NAD 3150 Stereo Amplifier Repair - Page 1
Forum section: electronics / repair

author: Smoky
author_rank: Frequent Contributor
timestamp: March 12, 2020, 12:23:26 am
content: 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 isnoresistor 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

author: SpecialK
author_rank: Regular Contributor
timestamp: March 12, 2020, 02:27:21 am
content: 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".

author: Smoky
author_rank: Frequent Contributor
timestamp: March 12, 2020, 03:20:57 am
content: 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:

author: Smoky
author_rank: Frequent Contributor
timestamp: March 12, 2020, 03:33:42 am
content: 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.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: March 12, 2020, 03:44:09 am
content: 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)/

author: Smoky
author_rank: Frequent Contributor
timestamp: March 12, 2020, 04:00:25 am
content: 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!

author: floobydust
author_rank: Super Contributor
timestamp: March 12, 2020, 04:20:50 am
content: 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.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 12, 2020, 08:36:35 am
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: March 12, 2020, 08:57:39 am
content: 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 heatsinkIt 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

author: ArjenCNX
author_rank: Contributor
timestamp: March 12, 2020, 09:50:17 am
content: 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.

author: bd139
author_rank: Super Contributor
timestamp: March 12, 2020, 10:03:36 am
content: Quote from: floobydust on March 12, 2020, 08:57:39 amI'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)

author: floobydust
author_rank: Super Contributor
timestamp: March 12, 2020, 08:33:44 pm
content: 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

author: Smoky
author_rank: Frequent Contributor
timestamp: March 12, 2020, 09:54:24 pm
content: 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 itThe 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.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 12, 2020, 11:51:31 pm
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: March 13, 2020, 05:44:57 am
content: 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.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 13, 2020, 07:29:12 am
content: 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.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 13, 2020, 08:18:53 am
content: 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.

author: Zucca
author_rank: Unknown
timestamp: March 13, 2020, 02:00:02 pm
content: For ESR testing you don't need to desolder them generally.

author: madires
author_rank: Super Contributor
timestamp: March 13, 2020, 02:53:00 pm
content: Quote from: Smoky on March 13, 2020, 08:18:53 amI'd suggest to replace them with film caps.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 13, 2020, 07:25:33 pm
content: 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, evensooner.Just having funThat 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!

author: Smoky
author_rank: Frequent Contributor
timestamp: March 13, 2020, 07:50:05 pm
content: 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 orderBUT 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 foundThere 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 MatsushitaWho is:The founder of Panasonic!

author: Smoky
author_rank: Frequent Contributor
timestamp: March 14, 2020, 10:30:43 pm
content: Guys, was there any doubt? You betcha. But she's back and breathing on her ownThe 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.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 15, 2020, 12:33:17 am
content: 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 paintI 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 cucumberGuys, 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:

author: GLouie
author_rank: Regular Contributor
timestamp: March 15, 2020, 06:16:43 pm
content: 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.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 15, 2020, 11:33:00 pm
content: 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

author: floobydust
author_rank: Super Contributor
timestamp: March 16, 2020, 04:39:21 am
content: Glad to hear the repair worked outNAD 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.

author: SubSub1
author_rank: Newbie
timestamp: May 06, 2020, 02:06:27 pm
content: Quote from: GLouie on March 15, 2020, 06:16:43 pmDear 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

author: Shock
author_rank: Super Contributor
timestamp: May 06, 2020, 08:55:47 pm
content: Nice repair smokey and good job documenting it. I also read yourcassette repair as well.

author: Specmaster
author_rank: Super Contributor
timestamp: May 06, 2020, 10:11:42 pm
content: 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.

author: GLouie
author_rank: Regular Contributor
timestamp: May 06, 2020, 11:10:09 pm
content: 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.Quote from: SubSub1 on May 06, 2020, 02:06:27 pm

author: cdev
author_rank: Super Contributor
timestamp: May 06, 2020, 11:26:30 pm
content: 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.

author: SubSub1
author_rank: Newbie
timestamp: December 05, 2020, 05:57:55 pm
content: 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

author: Sir Knight
author_rank: Contributor
timestamp: March 20, 2021, 01:39:21 am
content: Quote from: Smoky on March 12, 2020, 04:00:25 amQuote from: Smoky on March 13, 2020, 07:50:05 pmSmoky, 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 circuitC4xx group - phono circuitC5xx group - source voltage matching (or whatever its called - I'm still learning)C6xx group - input stageSo 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.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 21, 2021, 03:05:02 am
content: 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

author: Sir Knight
author_rank: Contributor
timestamp: March 25, 2021, 02:41:24 pm
content: 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?

author: cdev
author_rank: Super Contributor
timestamp: March 25, 2021, 10:30:19 pm
content: Thats probably denoting the taper on the pots.Quote from: Sir Knight on March 25, 2021, 02:41:24 pmHope you find them.

author: Shock
author_rank: Super Contributor
timestamp: March 26, 2021, 12:48:46 pm
content: An alternate method is adjusting fixed values with the pot, but keep the overall circuit value the same.

author: Sir Knight
author_rank: Contributor
timestamp: August 15, 2021, 12:31:19 pm
content: Quote from: cdev on March 25, 2021, 10:30:19 pmSorry 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-1-501LF3296Y-1-101Expensive 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.

author: davepinkham
author_rank: Newbie
timestamp: September 04, 2021, 07:59:29 pm
content: 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-1-501LF652-3339H-1-101LF

author: Sir Knight
author_rank: Contributor
timestamp: September 05, 2021, 01:20:09 pm
content: Quote from: davepinkham on September 04, 2021, 07:59:29 pmThanks 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.

author: davepinkham
author_rank: Newbie
timestamp: September 05, 2021, 01:40:43 pm
content: 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.

author: Sir Knight
author_rank: Contributor
timestamp: September 05, 2021, 02:28:25 pm
content: Quote from: davepinkham on September 05, 2021, 01:40:43 pmThats 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.

author: madires
author_rank: Super Contributor
timestamp: September 05, 2021, 04:26:10 pm
content: 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.

author: Sir Knight
author_rank: Contributor
timestamp: September 06, 2021, 01:43:55 pm
content: Quote from: madires on September 05, 2021, 04:26:10 pmThats 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.

author: David_AVD
author_rank: Super Contributor
timestamp: September 06, 2021, 08:40:34 pm
content: 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.

author: Sir Knight
author_rank: Contributor
timestamp: September 06, 2021, 09:46:20 pm
content: Quote from: David_AVD on September 06, 2021, 08:40:34 pmMy 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.

author: Smoky
author_rank: Frequent Contributor
timestamp: September 06, 2021, 10:10:49 pm
content: I tell you what would top-off this thread, it would be an image of the 3150 chassis through the lens of an infrared cameraEven 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

author: Sir Knight
author_rank: Contributor
timestamp: September 07, 2021, 12:55:03 pm
content: Quote from: Smoky on September 06, 2021, 10:10:49 pmYou'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 gethot, but there's not muchheat, 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".

author: Smoky
author_rank: Frequent Contributor
timestamp: September 07, 2021, 01:40:06 pm
content: 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

author: Sir Knight
author_rank: Contributor
timestamp: September 07, 2021, 02:11:49 pm
content: 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.

author: Sir Knight
author_rank: Contributor
timestamp: September 07, 2021, 02:16:55 pm
content: 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.

author: Smoky
author_rank: Frequent Contributor
timestamp: September 07, 2021, 03:02:51 pm
content: 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.

author: Sir Knight
author_rank: Contributor
timestamp: September 07, 2021, 03:12:35 pm
content: 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.

author: David_AVD
author_rank: Super Contributor
timestamp: September 08, 2021, 03:14:54 am
content: Quote from: Sir Knight on September 06, 2021, 09:46:20 pmIf you're not tacking onto existing cut legs, why are the legs being extended?

author: Sir Knight
author_rank: Contributor
timestamp: September 08, 2021, 01:11:47 pm
content: 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.

author: David_AVD
author_rank: Super Contributor
timestamp: September 13, 2021, 05:05:23 am
content: Ahh... gotcha. Thanks for clearing that up.