Showing off my mint NAD 3400 with eventual discussion

[Jan492]

Greetings,

Hope this is the right place for this.. I'm just getting into recapping (and occasionally repairing) vintage gear. I recently received this unbelievably mint NAD 3400 and wanted to share some photos of the inside and out. It has beautiful white circuit boards. It's really hard for me to believe this is from ~1989. I was hoping to see some date codes on the transistors.. Is 9F a date code?

Anyway. Thanks for being here. This website is super helpful and Im looking forward to posting about my adventures in NAD recaps.

Here's an IMGUR link.. It looks like it's higher res than the forum software provides.
https://imgur.com/gallery/b0OqHCA


-Jan

 

[Jan492]

Okay. I need some help now. There are three resistors that have heat/burn marks near them and on the reverse side of the board. I have attached pics. What do you think is going on there? Should I replace them with some beefier resistors and raise them off the board a bit?

Also has anyone had experience replacing the relays on any NAD units? The relays appear to be sealed, so I can't clean their contacts. There is some static/rustling occasionally on this unit that I am trying to work out.

Thank you!

-Jan

 

[SoCal Sam]

Easy stuff first, clean all front panel controls. Also, clean the rear panel switches. Bridge to mono switch is the usual suspect.

 

[Jan492]

I bypassed the relays and the problem remained. I deoxit'd the infrasonic, soft clipping, and mono bridge switches on the back and haven't heard the problem yet.. I think you were correct about the bridge to mono switch being the culprit. Thank you!

...I still think the pre-amp stage has something not quite right.. So bright.. so bright. Using the preamp stage from a 3020 and all is well.

Any thoughts on those burn mark resistors in the photos above? They are 1/6 watt 4.7k resistors. On the schematic it says that a certain serial number range (that this isn't one of) uses 15k resistors in those spots. I wonder if that was a correction they did. Regardless.. I tested the resistors and they measure correct.. I may still replace them with nicer 1/2 watt resistors.

Thanks!

 

[avionic]

Welcome to AK...

 

[Chilkat]

Did you ever get around to replacing the 4.7k resistors? Done any re-capping yet?

 

[Powertech]

Nice units. A great shame they didn't put a little more thought into the internal wiring. It looks like knitted spaghetti in there. Spoils a nice board layout.

 

[Chilkat]

Yes, I agree. It makes working on that model a pain, and the Monitor series had its share of problems. Of course we're talking a different series and the lowest end of the NAD spectrum, but look at the change in their design philosophy with the little 20-watt model 310 amplifier from the mid 90's. I just picked one up for fun w/ intermittent left channel - a simple deoxit repair. Neat little amp. Sounds great, simple to work on, easy to upgrade and room to play. I'm adding a custom phono stage for starters. Less is more, as the saying goes.

 

[kmatch]

I’ve got a NAD 3400 unit in front of me with the same burned resistor symptoms. I can’t comprehend form the schematic exactly how the bridge selector knob can kill those resistors. I see a 34 V rail involved but don’t see how it can cause a problem with those resistors and diodes.

I’m interested in any clues to point me as to why the bridge selector kills burns these resistors? Mainly I want to be sure I’m solving the right problem.

 

[jhjove]

One of the replies in this old thread was that the noise in the output was caused by the bridge mono/stereo switch on the rear plate.
DEOXIT contact cleaner eliminated this noise for the OP.

The OP showed pics of the resistor area. Looks like or could be a 1/4W fusible type. If the resistor is still intact and in spec I would at most raise the position above the PCB
a bit. But I would not change sizing. If you are having issues where this resistor is in check for solder joints going bad due to the heat/cool cycle.

 

[kmatch]

Thanks for the note. I am curious about how the circuit is arranged such that a flaky bridge selector switch causes these resistors to get hot. I’m just building my schematic debugging skills and I couldn’t understand the connection enough to explain “Why?”

 

[rothwellaudio]

Thanks for the note. I am curious about how the circuit is arranged such that a flaky bridge selector switch causes these resistors to get hot. I’m just building my schematic debugging skills and I couldn’t understand the connection enough to explain “Why?”

It doesn't say anywhere in the thread that the burned resistors did have anything to do with bridge selector switch. As far as I can tell, the two issues are unrelated.

 

[Oggioffshore]

Hiy
Just got my hands on 2 of 3400 's.
And also own one since it was almost new.
Thinking about recap it.
So i was curious on this thread.
Red some good advice on cleaning those switches on the rear panels.

 

[Oggioffshore]

Well... I did recap on my most used 3400, but sadly when re solder some spots. It managed to to short 2 legs on one of the small transistors wired to the heat sink.

So... Now it's in protection mode.
Burning resistor R055, if I'm remembering correct.

Also have those burnt marks mentioned.

But, now i got 4 off these amps, 3 working.

So I'll guess I'll try recap one of the others. That actually looks better inside, clean and neat.

The one with issues, i owned from 1990, and has been used a lot, almost never turned off for years. Got spoiled some beers inside during a party and so on... So the board does not look nice, burnt here and there. But i always have managed to keep it alive, been to workshop a few times those years though.

I'm not skilled in reading schematics either, and the 3400 I'll guess it's not an easy amp to repair....

I did some mods when i bought it.

Put a red led in the volume controll, easier to see in the dark.
And also put in thicker wires from the speaker terminal to the relay... Not sure if that gives any benefit though.

 

[Oggioffshore]

 

[Oggioffshore]

Well... I did recap on my most used 3400, but sadly when re solder some spots. It managed to to short 2 legs on one of the small transistors wired to the heat sink.

So... Now it's in protection mode.
Burning resistor R055, if I'm remembering correct.

Also have those burnt marks mentioned.

But, now i got 4 off these amps, 3 working.

So I'll guess I'll try recap one of the others. That actually looks better inside, clean and neat.

The one with issues, i owned from 1990, and has been used a lot, almost never turned off for years. Got spoiled some beers inside during a party and so on... So the board does not look nice, burnt here and there. But i always have managed to keep it alive, been to workshop a few times those years though.

I'm not skilled in reading schematics either, and the 3400 I'll guess it's not an easy amp to repair....

I did some mods when i bought it.

Put a red led in the volume controll, easier to see in the dark.
And also put in thicker wires from the speaker terminal to the relay... Not sure if that gives any benefit though.

 

[Oggioffshore]

Well... I did recap on my most used 3400, but sadly when re solder some spots. It managed to to short 2 legs on one of the small transistors wired to the heat sink.

So... Now it's in protection mode.
Burning resistor R055, if I'm remembering correct.

Also have those burnt marks mentioned.

But, now i got 4 off these amps, 3 working.

So I'll guess I'll try recap one of the others. That actually looks better inside, clean and neat.

The one with issues, i owned from 1990, and has been used a lot, almost never turned off for years. Got spoiled some beers inside during a party and so on... So the board does not look nice, burnt here and there. But i always have managed to keep it alive, been to workshop a few times those years though.

I'm not skilled in reading schematics either, and the 3400 I'll guess it's not an easy amp to repair....

I did some mods when i bought it.

Put a red led in the volume controll, easier to see in the dark.
And also put in thicker wires from the speaker terminal to the relay... Not sure if that gives any benefit though.

Works again