NAD 3150 Stereo Amplifier Repair

[floobydust]

Glad to hear the repair worked out 
NAD had an unusual design approach- minimalist in using very few discretes yet they get respectable distortion and slew rate numbers. Most manufacturers fell into the numbers game and piled on the feedback to get a lower THD spec, for the people that think it is the ultimate figure of merit and buy gear based on that.
If this was true, we have conquered THD long ago as today is spec'd in the ppm range yet old gear is preferred and can outperform modern cheap stuff. NAD spent the budget on dual power transformers and heatsinks. Worth noting is modern AV receivers with 7.2 channels weigh the same... they can't put out for long before overheating and ending up in landfill.

The R676 at 526 ohms I think is a failed resistor. I thought it may have been a necessity (low value) to prevent the relay from cutting out with main DC voltage sagging at high volumes. Although relays aren't critical of their applied voltage. A typical 24VDC part pulls in at >70% 16.8V and releases at <10% 2.4V so it should not care. I would not worry if the relay is underdriven somewhat.

[SubSub1]

TP 1 and 2 are the emitters of Q615 and 616. The book shows the same points, but on a pin of the dual resistor blocks IIRC. It is easier to just grab the emitters.

TP diagram attached.

Dear sirs,
Thank you very much for discussing and repairing this amplifier.  Very interesting reading and a very useful compliment to a home repair which I am also attempting on one of these amps.  GLouie, the diagram you have provided is the best quality by far I have found for this amp.  I wonder if perchance you have the diagram of the circuit layout with the same quality?  It would help me and certainly others who are looking to save these wonderful amps?  As this is clearly my first post, please forgive my rude intrusion.  I will however document the repair I carry out (should I achieve success) on this forum to help future dabblers.
Kind regards...Dan

[Shock]

Nice repair smokey and good job documenting it. I also read your cassette repair as well.

[Specmaster]

Yeah, good repair, but I didn't see anyone picking up on the high ESR of the 47uF cap, a decent 47uF should have an ESR of around 0.8 , it had also dropped a bit of capacity, which is sign on amps that they are becoming dry and slowly turning into a resistor as indicated by the reading of 29 . In the case of hum in amplifier 99% of the time it will down to bad caps, and if you want to cap its ESR then you do really need to de solder at least 1 lead other wise your reading will be affecting other components in the circuit.

[GLouie]

This is because I have an original paper service manual, bought new with my 7150. It is, however, some work to scan the foldout documents on my small scanner with high quality, so I do not do it very often.

Dear sirs,
Thank you very much for discussing and repairing this amplifier.  Very interesting reading and a very useful compliment to a home repair which I am also attempting on one of these amps.  GLouie, the diagram you have provided is the best quality by far I have found for this amp.  I wonder if perchance you have the diagram of the circuit layout with the same quality?  It would help me and certainly others who are looking to save these wonderful amps?  As this is clearly my first post, please forgive my rude intrusion.  I will however document the repair I carry out (should I achieve success) on this forum to help future dabblers.
Kind regards...Dan

[cdev]

I have an old NAD 7100 receiver, like this amp it has really excellent sound. "Tight" is how I would descrbe it. Well controlled. Good bass. Also the FM tuner is excellent.

Its a great receiver. Paired with good speakers, the sound is remarkably good.

Just a reminder if you have old equipment like this, it must be used, don't let it sit unused. Play it, or it breaks when you turn it on. If its been sitting for a while, get a variac so you can bring it and other equipment up slowly - reforming the capacitors, or they will blow up.  If its been off for a few years, it takes some time - a few hours, to do this right.

[SubSub1]

Evening, I hope you are all well.
I just checked back here last night to see if there had been any new posts on this one.  I wasn't expecting any as I thought that I had notifications turned on.  Lo and behold, GLouis, you  scanned and posted the document almost right away.  Out of hope, I have started trying to colour in and repair the faded ones I already had.  I must say that I am delighted and am more than ready to get stuck back into this project.  If I succeed I will begin a new thread explaining everything I did.
Thanks very much GLoius most appreciated, this is a lovely forum I must say.
Kind regards...Dan

[Sir Knight]

I moved over to what I believe is the input board. It is located in the rear right side of the chassis. It plugs in vertically to the main board.

I sorted and took notes on all of the locations of the worst capacitors I found in the NAD 3150.

The capacitors in the foreground were tested having over 30 Ohms ESR or "open circuit."

C801, C802, C525, C526 (47uf 25v) matched pairs in this amp that failed.

C419, C420 (10uf 16v) C407, C408 (10uf 35v) matched pairs on that small input board totally gone.

Smoky, I wanted to mention this in my PM to you earlier, but I think it would be better to post it here so everyone can make use of it.

NAD contracted the manufacture of this integrated amplifier to Fulet Industrial Taiwan. Following the Fulet numbering scheme, this is what your capacitor groups are:

C8xx group - power circuit
C4xx group - phono circuit
C5xx group - source voltage matching (or whatever its called - I'm still learning)
C6xx group - input stage

So that vertical card isn't the input board, its the complete phono circuit. Compare the 2150 and 3150 schematics and you'll see what I mean.

I just found a 2150 amp at a great price, I'm looking forward to comparing the two units in person.

[Smoky]

I've never seen so many cracked solder joints in a piece of stereo gear before!

Even on the "phono" board I found them. There, they mounted the through-hole components vertically which made them even more prone to stress cracks. After viewing the board under magnification, I went ahead and re-flowed all of the components.

No problems since 

[Sir Knight]

I agree. I think there had to be something their solder or solder processes did not like about age and/or heat. I have a 3155 integrated amp that had a bad dropout problem. Sometimes one of the channels might drop a little volume. A sharp hit on the nested two-gang volume/balance control would bring it back to normal but the thing would drop volume at random. Cleaning the pots seemed to fix it but the problem came back. I thought I'd have to buy another of those , but then I tried reflowing the solder joints on that control and the parts nearby on the PCB. Not only did the problem go away, the amp sounded better. When I recap that amp I'll reflow everything I can get to on the foil side.

Have you thought of replacing the DC offset and idle current adjustment pots? I'd like to put something in there that isn't so sensitive. A fraction of a degree and the DC offset swings 60mV!  The parts list calls the offset pots "variable resistors, Semi 8C 500B Flat" and the idle adjustment is "variable resistors, Semi 8C 100B Flat". I'm assuming that the "flat" part refers to these pots laying on their backs? Is "500B" another way of saying "500ohm"? The letter C in "8C" is actually a cent mark - what is that? I've never seen nomenclature like this.

I'd really like to find good Bourns multi-turn pots that will do the job. Do you have any recommendations?

[cdev]

Thats probably denoting the taper on the pots.

I agree. I think there had to be something their solder or solder processes did not like about age and/or heat. I have a 3155 integrated amp that had a bad dropout problem. Sometimes one of the channels might drop a little volume. A sharp hit on the nested two-gang volume/balance control would bring it back to normal but the thing would drop volume at random. Cleaning the pots seemed to fix it but the problem came back. I thought I'd have to buy another of those , but then I tried reflowing the solder joints on that control and the parts nearby on the PCB. Not only did the problem go away, the amp sounded better. When I recap that amp I'll reflow everything I can get to on the foil side.

Have you thought of replacing the DC offset and idle current adjustment pots? I'd like to put something in there that isn't so sensitive. A fraction of a degree and the DC offset swings 60mV!  The parts list calls the offset pots "variable resistors, Semi 8C 500B Flat" and the idle adjustment is "variable resistors, Semi 8C 100B Flat". I'm assuming that the "flat" part refers to these pots laying on their backs? Is "500B" another way of saying "500ohm"? The letter C in "8C" is actually a cent mark - what is that? I've never seen nomenclature like this.

I'd really like to find good Bourns multi-turn pots that will do the job. Do you have any recommendations?

Hope you find them.

[Shock]

An alternate method is adjusting fixed values with the pot, but keep the overall circuit value the same.

[Sir Knight]

Thats probably denoting the taper on the pots.

I agree. I think there had to be something their solder or solder processes did not like about age and/or heat. I have a 3155 integrated amp that had a bad dropout problem. Sometimes one of the channels might drop a little volume. A sharp hit on the nested two-gang volume/balance control would bring it back to normal but the thing would drop volume at random. Cleaning the pots seemed to fix it but the problem came back. I thought I'd have to buy another of those , but then I tried reflowing the solder joints on that control and the parts nearby on the PCB. Not only did the problem go away, the amp sounded better. When I recap that amp I'll reflow everything I can get to on the foil side.

Have you thought of replacing the DC offset and idle current adjustment pots? I'd like to put something in there that isn't so sensitive. A fraction of a degree and the DC offset swings 60mV!  The parts list calls the offset pots "variable resistors, Semi 8C 500B Flat" and the idle adjustment is "variable resistors, Semi 8C 100B Flat". I'm assuming that the "flat" part refers to these pots laying on their backs? Is "500B" another way of saying "500ohm"? The letter C in "8C" is actually a cent mark - what is that? I've never seen nomenclature like this.

I'd really like to find good Bourns multi-turn pots that will do the job. Do you have any recommendations?

Hope you find them.

Sorry to bring up this old thread, but I was reviewing it to refresh my knowledge and I found that I'd never replied to cdev's post. After a lot of searching I found the appropriate Bourns pots:

3296Y-1-501LF
3296Y-1-101

Expensive little things at $6.57 total for one of each value, but they do the job. I had to extend one lead on each pot due to the through-hole spacing vs the part dimensions, but thats why I save the trimmed leads off every cap and resistor that I install in my projects.

Even though these are multi-turn they didn't have the granularity that I expected in the adjustment - it was still somewhat of a struggle to get the values dialed in where I wanted them. Regardless, it was better than the jumpy nervous wrecks that were the original parts, and no need for nail polish or paint to hold them in adjustment after.

With those installed my 3150 project was complete, and once dialed in it sounded freaking awesome. I turned it over to its rightful owner, my uncle, who has it driving some Canton speakers and he's completely thrilled with it. I bought extra pots because NAD used these same values across most of their 1980-1987ish designs.

[davepinkham]

I've been working on my NAD 3150 and found this thread super helpful,  so thanks! In the vein of the last couple comments, I discovered (after watching a youtube video linked at the bottom) that both of the bias VRs are open in my unit and that one of the VRs for DC offset has previously been replaced by a tech. I will be replacing them all. Just wanted to chip in a bit of info in the hope it helps someone!

Replacement parts I ordered:
652-3339P-1-501LF
652-3339H-1-101LF

[Sir Knight]

I've been working on my NAD 3150 and found this thread super helpful,  so thanks! In the vein of the last couple comments, I discovered (after watching a youtube video linked at the bottom) that both of the bias VRs are open in my unit and that one of the VRs for DC offset has previously been replaced by a tech. I will be replacing them all. Just wanted to chip in a bit of info in the hope it helps someone!

Replacement parts I ordered:
652-3339P-1-501LF
652-3339H-1-101LF

Thanks for posting that video. I'm open to anything that will help me learn more about vintage NAD components.

As I mentioned above, when you install your new pots you'll find that one lead doesn't extend far enough to make it through the board, but the other two do. Just bend the leads so you get those two through the board and the third lead gets as close to the appropriate mounting hole as possible. I save the leads trimmed off all the passives that I install, so I pick the thickest lead that will fit through the board and solder that in the hole so there's a few mm sticking up. Put the pot in place, solder the two leads that will reach through their respective holes, and then you can simply solder the "orphan" lead to the new extension sticking out of the board. Trim, clean the flux, and you're good to go. I might have time to get back into my 3155 rebuild this week, so if you need pics detailing this mod let me know.

One thing that I forgot to mention - to save myself some time I measured the resistance of each pot that I pulled and then adjusted the new pot to that value prior to installation.

[davepinkham]

Here's a couple more, mostly on setting the DC offset and bias:



Interestingly both this one and the previous one had a problem with the main power switch.

I appreciate the advice on installing the trim pots! I've done something similar many times with old music gear. Just cut the old components out, twist the remaining lead into a loop, and solder the new component to the leads rather than heat up the traces.

[Sir Knight]

I appreciate the advice on installing the trim pots! I've done something similar many times with old music gear. Just cut the old components out, twist the remaining lead into a loop, and solder the new component to the leads rather than heat up the traces.

Thats a great tip. I think that could be better than what I suggested, the only drawback would be trying to keep the new part oriented properly while doing the solder work.

[madires]

That's a common practice by many repair shops to avoid removing a PCB, especially if it would involve some additional effort. The downside is that with a short lead to solder to the solder joint on the PCB can still get hot enough to melt partially or completely, which can cause problems, e.g. the cut lead can slide down a bit and touch something below the PCB.

[Sir Knight]

That's a common practice by many repair shops to avoid removing a PCB, especially if it would involve some additional effort. The downside is that with a short lead to solder to the solder joint on the PCB can still get hot enough to melt partially or completely, which can cause problems, e.g. the cut lead can slide down a bit and touch something below the PCB.

Thats totally correct.

But there's an easy way to prevent heat-slippage. Instead of using just enough wire to make an extension post, the tech should use the longest lead possible. When he solders the part to the extension, the longer lead absorbs more heat along its length, effectively cooling the lead faster and preventing the pad/part connection from slipping or otherwise loosening. Then trim and clean as normal.

Since I have a dish full of trimmed leads around an inch long or better, I used this technique with great results. Not one bit of slippage out of the four extensions I installed in the recent 3150, and since this and most other NAD products have removable bottom panels its easy to flip the thing and check the work.

[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.

[Sir Knight]

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. 

[Smoky]

I tell you what would top-off this thread, it would be an image of the 3150 chassis through the lens of an infrared camera

Even with those huge heatsinks in place, you could boil a pot of hotdogs above this amp!

The 3150 surely doesn't belong in a "closed" cabinet. It needs to rest on an open shelf or a countertop.

Thanks for adding the great repair tips too

[Sir Knight]

I tell you what would top-off this thread, it would be an image of the 3150 chassis through the lens of an infrared camera

Even with those huge heatsinks in place, you could boil a pot of hotdogs above this amp!

The 3150 surely doesn't belong in a "closed" cabinet. It needs to rest on an open shelf or a countertop.

Thanks for adding the great repair tips too

You're welcome Smoky, glad I could contribute.

I have to wonder if there's not something else happening with your amp. With the 3150 I just finished, yes the hot resistors get hot, but there's not much heat, if that makes sense. High temp but not a lot of thermal energy. The top cover gets warm but no more so than anything else in my collection. I'm wondering if you might have missed something in there, perhaps a cap slipped by that is seeing a huge ESR increase, or maybe there might be some incorrect resistors in locations around those hot spots?

Speaking of the hot resistors, I replaced mine with some 12W Mills, but if I could do it over I'd probably try something in their TMC-5 or -10 line, the kind with the integrated heat sink. It would broadcast more heat instead of passing it down the traces. Perhaps that would help the amp live longer if not sound "better".

[Smoky]

The heat wasn't radiating from any of the resistors. IIRC, the heat was rising form Q801/Q803 (2SD669/2SB649) and a voltage regulator. Those babies get hot. They cooked the electrolytic capacitors nearby. I added large heatsinks to them to spread the heat around. I tilted the capacitors away on their sides too

[Sir Knight]

Yeah sorry I forgot about your extreme duty heat sink (that was a couple pages and several months ago, you know how the old brain works...) I know your high temps weren't coming from the resistors but what I was wondering is if you have some caps going to high ESR, or perhaps some incorrect resistor values somewhere on the board, which could be diverting too much juice to those transistors. The idle and bias have been set and that should be the end of it, but the 3150 I had here had none of those issues - before or after the capacitor service was complete. Also, when the 3150 was new it ran cool. I don't recall a single heat-related issue with any of our demo units at the store nor did any of the customers report an issue. The later 3155 was the same way. But as others have said, NAD was famous for putting all their money into the circuit design, not the components. They produced stuff that sounded exceptional for a couple of years and then started to slide as the parts went out of spec, but if you spend the cash on high quality parts you end up with magic in a box.

Its too bad we can't put our amps side by side and look at current in various spots and see what gives with your amp. I'm betting something is moving current someplace that it shouldn't.