Just finished my longest ever repair. (NAD back from the Dead) - Page 1
Forum section: electronics / repair

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 19, 2016, 02:54:06 am
content: Just finished my longest ever repair. Approx 25 years it took me.Sorry about the photo quality.Timeline: -Maybe 25 years ago: from IIRCSalvaged this blown but nice looking NAD 3020A amplifier.Replaced every blown transistor I could find, which was the four output transistors and at least a couple more from the power stage.Turned on but awful hum.Tried to trace it but thought it was in feedback somehow. Hum seemed to be in power stage.Some years later.Decided to replace all 4 of the main rail electrolytic caps. It seems that the replacement caps that had a much larger form factor and probably much larger capacitance.Hum was still there.Some years later.I seem to have pulled out half a dozen electrolytics that had extremely visible signs of leakage. Then left them loose inside and done up the case again. ??!Some years later. Which was yesterday.Replaced all electros that looked dirty and all the missing ones with salvaged caps.... voila it sounds great. The faders still sound good too.Seriously I am not proud of this effort and I can't fathom what I was doing, for all this time.But I am happy that this amp is working as I really like the look of it, the feel of the controls and the classic simple amp UI.The 2 caps in the blue electrical tape in the bottom photo is to replace one 35V cap with 2 x 25V in series. I did this twice for the +- preamp power rail.

author: vze1lryy
author_rank: Regular Contributor
timestamp: April 19, 2016, 03:01:42 am
content: Very cool!I still have to go back to my first amplifier repair from 11 years ago. An HCA-1000a from parasound with a bad relay that.. still... has... wire... going around the relay.It's sitting under my desk to this day.50% of the time, it turns on. 50% of the time, it trips the circuitbreaker when I turn it on and my office goes dark. You can see the insides of the fuses bounce and light back and forth when I hit power too.. which never happened before bypassing the relay. It's always a surprise when I flip the power switch!How 16 year olds fix electronics)) Yours is looking much better than mine!! The 3020 is by far the best "Starter amp" for anyone who wants to repair this stuff, a great model to train on.

author: starphot
author_rank: Contributor
timestamp: April 19, 2016, 08:16:14 am
content: Congratulations! Electrolytic caps do go bad just by sitting there with no power. The NAD 3020 was a classic. My first amp was and are a pair of the Southwest Technical Tiger 01 amplifiers. These came in a kit. I put them together while on board ship in 1973 and bought two Pioneer CS-88A speakers from the exchange for them. I sent both home and put the system together when I reached home port with the 198/A preamp to go with it. A heathkit tuner was built and a TEAC cassette deck bought as well. Only one repair in 1974 when I came home on leave and discovered that my brother, a teen left at home alone, had a wild party. A bullet hole in one of the woofers and I believe the two reference zeners and one of the buffer transistors blew in one of the amps.I still got both of the amps but haven't fired them up in a few years. No stereo components replaced since the theft 32 years ago took away the rest of the system, including a good turntable with the better Audio Technica cartridge. My job in electronics repair and the progressive bad hearing kept me from replacing the system."The cobbler's children have no shoes."The same with us professionals of various stripes.My job in the Navy was aircraft fire control radar repair. My civilian job was repairing RF transceivers, cellphones, the large stereo receivers, CD players, scanners and more. The knowledge gained by putting together those Tiger 01s helped me in my career after the Navy repairing those large receivers. Complementary/symmetry Push-Pull circuits acting like an op amp on steroids.http://www.tigersthatroar.com/?page_id=12Joe

author: MrSlack
author_rank: Frequent Contributor
timestamp: April 19, 2016, 08:49:49 am
content: Congratulations - nice work!NAD stuff is lovely to work on. Schematics generally available, nice clean designs. I've got the distant successor, a C320 which will no doubt fall over at some point in the future. Wouldn't part with it - lovely amp.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 19, 2016, 09:46:23 am
content: Quote from: starphot on April 19, 2016, 08:16:14 am"The cobbler's children have no shoes."Yeah that's me all over.Making up amps and PSUs it's a worthy rite of passage.It's quite nice working on discrete through hole stuff for a change. I wish the NAD was a bit louder though, only 30V rails IIRC. While working I even cranked up the HP1725 to trace the signal, though it seems to have more buttons than I remember.Quote from: vze1lryy on April 19, 2016, 03:01:42 amThat don't sound good!! Try disconnecting sections if you can't find the short any other way. Intermittent faults are tricky but if you can get a fairly regular fail then you are in with a chance.

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: April 20, 2016, 02:37:35 am
content: Any more stories of long term repairs or projects?We could have an award for the longest time span !To start off: In 1975 the UK magazine 'Wireless World' published what looked like a complete schematic for the Quad 405 audio amp. My PCB draughtsman and myself immediately set about laying out a double sided PCB and etching a few copies.I built up 4 but never powered them up. Last month I found the boards in a box in the shed (in a different country and hemisphere now) and, seeing how much the Quad 405 is fetching these days, wired up a pair to a PSU and a pair of Allison speakers. Bliss after 40 years.Regards, BT

author: MrSlack
author_rank: Frequent Contributor
timestamp: April 20, 2016, 05:54:12 am
content: Not a repair story but I built some active speakers based on a very simplified Quad 303 design (substituting the entire front end and integrating a preamp with an NE5532 and basically using any transistors I had lying around) back in about 1994. They gave out a couple of years ago due to overheating so I have eviscerated the cabinets of electronics and am using them as dumb speakers while they await my class D implementation.Quad 405 is a nice design!

author: macboy
author_rank: Super Contributor
timestamp: April 20, 2016, 03:02:45 pm
content: Quote from: vze1lryy on April 19, 2016, 03:01:42 amThat's a bodge not a repairFind a NTC thermistor from an old PC power supply and wire it in series with the mains on the transformer primary. Put the thermistor somewhere heat-proof. It will limit inrush current then get hot and just sit there dissipating a watt or so and not affecting performance.Quote from: BurningTantalum on April 20, 2016, 02:37:35 amI have an unfinished project on a breadboard. It's been there for 14 years. It was going to be an IR remote control for my MiniDisc Walkman. That thing has been in a drawer as long as the breadboard has. I think it's time to liberate the parts.

author: 4cx10000
author_rank: Regular Contributor
timestamp: April 20, 2016, 04:48:04 pm
content: Nice work!Started to play with a dead 3020 six weeks ago, but got ill so its still on a shelf in my workshop. One of the transistor (2N3055) in the output stage was gone and driver 2n6551 as well. I did not have time enough to investigate further so coming Monday I'll start up again. I noticed that it was not an easy task to find nor the original driver or a good replacement. I Also found out, by looking att the schematic, that it was a bit more tricky to adjust the idle current than those with a trimmer, I may put in a trimmer instead of the fixed resistors for the bias circuit.Did you replace the drivers as well?

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 21, 2016, 08:26:10 am
content: From memory, I am pretty sure I replaced the to-126? packages as well as the t-03. That was maybe 25 years ago. I cant remember having trouble finding them. Hopefully I didn't just whack in any transistor, I really doubt that I would have.Find chips found this link, seems to have 21 in stock.http://www.onlinecomponents.com/national-semi-2n6551.html?p=44384111&utm_source=findchips&utm_medium=inventoryrefferal&utm_campaign=findchips

author: 4cx10000
author_rank: Regular Contributor
timestamp: April 21, 2016, 02:56:25 pm
content: Quote from: HackedFridgeMagnet on April 21, 2016, 08:26:10 amHackedFridgeMagnet thanks for info!I will check that out! Hopefully it will not take 25 years, but you never know

author: vze1lryy
author_rank: Regular Contributor
timestamp: April 22, 2016, 09:59:23 pm
content: Quote from: macboy on April 20, 2016, 03:02:45 pmI want to make time to find a proper relay.I've been telling myself I will do that "tomorrow" for 11 years now.I never make the time for myOWNstuff!!A habit I hope to break before I get to my 30s.

author: 4cx10000
author_rank: Regular Contributor
timestamp: April 22, 2016, 10:11:38 pm
content: Hm ... I am nearly 60 and haven't manage to either fix the code "I don't have time right now" or make things in time for myself...

author: Riotpack
author_rank: Regular Contributor
timestamp: April 24, 2016, 08:50:45 am
content: I have repaired 2 of these.1) glue had become conductive/corrosive2) transistor driving 2n3055 randomly went short causing random speaker pop+blown rail fuse. When replacing output stage you must add 0.1R emitter resistors to all output transistors. The original transistors used a epitaxial construction and a modern replacement can go into thermal runaway without adding emitter resistors.

author: Riotpack
author_rank: Regular Contributor
timestamp: April 24, 2016, 08:53:24 am
content: I have upgraded output stage to MJ21193/4 with resistors mentioned above. The heatsink in these amps gets quite hot. adding a small fan will increase life.You can run a 12v fan at 5-6v and it will do the job while remaining pretty much silent.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 24, 2016, 09:25:12 am
content: Quote from: Riotpack on April 24, 2016, 08:50:45 amThanks for the tip.What glue?Luckily I have a few old ones. That I am probably never going to use.

author: jitter
author_rank: Frequent Contributor
timestamp: April 24, 2016, 09:38:27 am
content: My longest repair took about eight years.My CD-player (a Swedish Primare D20) started spitting out white noise randomly and intermittently, needing a power cycle to make it stop doing that.It started around 2002-2003. At first I had the shop that sold it to me have a go, which cost me € 150 but didn't cure the problem. Then I decided to have a closer look myself, startingthis threadin 2008, only to find the real cause two years later.This player has quite a few separate power sections, the DAC chip alone getting two separate 5 V supplies, one of which turned out to be faulty, intermittently. The cause was a bad wire connection inside one of the two pcb mounted transformers. Because of loud mechanical hum, I took them off the pcb and mounted them on rubber grommets inside the case. The white noise fault had already started at that point in time. After doing this it had cured the problem, but only temporarily.When the fault returned again it became more persistent. As a result I was finally able to trace the problem to one of the 5 V sections to the DAC going out of regulation, which in turn was caused by a drop in the AC voltage supplied by one the transformer taps. This transformer being a resin sealed type meant repair was not going to be likely, so I added another one I found in a pile of scrapped pcbs at work.It continues to work fine ever since, now being about 18 years old.I can only assume that the fault lies within the transformer, which still works fine one the other taps, though.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 24, 2016, 09:51:54 am
content: Quote from: jitter on April 24, 2016, 09:38:27 amThat would have been a hard one to solve.

author: 4cx10000
author_rank: Regular Contributor
timestamp: April 24, 2016, 07:15:53 pm
content: Quote from: Riotpack on April 24, 2016, 08:50:45 amThanks for info!Did you make any modification to the idle current adjustment? I'll add those 0,1R emitter resistors to be on the safe side. Good idea to put a small fan inside, I'll think of that too.