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NAD 3140 Amplifier

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Technotronic
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This one had been sitting in a barn for years. Powered it up and the large power transistors on the heat sink would get hot
after a time....and the heat sink would also get quite hot. Speakers sounded like they had some DC on them....with a loud thump

Found the manual on Hi-Fi engine,
http://www.mediafire.com/file/b8mlsu7q3pz0bvf/hfe_nad_3140_service.pdf/file ]
and saw a video on youtube saying lots of capacitors dry out
due to their locations near resistors that get very hot, I guess sorta a poor
design. Went in and recapped almost the entire
product save for the (4) large 50V PS caps which appear to check ok.None of
the high power or driver transistors
mounted to the main heat sink were shorted..

Powered on but no audio, and noticed the fuses to Bridge D805 were melted open.
I believe they were ok before.
Replaced them and they started to light up like fiilaments, so shut down,
removed the bridge D805, then checked it, its ok.

There is about a 16 ohm short on the + / - lands where the bridge was. It
seems like something I did maybe is causing this,
like a capacitor in backwards, which I am doing alot of lately for some
reason.

Turns out, yep I put C805 (470/ 63) In backwards ! Capacitor still checks
ok, so will re-use it.

Short is still there though Im guessing R803 is adding in 5.6 ohms or so,
that means theres a 10 ohm short someplace. Q801, 802 arent reading short,
and looks like this becomes a +43V ps feed.

Any ideas out there on what to check next? I am guessing this cap in
backwards caused the voltage applied to a component to be
reverse and its shorted. Or would this be normal with a reversed cap installed,
and putting it in right would be how to fix it?

Steve Hearns
Technotronic Dimensions [USA]
WWW.TECHNOTRONIC-DIMENSIONS.COM
 

Comments
Kari

Hi Steve,
we have at ET-database two different manuals for you :-)
rgds, Kari

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lcsaszar

Steve,

Measure each transistor out of circuit, i. e. two legs disconnected from the PCB. Not only C-E can be shorted, but also B-C.

Good luck, lcsaszar

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Technotronic
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Thank you. I was just curious if someone had a good place to start checking. The final outputs seem to be ok. With that cap in backwards, I guess its suppose to help derive
a plus or minus 43 volts, so would that mean it develops the opposite intended voltage if installed backwards?

Strange thing is the fuses didnt blow like a dead short, they actually lit like filaments, like that cap was gradually charging to be a short.....

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lcsaszar

"would that mean it develops the opposite intended voltage if installed backwards?"

No, it does not work like this. I would replace the capacitor in question anyway. 

- lcsaszar

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Technotronic
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Ok very good - I put the cap in the right way, and for safe move put new fuses in. Amp powers up, and seems to be working through the headphones 
at least!

The main output transistors get kind of hot but could be normal for this design. I will need to check bias pots next. But for the most part shes up and stable.

Thanks for the help !

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Technotronic
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Ok did the DC offset calibration ok, but the Rt side idle current dont vary with the pot at all, its 0v.

I see some service bulletins in the manual to help the trimmer pots be less touchy, and this unit could I guess use the mod.

Other service bulletin for the output stage (left) says MJ15003 or MJ15004 - but those are in a T0-3 package, and what is in there now is as follows:

Q626: 2SA1215
Q624: 2SC2921
Q625: 2SB755
Q623: 2SD845

They look original but I dont see any reference in the manual as to why this configuration should be there. They seem to recommend 2SC2565 for Q623 and I guess Q624
and 2SA1095 for Q625 and I guess Q626?

 

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Technotronic
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Replaced all the output transistors including the drivers, but used 2SC1494 and 2SC3858 as per something I read on an old internet  newsgroup post.
Probably should have checked that info more closely, but they are in now, and dont seem to be overheating.
Found I put in yet another cap backwards, this time it was C621. Corrected that and got the DC offset cal set ok.
Went to the current calibration and Im getting 0V on the test point.
Noticed transistors Q609 and 610 get pretty hot.
The S-B in the manual
says to use 2SC2565 for the MJ15003 and 2SA1095 for MJ15004,
and its also saying remove some resistors and replace with jumpers, which I see have been done.
Q619 - 622 have been removed but Q615-618 I cant be too sure as the markings are faded.
Did I make a poor choice for the subs?

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