NAD 7240 Receiver Part 2 was like a kick in the NADs

[@MrElectrowhiz]

That was a tricky repair. You done a great job on diagnosing the amp and getting it to work. On a side note, I noticed the TDK AD tape box on the bench. That is about 1972 or 1973

[@tonybalm1513]

You remind me so much of my late father. You said his exact words for D.C coupled stages. He spent many an hour fixing amps like this!!

[@JImmY1982MinG]

Thank you for continue to upload these great videos. I have a few NAD pieces that stopped working, watching your videos had helped me to understand why people did not like NAD stuff. You have inspired me to learn more about electronics and hopefully I will be able to work on my gears in the future. Until then, it'll at least give me an idea why my electrician always charges me a few hundred dollars on every piece I bring to him.

[@johnb5519]

Good , tenacious detective work, on that one. That kick in the nads had you bent over for a while there.

[@bwalsberg]

Terrific repair....I have the same amp that I picked up in an ebay deal for parts ( $20). I noticed all fusses blown started there...nope. Now onto transistors....oddly it was the left channel (the opposite of yours ) that were bad.....found that both needed to be replaced. Crossing fingers that that is it ....lol....probably not!!!! Your channel inspired me to tackle this, so thanks.....hahah! Also, bought a Hakko and sucker pump. This is very therapeutic especially during our “pandemic”.😃

[@raceingdemon6464]

Lovely Job !! Your Such a Champion For All Us Guys That Like To Fix Audio Gear Dave..Stay Safe My Friend Regards mike.

[@waynemckinnon1238]

I feel the same, love saving stuff. However, sadly, it's because of me that Dave's nads were kicked! Sorry Dave...

[@heinrichlubbe488]

I’ve been watching several of your videos and I must say you know your thing. This video relates to the problem I have with a NAD 3240pe. It had a small distortion in one channel and unfortunately when I was measuring I made a short at the output transistors and BOOM. After that it sat on the shelf for 2 years and now after seeing this video I have hope for it again and it’s off the shelf. Just waiting for parts. Thanks you are a genius at what you’re doing. Keep up the good stuff.

[@mickspencer4171]

Excellent video Dave,I wish I had your fault finding experience.

[@onemat2000]

I just picked one these NADs this week (in 2024). The unit is working well except the volume and balance pot acts up a little. That should be easy, but I am going to look and check for many of the things you found, and fix as needed. I'm actually using the Preamp out running to my Adcom power amp, as my Adcom Preamp needs work. I enjoy watching you work on these devices. It's very helpful. A few videos ago, I asked what model Casio you are wearing .... Please let me know.

[@buriedbits6027]

This was a great video. I have about 2% of your knowledge but I loved it! I have this amp and the thing smoked when I turned on two years or more ago. It's been sitting there. The part, or a part fell out of the unit when it smoked and I taped it to the back. I just took a look inside and yup, scraps of paper all around C422. The cap blew up two years ago, only now I was reading the board. I now have to read the board, and maybe desolder parts and measure them out of circuit. I have no clue how to measure in circuit. I know it may seem funny on how I might approach this but like I said, I got probably 1-2% your knowledge. If I get stuck I have to send it out for a repair. Love the 7240PE but looks really complicated. I spent 3 hours watching this and the last video, schematics on screen, marking them up to follow along. Quite a journey!

[@12voltvids]

It is complicated. It's a class H if i remember. 2 level power supply and it switched to higher power for peaks in the music. NAD call it power envelope.

[@jazbell7]

Having designed a number of those amplifiers (some time ago), I know that when an output transistor shorts, it takes out anything in series with it. Including any low value emitter resistors. In the old days, we had to deal with "second breakdown", which was a serious limitation.

[@FLACguy]

I really enjoy watching your videos and they have helped me repair several amplifiers and TVs so thank you very much for that. Yesterday I got my hands on a NAD 7240PE receiver and while checking it out I was able to set bias following the service manual procedure but noticed that the DC offset in one channel drifts significantly once the receiver has warmed up. I even cleaned the VRs with Deoxit but still one channel's DC offset drifts to more than half a volt after 30 minutes. Ideas?

[@DENI-ki5sx]

You must to make video for adjustment bias and center dc.really whe need that.thanks for a great video.

[@Budro4764]

When you added the 2 fuses I saw something funny looking to the north west of them ....... It might have been a shadow or smoked resistors ????

[@lionnelc13]

if i remember correctly, nad amplifiers have 2 voltages comming from the transformer (each wirh their own set of fuses), and the independence selector simple switch between them. THe missing fuses are only used in 4 ohm (or maybe 8

[@12voltvids]

@@Runco990 They supply the 43 volts to the outputs. The 70 volt supply goes to q432 and 432 and are part of the "envelope modulation" that will modulate the b+ and b- for peak power to allow instantaneous voltage peaks to prevent clipping. It's an interesting and overly complex solution that makes these even more difficult to service. Make no mistake these are a kick in the nads to work on. Very complex design. It gives the amp a 3db dynamic boost. With the 43 volts missing the outputs were drawing all their current from the 70 volt supply and that is likely what popped that 4th transistor because it definately was not shorted when I made my parts order. But looking at the schematic with loss of the 43 volts and that open 3.3 ohm resistor all if it was being drawn through that transistor. It upset the critical balance of this circuit.

[@EastAngliaUK]

my mate has this nad I have the 3020i mine sounds very good.

[@mordyvontessimus6403]

Hi! I am having an issue with a VCR. Can I send you a video of the symptoms for your advice? Thanks in advance.

[@barryinkpen6026]

I've had a 7240 for years and it has been a good unit... except I also had the output blow; twice! I couldn't find a root cause initially but decided to bypass the speaker switch to the "A" speakers which means the amp is always connected to the "A" speakers and I only switch the "B" speakers if used. Never had another issue in 15 years. Go figure...

[@joshm264]

I feel your pain about neighbors playing and swearing all the time, and I'm 15! I want to make videos at my bench but it's either them or a lawn mower. Also, can I get a link for the song at 13:17?

[@williamsquires3070]

Thinking about your problem with the signal getting through despite the lack of B+ (missing fuses), if this is a direct (DC) coupled amp and the 1st transistor had a B-C short, the signal source itself would become the B+ through the collector resistor, though it would be highly distorted as the signal source would see the big filter caps in the supply rail which would try to filter it just like ripple. This might be enough to allow the signal through. Also, the signal could make it through a common-emitter follower if the signal amplitude was large enough to induce a voltage drop across said emitter resistor to turn on the next stage.

[@12voltvids]

It was pulling power from the higher b+ source. This is a power envelope amp. Dual dc supplies.

[@ricokristensen3217]

Nad has power envelope. it is a higher supply to the output transistor in peak situation. that why there is som extra transistor in the output stage. and i think the extra fuses is for that. it can work whitout it and work like a normal amp, the peak output is just lower.

[@12voltvids]

Yes I already have gone over it.

[@snowman307]

Hey wondering what you charge to fix a Nad 7400.It worked for a while but then .It went into protection mode?Maybe a good video.

[@patrickjmorgan]

Well done- Id have been beaten a long time ago given the heat those boards have seen.

[@12voltvids]

It's a power amp they get warm.

[@Synthematix]

This always bloody happens dave, once those dam transistors pop the feeding resistor goes with it

[@12voltvids]

This is what happens in direct coupled amps. One of the major reasons that many manufactures moved fo ic power modules. They are dc amps. When something blows you change the module and move on to the next one. With discrete parts you spend much time finding all the blown parts.

[@Synthematix]

@@12voltvids Very very rarely in these amps does a single part blow, its usually at least 2 parts yep, but as you say they sound really good when theyre working properly, sounds silly but some of the best sounding amplifiers ive ever heard have the STK MKII modules

[@DeadKoby]

I still prefer guitar amps, since they aren't quite as over-engineered. Good work as usual sir.

[@12voltvids]

Lol a guitar amp is about as simple as they get. Especially the vacuum bulb type.

[@skdevanytube]

very informative--- thank you very very much

[@robinsonsoto8471]

Nice job.

[@jameskrivitsky9715]

Dave, at 29:24 you said jump on the Harley ? What bike do you have ? I'm a Hog rider too. 93 Dyna Wide and 2003 Ultra Classic. Maybe that is why I like your channel, besides fixing electronics.

[@12voltvids]

It's an 07 electraglyde full dresser. Get out on is whenever I can, pretty much every nice day in the summer.

[@rogueninja1685]

May be a dumb question, but why are mica sheets used on a heat sink? I thought mica was an insulator. Can you install the transistors without them for better heat dissipation or does that mess up the signal or anything else?

[@12voltvids]

It's mica is an insulator. That's why they are there. The metal back is the collector. Guess what would happen if you were to mount the metal back transistor with the b+ rail on it to a grounded metal heat sink. I will give you a hint. It involves magic smoke.

[@rogueninja1685]

Awesome. That's the answer I was looking for. My 7240 already had plenty of that. Great vid too BTW@@12voltvids

[@keithking1985]

you were saying that parts guy was working from home and his neighbors complained... i wish i had a place to get stuff a half hour away from my gaff..

[@alexispieltin9379]

Nice job again! These Nads are sometimes tricky to repair as you generally have an avalanche of components failing with limited protections and a somewhat inconsistent customer information. Once you've solved one part, the other is not fine, or you discover other problems. They sound good, but have a nasty reputation in the repair station. And once again, you're lucky to easily source "Japanese" reference transistors. It's always been a pain in the - - - to find a store here in Europe that has a large amount of these references. Digikey or Mouser Europe have a limited choice here, and are extremely expensive else you order gigantic amounts... So you generally ends up ordering parts by at least two or three stores on the continent, and pay equivalent on postage fees and for components. There was a time a paired output transistor like these should have cost 10 times that, and if these were in standard reference, they were available at the local store downtown. You now pay more for the service, and special delivery in 24 h at your door!

[@12voltvids]

Oh believe me I am aware of how they can go nuclear real quick.

[@williamsquires3070]

4.7ohm || 10 ohm gives roughly 3.19 ohm. 😊

[@nickfrench7372]

What is that white stuff that u put on the back of that transistor? What does it do? Is it like glue to that heatsink?

[@12voltvids]

It is heat transfer compound that fills the slight gaps between the component and heatsink.

[@nickfrench7372]

@@12voltvids Ok,,will have to look at getting that heat transfer compound for a few stereo units i have here with repairs due to sound troubles.

[@tommost1]

0:55 why I wear disposable gloves when I use thermal paste LOL.

[@12voltvids]

That's what soap and water is for.

[@tommost1]

@@12voltvids Most thermal paste isn't soluble in hand or dishwashing soap.

[@12voltvids]

@@tommost1 try baby oil. It will take off just about anything.

[@tommost1]

@@12voltvids I haven't oiled a baby in years but I'll try it thanks.

[@12voltvids]

@@tommost1 a roofer told me that trick for getting roofing tar off hands. Works better than lava soap. Leaves your hands as soft as a baby's but. Great for the grease in communications cables too.

[@jeremylister89]

That 47k resistor is dissipating best part of 400mW. Looks a bit small for that power level. It'll certainly run hot by design but has obviously survived a long time then finally given up.

[@CliveTrezona]

Really great repair. Would be nice to also be able to hear as you pan left to right. Does your camera have 2 separate stereo mics?

[@12voltvids]

It has a stereo mic on top.

[@mitchlazer7948]

nice save and thx for the parts guy referral

[@12voltvids]

Get your hard to find parts there. He has quite a good inventory. Keep him in business.

[@rawr51919]

@@12voltvids Especially right hot off the heels of a pandemic. Dude's gonna need every bit he can get.

[@pliedtka]

Dave, where can you get matched pairs Sankens, Toshibas, Motorola output tranies. Bryston used to sell matched Motorola output transistors. Once I got amp with everything past differential input gone: VAS, Bias Adj, drivers, output transistors. I noticed the heat sink is kind of undersized - no fins for +/-70V rails. So most likely the owner cooked the outputs - with today's music being so compressed dynamically, I wouldn't be surprised. Or too low speaker impedance, who knows. You make me look for the schematic, to study the sucker. So, no current current flow through differential input pairs for the second ch., maybe not enough wattage rating on resistor. Good only a single resistor, no semiconductors that are gone. You got lucky. OK, kpcomponents, Surrey, BC. Thank you F... Digikey shipping... This guy is awesome.👍 Thank you again, he has belts, motors, transistors, etc.

[@pliedtka]

BTW, looking at the schematic it is a well designed low distortion amp. Hopefully the owner will take care of it.

[@janchristensen9858]

Just asking. Did you readjust the Bias after replacement of the blow channel. Because you did not show it.

[@12voltvids]

Yes, camera wasn't running the entire time. Lots of head scratching and hair pulling on this one.

[@lyntonprescott3412]

Well done sir.

[@tyler2610]

Hi Dave, I have a question about a Sony DVD/VCR unit. I sold a new unit on eBay and the buyer is telling me that both the DVD and VCR will play for a few seconds and then goes to a blue screen. I tested it some time ago before selling it and it worked fine for me. I am not sure if he is trying to scam me but I am trying to avoid having to have him return it and lose $40 in shipping both ways. Do you think it is a mistake in the way it is hooked up or have any ideas that I could have him try? It seems odd that both mechanisms would have the same problem and I find it odd it would go to a blue screen on the DVD portion.

[@12voltvids]

It is probably his TV giving the blue screen for loss of signal. Does the unit have hdmi out? If so suggest he connect with av cable and see if it works. Hdmi has many headaches especially with older equipment.

[@tyler2610]

@@12voltvids No, it wasn't HDMI. I will see if he has any other component cables he could try.

[@williamsquires3070]

(@12:47) - it looks like something got too hot. 🤔

[@12voltvids]

They all look like that.

[@skyoreece9805]

Awesome

[@Madness832]

A 3.3-ohm measuring 24k? Holy horse-mess, Batman!

[@simonspeaker]

New As Defected "NAD"

[@12voltvids]

Never any distortion.

[@keithking1985]

when you were talking about the chain reaction i remembered the schematic from the last video(which i just watched) and i see what you mean by its all just transistors and resistors, coz it "IS" fuck loads of them. im half way through your video and im guessing maybe a blocking diode.. so lets see!! : ) well i was wrong. but then again the answer was there when voltage was creeping up on the transistors base that it was a biasing resistor.... by the way was that one of the transistors that blew???

[@zx8401ztv]

Well done :-D A very twisty repair, that was a good one :-D That schematic looks horrible to my eyes, i'm use to well layed out classical designs. How the hell you make sense of that mess i don't know. When i've repaired dead amps i connect two multimeters to the outputs (before the output caps) and both should show identical values, if not its powered off very quick. A kick in the Nad'S, sounds painfull :-D

[@12voltvids]

Typical british amp design. Overly complex. When they work they are great. When they break most will run because they can turn into dogs pretty quick.

[@zx8401ztv]

@@12voltvids I admit that i would not know, nads were a bit too expensive for my taste and i always thought of them as being for audiophiles with too much money to waste. I was never that, i didnt mind the old realistic music centers or most japaneese designs. I was never into loudness, just a balanced sound.

[@12voltvids]

@@zx8401ztv yes audiophools is right. I went the luxman route. Again an audiophool amp.

[@RoughJustice2k18]

@@12voltvids Luxman is good and so is Denon. The rack-mount systems were considered "mid-level-to-high-end" consumer gear a few years ago. My old man used to service those for a living.

[@jsjs1709]

Sony 👍

[@wendellporter4875]

those nad amps can be a bit moody i have worked on my share of them

[@12voltvids]

Put your probe in the wrong place and they go boom, and no wet finger to look for missing stage on these or you will upset the balance with the same nasty results. I used to hate them. Without a print you are screwed.

[@enigmaticmf]

Part 1: kzbin.info/www/bejne/eoK5gHeFiNuLiMU

[@jdmccorful]

Good job!