigslist trade in which the other party ripped me off. I was an idiot and didn't test the equipment first. As soon as I got home and plugged it in, realized one of the RCA connectors needs to be jiggled just right or the channel cuts out. Despite this, I absolutely love this stereo amp over the feature-heavy Sony 7.1 surround receiver that I traded to that fat lucky jerk. A couple years later my stinking cat jumped and landed on my beloved 902, breaking one of the speaker connectors. This nice little amp has seen some rough times, but I think the problems are minor. Right now zip ties are putting just the right tension on the RCA connector, and the bare wire on the speaker output is held in place with electrical tape. I don't have any soldering experience, but I did find a service manual. Can you still buy OEM parts for these early 90s models?
First of all determine what is causing the erratic connection in your RCA jack. If you can repair it, that is your best option. If it is damaged beyond repair, then you will have to try and find a substitute part. You can buy many parts for older amps, but you may have to do some searching to find ones that work in your amp. In some cases exact replacements will be available - components such as resistors, capacitors, transistors. Other parts such as chassis mount connectors you will have to search vendors such as Mouser Electronics or Digikey to find parts that are close in form, fit and function.
@@jamelectronics Thanks for the detailed response. The erratic connection is fixed by jiggling the wire around at the connection. I'll invest in a soldering iron, and then a new amp depending on how that goes.
Sorry to say but after watching several repair videos I find a Fundamental problem. When working on a amp it seems no one gives a summary of what is actually wrong with the amp.
Danny, thanks for your comment. I agree it would have been better if I had said, up front, that the basic problem was that the amp instantly blew the mains fuse whenever it was powered up. I do talk about what the failure is at 1:23, but should have summarized early on. I usually do that, but forgot to for this one. Thanks again!
On some amps the fan might run continuously, on others the circuitry may sense the heat sink temp. and turn on the fan when the temperature gets to a certain level. First, you should determine if yours really doesn't work or if temps. just aren't such that it is coming on. Then, if there is a problem, figure out whether it is the fan itself or the circuit driving the fan . Replacement fans should be available from the manufacturer or equivalents can be bought from electronics vendors like Mouser or Digikey. Good luck!
Good day Sir. I have a NAD 2200. The minute i plug an input in the right channel, the relay click when i go upper with the volume.At low volume, it will play, but at high, the relay cuts. This amp has low voltage 62V at low volume and than when i go up the volume, it goes up at 92.7 volts. Two voltages. My problem is that all the components, output, drivers, resistors, etc.. are good compare with the other side that works. I even interchanged the relays and same problem. Now the only thing i did not change are capacitors but all voltages are good. My question is: Is it possible that even if a transistor have the right voltages and work good, it can be defect. Secondly, when i bypass the relay i go up with the volume and it works good exept when i shut it off without the relays i get a big DC voltages boummm in the speakers. i don't know where to look anymore. Help would be appreciate. By the way i'm a certified electronic technician but not a lot of experience. Can you please give me some advises? Thank's in advance. It's a NAD 2200, right side cut relay cut when i put the volume up. Excuse my english Sir. Paul. Best regards.
Here are some ideas. Check the Center Voltage is set to "0" per the alignment procedure( adjust with SVR202 for right channel SVR 201 for left). Also, check the Idling voltage while you're at it( part of alignment). IC 403 is the Relay Drive Protection Circuit and must have correct voltages on its inputs or it will protect the circuits. Pins 2,3, and 7 should be "0" volts. Q336 and Q338 are the overcurrent detection transistors, so these need to be checked. Any component in these circuits could be bad or marginal, so it can be very difficult to find the one. Good luck! Your English is fine!
