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NAD 7150 Issues: Set speaker on fire!

honthro

honthro

New Member
Hi guys,

I was hoping some of you would be able to help me out with my NAD. I got this baby for $70 and the sound seemed ok, but needed work. So I opened her up with the sole intention of Deoxing her.

Having done so, I realized she had a number of caps burnt out (I've attached the pics in the album to show which ones). Note: the construct of the NAD is such that there are two electrical boards - one for the radio, and one for the amp - removed the radio to work on the amp. I went ahead and replaced these caps with either exact same ones or ones that had the same microfarad rating but a higher voltage (I was told this should be ok).

Go to plug it in...*POOF!* a part of the main amp board goes up in smoke and fire. Turned it off to find one of the transistors had been set on fire (last pic in the album). Went and replaced it and so far nothing catches fire.

EXCEPT for the left channel - when I plugged my speaker in the driver produced a very low tone and extruded way out to the front, almost breaking the seal. Confused, I left this on for less than a minute and the thing literally caught fire! The right channel is fine though.

So I went ahead and replaced all the other burnt out caps on the radio board as well and put everything together, but nothing seems to fix the left channel. I even tried setting the DC offset and idle current in the amp, but all it proved to me was that my right channel was around ~150mV (which I adjusted to ~15mV), my left channel was actually NINE VOLTS! No wonder it blew the speaker!

So I ask you for help because I've run out of ideas of where to look. My limited experience in electrical work leads me to believe something died (maybe a resistor) but it hasn't set itself on fire so I can't find it :no:

Album:
NAD Pics

I have the NAD manual but I can't seem to upload a pdf, so here is the link:
HiFi Manuals Access to manual

:tears:
 
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Bumping again for any additional insight/input.

I've ordered replacements SoCal; will report when I replace them
 
petehall347 said:
you maybe put one or more caps in backwards or you have created a short circuit .
Click to expand...

I went back and checked all of the caps I replaced and they all seem to be correct. Soldering-wise, I don't think I created anything that might create a short circuit.
 
That black stuff is assembly glue, used to hold larger caps steady during manufacturing and transport. I see in several pics many "bald" caps, where the jacket has shrunk due to heat. These should be replaced.
This may not fix your issue though.
 
ryuuoh said:
That black stuff is assembly glue, used to hold larger caps steady during manufacturing and transport. I see in several pics many "bald" caps, where the jacket has shrunk due to heat. These should be replaced.
This may not fix your issue though.
Click to expand...

Hmm so you're telling me I might have replaced caps that weren't busted?
 
Well, I would replace all the electros, just for reliability, but yes, the bald caps have been overheated, the caps with glue might have been good, but since they are cheap, why not replace them all.
Don't mess with the tuner board though. Those caps lead an easier life.
 
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