NAD 3150

gpounce32768

Active Member
Hi all,

I recently got this from ebay, seller says it doesn't work- hopefully something is blown out. It has the oft-reported heat issues related to the protection relay driver circuit- so I'll dig into that- some other heat issues look to be related to the regulators so some work may be needed there as well.

The controls are de-oxed, they were very stiff but reasonably lubricated now. The buttons feel great. I think I prefer Sansui/Kenwood engineering- certainly wrt some of the parts selection (trim pots) but we'll see how this one goes once I work it up. OTOH like the bridging & clipping enable switches in the NAD design, great touch there.


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Reminds me of my 3140 amp. But no protection relay. What are the protection relay heat issues?
 
The x0150 series along with the older New Acoustic Dimensions models are my favorite NAD's. Didn't know the 3140 did not have protection relay so that one if off my list.
 
Apparently the protection relay driver circuit includes a resistor that runs hot, the issue involves the resistor seen mounted on the underside of the board- it seems a weird place for a heat issue but its been reported several other 3150 threads. The regulator heatsinks are also on the small side but well integrated, so the fix there is probably just larger sinks.
 
Had it since new, no problems. Just have to recap it. Hasn't been used for a few years. Not sure trusting the caps after not using it. As for heat sinks check the Avid site for different heat sinks for the regulators. What is the value of the resistor mounted on the bottom of the board and where in the circuit is it connected to (R?)?
 
Yeah the bigger heatsinks on the regulators are no problem. I'm in the schematic now. The resistor in question is a 680 ohm 2watt affair, I guess they're burning off a lot of current since the relay has a 24v coil and is hanging off a 45v dc rail. I may do some redesign on it- we'll see as a dig thru the unit. I haven't figured out the one on the underside yet...
 
If I get a chance, it might be possible to add a output protection relay to the amp. The regulator heatsinks could go higher to increase radiating area. Also check the compound between the regulators and the heatsinks.
 
Maybe upping the resistance might help. Lowering the current thru the resistor. Or upping the power of the resistor, 2W to 3W. On the protection relay circuit would like to check out the one leesonic used for his 3020's.
 
The detect side looks fine, just a lot of drama on the relay driver side. The resistor on the underside of the board is across the relay coil terminals, 2watt resistor is the current limit- and the spec is for a metal film for some reason... why not a 5watt wirewound and be done with it... hmm
 
There is plenty of room to raise the resistor off the board with the leads kinked or maybe some tubing as standoff. I suppose the resistor under the board is there to reduce current thru the coil- meaning a higher value series resistor is likely a good fix. Perhaps it was cheaper for NAD to fiddle around with metal films likely already on hand for the unit than to spec an additional part. I do like how easy it is to work on the 3150 though.
 
Yeah thats one of the threads I found which gave me some hints as to what to look at. Those resistors on my unit are pretty badly cooked and the board is considerably darkened in the heat affected area... may be a high hours unit, I'm certainly going to look into the caps. Nothing bad is obviously going on but I've not sampled any of the caps yet.
 
Kink the leads and use a spacer for the resistor to keep it off the board. Will help to put less heat from the board. Use the same technique at work. As for the caps plenty of years gone by.
 
Yeah, caps are funny that way. Easy to find many that are still fully in spec, and then on some units most of them are a mess...
 
A lot of times it is the small low voltage caps that fail first, the 0.47 - 47 , 6.3- 16 vdc caps. So at times replace the .47-4.7mfd caps with film caps (WIMA). Leesonic's done this in his 3020 rebuilds. Going to do this in my 3140 where possible.
 
Just pulled the R676 series resistor and coil bypass resistors, R676 was low at 370ohms, bypass resistor was ~980ohms- no wonder R676 was running hot the schematic shows it as 680ohms... yikes.
 
Put a new 680 back in. Being 300 ohms low almost twice the current through the resistor. No wonder there is so much heat.
 
With the resistor 300 ohms low, almost twice the current through the resistor. No wonder so much heat was generated.
 
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For petes sake, got out the bin box of caps and the @*$% thing was half full of water, each of the baggies fully soaked- probably 30 different values & voltages all good brands represented... grrr. We had some heating issues last month, I think water leaking out of the radiator valve ran down the pipe, dripped off the elbow right into box. Might be a complete write-off... washed and rinsed them with alcohol I guess we'll see how they end up after they dry.
 
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