NAD 3150 Stereo Amplifier Repair

[Sir Knight]

Speaking of the "old brain", I have to clarify something: The 3150 is actually my uncle's, I have a 3155 and a 2150. Forgive me if I get numbers or ownership mixed up here and there.

[Smoky]

When I seen parts that didn't look factory and the burned spots in this amplifier, I thought the best route would be to clean everything up and replace the components with the original spec'd parts. I didn't replace any of the original transistors or voltage regulators because they tested good on the bench. All of the electrolytic capacitors were replaced. Nothing else looked out of line.

If you say that the voltage regulators and transistors shouldn't be hot then there is still a problem in the amplifier.

[Sir Knight]

I don't know for sure, but thats something to look at. Some other things to look at would be to verify each of the resistor values that you can get to. You can do this in-circuit so its not rocket surgery.

I'd also switch out any diodes with UF equivalents, or if those aren't available, new production IN. They're cheap, I buy diodes by the hundreds and swap them out as a matter of course now. In those quantities they're like 2-3 cents, so $3 tacked on to one of my semi-regular cap orders gets me hundreds of little bits of piece of mind.

[David_AVD]

I hate seeing new components soldered onto the legs of old ones. 

Taking to board out (or getting access to the underside) lets you do the job properly. It also allows you to check for cracked joints, etc.

My post probably wasn't clear.

I take leads trimmed off new passives and use those as extension posts. New parts are soldered to those new posts. I even use new solder. 

If you're not tacking onto existing cut legs, why are the legs being extended?

[Sir Knight]

Hi David,

A new part might be the perfect thing for the job electronically, but mechanically is another story. The package might not fit in the orientation or location of the original part. Occasionally you might run into clearance issues because a part is too large, but even a part that is too small can cause issues. In this case, the leads of the new part did not reach their solder pads or lands.

The original adjustment pot has three leads in a peaky triangle footprint. Two leads about 4mm apart (I'm guessing) and a 3rd lead that is about 6mm away (guessing again), centered on the other two. The new pot has a perfect triangle footprint. All three leads are 4mm apart. As the leads on the potentiometer are too short to fit through all 3 holes I try a different approach. I solder in the two leads that fit in the holes spaced 4mm apart. Then I bend the remaining lead so that it points to its mounting hole 6mm away. It won't reach into the hole but it should come right up to it. Take a fresh piece of trimmed lead from another part, the longest piece I can find, and solder that in the 3rd hole. I make sure to attach the piece so that half is above the board and half below. Now use that as a post to solder in the 3rd pot lead. Clean and trim the finished mounting to suit your preferences.

On the NAD amp, the lead would reach to the hole but run out of length so it couldn't even be bent into the hole, but it would be close enough to touch that post. On another amp I might have to bend the new post towards the part. In either case it works and its very sturdy. Its really no different than doing point-to-point on an amplifier or a crossover.

When I can get some bench space to finish my 3155 I'll take pictures and post them here.

For the record, I'm also not a fan of tacking things to old cut legs as well, from an aesthetics point of view. However you have to understand that there is really no difference between doing that and placing a new part in an old board. You may have a new part in there but you're still soldering those parts to an old trace. 

[David_AVD]

Ahh... gotcha. Thanks for clearing that up.