Nait 1 Repair

01/11/05

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Nait 1 Repair

In the Beginning

I bought this Nait off Ebay knowing that it was 'not working' in the hope that it was 'something simple' and therefore well within my capabilities to repair - Hah !.  No such luck, this turned out to be another 'Saga' involving replacement of more than 10 Transistors, and two resistors (more about that later).

Having opened the can, and powered up, I could see that I was beaten before I started, even the 24V supply rails looked low, so I decided to remove the circuit board and try some 'cold checks'.  The picture at the top shows this stage quite well so I won't repeat the photo here.

A few hours later and the cold checks revealed that the main Output trannies were trashed on one channel.  Not being too adept at this kind of thing, I did the most logical thing and posted on the PFM site for help.  Thanks to LesW, RichardH and Andy Weekes for their sterling advice, I (we) decided that the poor little Nait had gone DC (what ?) so I started looking about at the rest of the board.

This photos just goes to show what a good digital photo can reveal, I did'nt even notice the burnt out resistor and exploding trannie with the naked eye (well mine need reading glasses these days but you catch my drift).  You can see how many 'bits' needed to be replaced on the last photo by the holes in the board, lucky it was only one channel and I had something to measure against.

   

If you look close enough there are two knackered trannies and a cooked resistor for starters.  Andy advised that these are part of the protection circuitry (well they seemed to have committed hari-kari right enough) put in there to protect the O/P pair.  OK, so they failed miserably but who knows what else might have happened had these 'fuses' not blown with such gusto.

To cut a long story short, the total count was (IIRC), 2 off MPSA56, 2 off  ZTX384's, and several ZTX752's ..... plus the output trannies of course.  I phoned Naim direct for the output transistors (they being Naim's own brand and was advised by a very nice chap there (can't recall his name unfortunately) that they dont supply them as a part (predictably) but he was kind enough to advise on a suitable replacement - more later.

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This site was last updated 01/11/05