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 NADY AUDIO XA-900 So I found another 'broken' amp for <$20, and waited mostly patiently until it came in. I didn't get any pictures at the time when it showed up (mid 2009), so these are more recent. Looking inside, I see nothing obvious, as was with the 2nd DECA. (see DECA page for more info)  After checking and replacing the blown fuses, I plugged it in, and tried to turn it on. It immediatly blew both fuses, so it seemed time to look closer. That's when I noticed burnt marks and fried parts.  They are hard to see now, because the fried components have been replaced, including all PA, driver, and switching transistors. So after the replacements, I tried it again. This time, channel A worked perfectly (as I had thought), but channel B (that was previously fried) did not. It would pass whatever input signal I gave it, but it wasn't clean above a whisper with a signal that included anything < 120Hz or so. At that time, I had already started moving my desk from the dining room, so didn't really have much room for the equipment I needed to diagnose the problem. I did set up the scope and a signal generator to start tracing, but didn't get very far with it. I have to wait till my repair desk is complete and moved in before continuing with this project. This is a project I'll have to put audio and video clips up for. Straight pictures just won't do it justice. **UPDATE** (09/02/2011)
After a bit of time and numerous other things being completed, I was once again looking for a project.With my repair desk being *mostly* completed, I decided it was time to look into the Nady again. Remembering where I left off, I dug right back into tracing the signal problem, but this time I'm set up for more through testing. I was not able to get pics or video because I cannot find my memory card for my camera. (it WAS in the wii...) Tracing the signal, it was clean thru the input, gain circuit, protection circuit, and into the predriver stage. As the signal went into the driver stage, it began to distort. Once into the PA stage, it became worse. I guess a more experienced tech would have seen it sooner, but for some reason, it didn't even cross my mind until I was sitting there just staring at the 'scope. The signal was clean at the (mostly) top half of the wave, and very distorted at the (mostly) bottom half. Well, this IS a push-pull class AB amp. Stands to reason that only half of the output is conducting as it should. Let's look into that part. Warm soldering iron and a few minutes later I had driver and final base resistors and pass resistors out and ready to check. A quick hit on the meter shows the base and pass resistors for the driver and one final on the +vcc side, and only one pass resistor on the -vcc side were close (still wrong). Four of them were completelyopen, and the other 8 were burnt well out of tolerance. This totally explains what I was seeing on the scope, and how it sounded when connected to a speaker. Now, I am waiting for parts to arrive. and still looking for my missing memory card... **UPDATE** (09/14/2011)
My parts came in monday, so later that nite, I was able to install them. As it turns out,that wasn't the entire problem. Tracing the signal now, it's distorted throughout the entire channel, but only when it's loaded. And the distortion is worse as the load resistance decreases. I'm still looking into it, but still clueless. I did, however, find my missing memory card. It was hiding in it's lil plastic box on my desk..... right where it was supposed to be. Here are some videos of what I've got after installing new parts. The picture quality is poor and resolution is small, but it's all I have. (great stills, sucky video) They should stream using QuickTime. Movie 1 (9.4Mb) Movie 2 (13.6Mb) |
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