NAD 3150 Stereo Amplifier Repair

[Smoky]

A NAD amplifier found its way onto my workbench recently. A friend found out I was dabbling in electronics and asked if it was possible that I could bring it back to life. I couldn't guarantee him 100% but would try.

First off, when I lifted the lid on this thing, I knew it was going to be a challenge. Darkened PC board areas due to components overheating and the obvious signs that someone else had been working on it before. Not good. So I'll keep this thread short and show pictures of what I've found and what I've started to do to turn things around in it.

I lack troubleshooting knowledge for this kind of stuff but I'll keep focus on the obvious and hopefully it will lead to a happy ending!



The first thing my eye noticed was this burnt area near a relay and diode D615:



My first thought "this is not factory!"

First, a large 1K Ohm resistor was in parallel with a 1N4002 (D615) diode.

Second, another large resistor next to it has a burn spot and unrecognizable color rings.

So, I removed this stuff to include the small black capacitor nearby. The capacitor didn't match any of the other capacitors in this chassis since the rest were original and colored light-blue. Plus, this black capacitor had no name on it, just numbers.

Here is what the area looks like right now after a good cleaning. The two new black Nichicon 105c capacitors with the yellow dots next to them are what I just installed. The rest of the capacitors for the entire chassis are on order.



I was able to download the schematics from the internet but they are very fuzzy and "write-protected" so I'm unable to share small portions of it.

D615 is a 1N4002 diode. R676 is supposed to be a 2 Watt 680 Ohm metal oxide resistor. There is no
resistor anywhere near this area that is supposed to be a 3 Watt 1K Ohm resistor!

Here are the pulled parts. The reisistor with the yellow dot is R676. It measures 526 Ohms! Not the 680 Ohms as stated in the parts list of the service manual.



I can already tell that this project is going to be fun!

...and here's a sample of one of the original 10uf 50v 85c capacitors. It is stamped "CE" with a triangle. NAD mounted these small capacitors right next to transistors with heat-sinks 

[SpecialK]

What were the reported symptoms it came in with?  Did you test the unit?

It doesn't look like the resistor value should be that critical.  It seems to limit the current through the relay coil.  I believe coil would tolerate the lesser current the 1k resistor would provide.  It does look like they drilled for two different footprints for R676 too.  Perhaps there was a service bulletin that prescribes certain "fixes".





 

[Smoky]

SpecialK, I was told it had loud hum, and when switching between some inputs, there would be no sound at all.

I did not power the amplifier on after I took a look inside of the chassis.

The first step I took was to remove the bottom cover and de-solder the power transistors and test them.

There are two 2SA1215 (PNP), two 2SC2921 (NPN), and two BC556 (PNP) transistors attached to the heat sink.

They all pass the basic diode tests.

I also set the multimeter to the 2K Ohm setting and found no shorts from the emitter to the collector on the 2SA1215 and 2SC2921 transistors.

The amplifier has one fuse and it is good.

Are there other ways of testing the transistors or were my tests good enough? Thanks!



By the way, here's the typical reading of 47uf caps in the amp:

[Smoky]

This picture is only for comparison.

I found this picture of a NAD 3150 on Google.

Notice R676. I can not read its value. It looks larger too.



I only have fuzzy schematics and a parts list. No repair or revision notes.

All I can say is that I ordered a new Panasonic 2 Watt 680 Ohm metal oxide resistor based off of the schematic.

[HackedFridgeMagnet]

Not sure if it is much help but I repaired and still use a NAD 3020A ?

It did have hum and replacing the caps fixed it.

https://www.eevblog.com/forum/repair/just-finished-my-longest-ever-repair-(nad-back-from-the-dead)/

[Smoky]

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 removed the old capacitors and cleaned the board. I installed four more new 10uf at 50v Rubycon ZLG capacitors. The 1000uf at 6.3v capacitors should be here tomorrow. Most of the capacitors are listed in pairs, so I closely matched all pairs of capacitors.

Since NAD has all electrolytic capacitors listed as -10/+50, I also ordered 1200uf capacitors in the event the 1000uf caps come up short.

I also took the same route for the large filter caps on the main board. So in addition to the 4700uf caps, I ordered 5600uf capacitors too.

I also tested all of the small transistors and they appear to be fine while in circuit.





Thanks for the link HackedFridgeMagnet!

[floobydust]

Check the HV regulators are OK, I believe Q801 generates the +45V rail and R676 is the dropping resistor for the loudspeaker protection relay by dropping 45VDC down to the 24VDC for the relay coil- check the relay coil resistance is still good too. The schematic gives 830R for 29mA which is a little piggy for power and the 680R dissipates 0.58W for the 2W part, so not really the heat that is seen.
If Q801 and related IC801 have failed, it would overvoltage things. Also be careful with ESD as the JFETS are sensitive and unprotected.

[Smoky]

Good to hear from you FloobyDust!

Here's a shot of Q801 surrounded by a bunch of those failing 47uf caps.

I put a yellow dot above a hidden four-legged IC behind a capacitor too.



As for the coil in the relay, I followed the two diode solder pads and I do get continuity across those two points with my DMM.

[floobydust]

Schematic says Q801 is a 2SD665, a big TO-3 part, so I'm a bit puzzled what is that is a tiny TO-126 part on a tiny heatsink   It could not take rock and roll.
From the messy extra silicone thermal grease, somebody has replaced Q801.
Q801 is a 2SD669 TO-126 part for Vreg supplying driver stage and prot. relay.
When you power the unit back up, can check the output voltage. If the HV regulator is shorted, then it overvoltages the relay circuit and C804 to over 60V. I would check Q801 C-E and hopefully it is not shorted. I think the pinout is ECB for whatever part is in there.
The relay I wondered how many ohms the coil is, if low that explains the heat.
This NAD reminds me of Sony products where they have hot running parts on purpose just to shorten lifetime.

edit: had wrong p/n for Q801

[ArjenCNX]

just a reminder, those caps are tested at 120HZ, if you test them higher, the value drops, and it looks like you have a bad cap, but its A-OK.

[bd139]

This NAD reminds me of Sony products where they have hot running parts on purpose just to shorten lifetime.

I've repaired a few NAD and Sony items over the years. I agree with your assertion. There is a lot of marginal engineering in their historical products. Even Realistic brand stuff was better.

They seem to have improved over the years and the later products such as the C320 and on are actually quite nicely engineered (apart from those horrible microphonic green blob poly caps they still use  )

[floobydust]

Sony audio equipment I've repaired frequently has a lonesome IC/transistor running at +85C a finger burner.
It's not like the engineers didn't check the prototype, I think they ran out of product budget money or want the product to last only a few years.

The NAD 2150 used the same circuit here as the NAD 3150 (680R 2W).
I see some of these cooked many parts and people added extra heatsinks as well. I think the HV Vreg must fail?

WARNING loud corny music and roastie parts

[Smoky]

This is good stuff! I just wish that I would've started this hobby 30 years ago.

ArjenCNX, I have three capacitance meters and they all show the old CE electrolytic caps as failing. An LC53, LC102, and the Peak ESR 70.

But who knows? Well, when you mentioned testing at 1kHz, I'm new to this hobby and not sure how those testers come to their result.

But get this, I was given a Huntron Tracker 2000 a year ago and it has a section in the handbook showing how to test resistors, capacitors, and inductors. So I set up a comparison test. I'm using a brand new Nichicon UHE 10uf 50v to compare against the old CE 10uf 50v cap in the picture above being tested by the Peak ESR 70.

Give me a few minutes to put up pictures of what I discovered...

I set the Huntron Tracker to 2000Hz and selected MED 1 range. Here is the old CE capacitor:



I then attached the new Nichicon capacitor:



From what I've read in the Huntron handbook is if a signature tilts on an angle, that is the level of resistance.

The new Nichicon capacitor has zero, or close to it, of ESR.  And to let you know, its signature is an elipse shape but it's so "fine" my cheap Walmart phone can't see it

The handbook also reads that if the Huntron is set to the "Med 1" range, the angle of 45 degrees represents 1K Ohms of resistance. In the "MED 2" range, 45 degrees represents 15K Ohms. In the "High" range, 45 degrees represents 50K Ohms. And in the "Low" range, 45 degrees of tilt represents 50 Ohms. Awesome!

I'll be putting in all brand new electrolytic capacitors into this amplifier.

[Smoky]

FloobyDust, I pulled and cleaned Q801.

It is a 2SD669A-C.

The schematic reads 2SD669C as being the correct part. I've read that the last letter "C" is the classification of hFE (100-200).

I gave it a diode test and an emitter to collector short test too.  All seems OK.

Fresh silver heatsink compound and she's back in place.

I'll test every transistor.





I also measured the resistance of the coil in the switching relay with a DMM. It measures 750 Ohms.

[floobydust]

OK I misread it, the first NAD 3150 schematic is a blurry pic and "5" vs "9". Q801 is taking in +60.6V regulating down to +46.2V to power the driver stage and relay.
The NAD 2150 3150 4150 7150 schematic shows 2SD669 for Q801.

Sorry didn't mean to give you a goose chase but some of these amps look fine while others have cooked parts inside, so I suspected the Vreg is failing and causing that. Might be best to check all the rails when you power it up. Many caps are running near rated voltage too, 63V at 61V and 50V at 46V so no room for anything to go high.

[Smoky]

Guys, everything is going really well!

The input board is re-capped and installed:



I removed every IC and transistor that had a heatsink. I cleaned them, tested, and with new compound, they were installed. I think they are original. The old compound was so dry it flaked off the components. Notice the IC, it has the same company logo as the old capacitors:



I also flipped the heatsink around on the small transistors to get some air to their legs when installed. The landscape looks much more airy now:



I also went with the Nichicon LGY 5600uf 50v main filter capacitors. They measured 5200uf on the Sencore. The original capacitors were of the 4700uf type:



FloobyDust, I found four of the old capacitors that wouldn't measure on my capacitor testers for capacitance or ESR. They aren't shorted because I used my DMM checking for resistance. Weird.

[Smoky]

Hey guys, I need your opinion on these capacitors as to what they are.

I went to the schematics and part's description section of the service manual, I found out that there are six caps in this amplifier that are described as "electrolytic-solid" capacitors.

C543 and C544 (.68uf 16v)

C601, C602, C603, C604 (1uf 16v)

Looking at the schematics, I'm guessing that these are "coupling" capacitors.

Are these types of capacitors known to go bad? I'm also guessing that they are a tantalum variant.

[Zucca]

For ESR testing you don't need to desolder them generally.

[madires]

C543 and C544 (.68uf 16v)
C601, C602, C603, C604 (1uf 16v)
Looking at the schematics, I'm guessing that these are "coupling" capacitors.

I'd suggest to replace them with film caps.

[Smoky]

So I pulled one of the Sanyo 1uf 25v capacitors to test it out of circuit. It reads 1.57 Ohms for ESR, which is still very good.

I read online that these capacitors are of a "polymer" type known for long-life, low-ESR, and excellent noise reduction.

So, these are indeed coupling capacitors.

I'm going to test each one to ensure that they are good and leave them alone for now.

Notice how one of the leads comes from the edge of the capacitor:



I just found one of the four 1uf Sanyo capacitors measuring slightly higher in ESR than the others.

I notice slight differences as caps alternate on the Huntron Tracker.

I'll put them back in but will investigate good replacements in the future, or if noise is obvious, even
sooner.



Just having fun

That Huntron is so cool! You set up one good capacitor with one set of leads and then use the other set of probes to compare other caps while looking for differences on the screen as they alternate.

If you look closely at the picture, I put the left side of the oval dead center on the horizontal graticule line. You can see the slight rise of the oval shape on the right.

Using the horizontal and vertical controls, I can adjust the placement of the signature anywhere on the screen for better reference too. I'm just scratching the surface at what this Tracker can do. What a great tool.

I bought good quality E-Z Hook grabbers recently, but I can see, I'll need more styles like micro-grabbers for tight components for in circuit testing and a cradle device to do more hands-free bench testing.



Electronics repair is an addiction!

[Smoky]

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.

C805,C807, C808, C641 (10uf 50v) burnt capacitors on the main board near transistor heatsinks.

I replaced all capacitors with higher temp 105c types. I replaced these and others with higher voltage specs too.



Next, I re-attached the two banks of power transistors. I made sure that every bit of flux was removed between the pads too:



What bums me out is I don't want to re-install that old 1N4002 diode near the relay. It tests good but you can tell it's seen some heat. On top of that, not one local shop in Raleigh has any for sale. I forgot to add a few to my last Digi-Key order 

BUT WAIT!!! You guys are not going to believe this. I have very little in new electronics parts. Most of what I have is high-voltage tube amp stuff. What I have in the corner of my rec room though, sitting next to an old microfiche viewer, is a Sencore SC61 donor machine. So I ventured inside of it.  Look what I found



There are no less than 12 Motorola 1N4004 diodes in perfect condition! Two are right on the egde of the power supply board. Now they have a higher voltage rating but shouldn't they still work in place of a 1N4002?



Alright, the Motorola diode is installed and a 3 Watt 680 Ohm resistor is in place. I have the resistor mounted up off of the board to get some extra air around it too:



I also installed a jumper wire from pin 6 of IC701 (TA7317P) to R676 (680 Ohm 3 Watt). The trace on the back of the main board necked-down really narrow from the IC to the resistor. The other end of R676 is just fine since it goes to a super-wide trace that ties-in the new 1N4004 diode and the relay's coil. So things are beefed-up pretty good on the back side.

I have just a couple of small things to tidy-up and then it's time to bring her back to life

...and I also found out who the "triangle" logo belongs to:



Konosuke Matsushita

Who is:



The founder of Panasonic!

[Smoky]

Guys, was there any doubt? You betcha. But she's back and breathing on her own

The amplifier has been running for about 45 minutes now an it's stable as can be. I tested R676 a few times now and it sits steady! I've touched R676 a few times now too and it seems to be at room temperature. And there is no noise from the speakers while it's at idle. I have two small Bose 101 speakers on the bench.



Now, my uneducated guess is that when the old C800 series of capacitors started to burn up from the heat of the transistors, it sent the voltage sky-rocketing to the Moon and that's when the resistor burning began. I can feel the warmth rising from that group of transistors with heatsinks as I type this. A fan would do wonders.

Before start-up, I cleaned every switch with DeoxIT Fader F5 and went through the amp to make sure that no two components touched each other.

I'm reading the service manual about how to check the bias. This will be my first time doing that.

FloobyDust, remember that I mentioned that the old R676 measured 526 Ohms? Would that value have brought the voltage at the relay closer to 24v? I was just wondering if that was a selected value installed at the factory. No big deal unless you think it needs to be closer to 24v.

Anyway, the amp is playing a sweep between 20Hz and 20kHz. I have only one BNC to RCA cable, so every now and then, I just press the "mono" switch. It's music to my ears



...and just moments ago, I put the amplifier on its side while I touched every power transistor. They are all just lightly-warm and even. The amp has been putting out about 7 watts per channel for the past hour now.

[Smoky]

This is so cool!

Here is the short paragraph on the alignment procedure for this amplifier. This amp does not have a tuner:



So, I start with the DC Off-set Alignment. I find VR601 and VR602 totally covered in paint I slowly, with a small stiff brush of lacquer thinner, I scrub until it allows me to turn it with my "modified" screwdriver. Look how the paint even oozes out from under the disc.

I put a drop of DeoxIT Fader to clean up any scratchiness. Let's see if this amp will get to 0mV!



Definitely touchy, but, I got the right channel flopping between +.7 to -.7mV depending on how much I breathe on it!



I forgot to mention, the right channel measured -167mV and the left -220mV when I started.

Not bad, the left came down nicely too. That DeoxIT really smooths-out the movement. I'll put a dab of acrylic paint over the VR's when I'm done. Both VR's have plenty of adjustment both ways, so that says to me, the circuitry must be close in tune.



...and after three hours, R676 is still as cool as a cucumber


Guys, I'm stumped on the next step. It reads that there is a TP1 and a TP2 on the foiling side of the board. I imagine they mean the trace side.

On the schematics, it shows the test points not far from the matching VR's, but, I can't find these test points to save my life!

This amplifier doesn't have fuses for each channel which would ease this process.

So I shifted to a test that goes like this:

I set my waveform generator to 10kHz at 50mVrms. I fed that signal to the Tape 1 inputs of the amplifier. With my DMM set to AC millivolts, I measured the left and right channel speaker output terminals.

The left channel measured 67.8mV and the right channel measured 67.2mV.

It seems to me that both banks of output transistors are still closely in balance. And since no transistors were replaced, could the original Idle Current Alignment still be acceptable?

I'm going to do a couple tests with my oscilloscope tomorrow.

Good stuff!

*Update*

So I let the NAD 3150 amplifier play for over four hours. During that time, I noticed a transistor and a voltage regulator remaining very hot. I found that they had very small heat sinks attached to them to address the dissipation of heat. I fabricated two heat sinks much larger than the originals and they work like a charm!



On top of it all, check for cracked solder joints around the perimeter of the PC board. Here are a few of the ones that I found:

[GLouie]

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.

[Smoky]

Thank you GLouie, the schematics available online are no way near as sharp as yours. Both VR's were adjusted so both channels read 5.6mV, just about in the middle of the range suggested by the schematic.

And, the amplifier made it back to its owner today too!

We hooked up the NAD 3150 to a vintage pair of Boston Acoustics A40 bookshelf speakers and a CD changer loaded with a little bit of Elvis Costello to break it in. It sounds terrific!





Next is the repair of a NAD 6130 cassette deck