Hi all. I've been doing a lot of googling and I found a similar thread archived here with some pretty insightful responses (way to go guys!). I think this might get a bit long, but please bear with me. Hopefully I'll be able to contribute more to the forum over the next couple weeks.
So here goes. First off, a bit of history. This receiver was given to me about a week ago. The previous owner (family member) said it was used a decent bit up until about 7 years ago when the observation was made that it was running hot. At that point, it was put away until it was rediscovered about a week ago (and thus gifted over to me). So after hearing this story, I thought I'd give it a try. Sure enough, she was running rather hot, but playing beautifully. I thought maybe it was a ventilation problem (although my stand has plenty of space above the receiver), so I left the receiver out in the open. Same problem. After about 4 or 5 minutes at a low to moderate volume (hooked up to B&W DM602 S2 speakers), with the impedance selector at 8 ohms and the soft clipping switch off (it heats up with soft clipping on as well), it got extremely hot and stopped playing (did not turn off).
So here's what I've done so far. Honestly, I think there may be more than one problem. I tried hooking the preamp out to the amp in of another amp I have (Dussun V8i). I tried playing both radio, and cd through the preamp section of the NAD with no luck. I can't get anything from the FM radio. When playing a CD, I can definitely hear it playing but it's extremely scratchy and distorted. I can tell what song is playing (because I'm familiar with the CD) but just barely. It doesn't matter which input I have selected, CD, tuner, tape, the sound is exactly the same. I also tried running the preamp out from the Dussun to the amp-in on the NAD. Absolutely no sound at all. Not even a faint hiss.
Also, a couple of observations before it died. The heat seemed to be centered towards one side of the heat sink. Since it's symmetric (output transistors mirror each other on each side of the heat sink). That being said, I'm thinking it was one of the output transistors. Looks like the thermal heat sink grease on the back of the big one (a Toshiba 2SA1302), well, one of the big ones was nearly completely dried out. Maybe it had been running hot for a while and just finally fizzled? So the output transistor(s) is fried, what caused this. I'm pretty sure nobody has adjusted the bias. Could something else have aged to cause the bias to be thrown off and thus the transistor to fail? (Because if so, and I just replaced the transistors (a couple $$), they'd likely fail again.) Might the biasing pots have failed? I'm thinking I might just want to replace the output transistors and the pots, rebias them (I got a hold of a copy of the service manual, with proper values).
Do you think it could be something else? Should I just bite it and take it in to a tech? If I told a tech this story, do you think they'd be able to diagnose things rather quickly? Where is a good place to buy these types of output transistors? I've heard that there are some counterfeit ones floating around, and that really wouldn't help anything, were I to end up with one of these. I work at an aerospace research lab and have access to all the test equipment and thermal paste I may need.
Thanks in advance to anybody who can help or shed some light on this. I can email relevant sections of the service manual if you think it may help you out.
I have the 7155 which is probably mid 1980s and I love it -- but like yours it was a "find" (actually dumped in the street).
Whether I would spend serious money getting it fixed is something I haven't yet had to face -- but, given the frequency with which old and durable Japanese made stuff of this period turns up, I suspect I wouldn't.
Not sure the cost, but you would probably have to have all the output transistors replaced with an acceptable replacement part if one is broken. At the age of this, the power supply caps are probably not up to snuff either. If you dont need this thing, dont waste your time. If its nostalgia, do whatever you thing is right. There are no working ones one ebay at the moment so no idea what the value is of this. Good luck