Nad 2240pe

[Gagarin]

Hi! I am new here at DIYAudio.com, and it looks like a great place to hang out!

As for now, I am struggling hard to bring my NAD 2240PE power amp back to life.
It was dead on arrival (bought used, excellent outer condition), and burns all four fuses immidieatley when turned on.
I downloaded the service manual, and discovered to my big surprise that five (yes 5!) semiconductors in the right channel section was replaced with totally wrong sorts.
Three of them were shorted.

My questions:

1. Is there any way to make sure if its just the faulty semiconductor replacements that blows the fuses?

2. Is there any risk involved in removing the faulty semiconductors and try to power up the amp w/o them?

3. How can you tell if a rectifier is blown/shorted? I have a measuring instrument.


There are so few components in this amp, so I really think that a happy amateur like me should be able to get it up and going!

I uploaded the service manual on my website if anyone feels like taking a closer look:

Nad 2240PE Service Manual

The faulty semicon´s were in locations: Q424, Q426, Q432, Q434 and Q438.

Any help at all would be very appreciated!!

[analog_sa]

1. No. You have to check all transistors, starting from outputs and drivers. Being lazy i would just check for shorted pn junctions before unsoldering.
2. No risk. You shouldn't randomly apply full power though. If you have a variac use it, otherwise connect something like light bulbs in series with the power line to limit the current.

3. Draw a bridge rectifier and see which junctions you can measure. Obviously measuring from the AC side it will behave as an open circuit which is usually enough to apply power and see if the PS is ok (Provided you can disconnect the output from the blown amp).

Once you are pretty confident that all faulty semiconductors have been replaced apply power, but again, very carefully. Use either low power 30-100ohm resistors after the PS (it may not be easy) or a variac/light bulb in series with the mains transformer primary.

Once it shows signs of life you'll have to adjust bias and offset

[Lou914]

I just bought a 2240PE which I hope to use as the tweeter amp on my Orion speaker system. I'd like to ask Gagarin to post that service manual once again so we can check out the procedure for setting bias and offset on this amp. We never know what time and use has altered or the previous owners have tweaked...