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Old 12th September 2013, 10:58 PM   #1
noexit is offline noexit  United States
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Default Repairing an NAD 3140

I'm re-capping my NAD 3140, and there appears to be a discrepancy in the parts list vs what's actually on the board.

Looking at the parts list, C643 is a 3300PF 50V mylar cap, but on the board I think it was a 47uf 100v electrolytic cap. Unfortunately, instead of documenting each cap as I removed it and ordering based on what was on the board, I ordered off the parts list, and didn't realize that I had a mismatch until half way through.

Also, there was a group of caps in the regulator section that were fried looking, with dark, melted off wrappers. Is there any way to upgrade this section to prevent it from burning out?

I have some pictures attached.
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File Type: jpg hfe_nad_3140-1.jpg (240.4 KB, 158 views)
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Old 12th September 2013, 11:38 PM   #2
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"there was a group of caps in the regulator section that were fried looking, with dark, melted off wrappers. Is there any way to upgrade this section to prevent it from burning out?"

thats not good! sounds like there might be some troubles other than caps.?
did it work OK before you started?

you can go up in voltage as long as they fit. and a higher temp rating will help them last longer. but i have a feeling the heat came from the outside. could be wrong but (in my limited experience) caps bulge, pop the top off, short and burn till open but, Ive never seen one go crispy on the outside from itself and work after.

the 800 Q's in the regulator cir, have they been changed? they might be bad or where at one time. might have caused some heat.

good little setup IMHO. phono stage is allot better than the ones in integrated now-days. watch those jfet's in there, don't zap them. hard to find now-days.
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Old 12th September 2013, 11:45 PM   #3
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btw, if you got it open. i recomend cleaning all the switches real good and pulling the pots, mesuring them, and replacing with multi-turn sealed type. be careful to set the new ones to the same setting as the old. it will give you a start point to re bias and set the offset. also going up a bit in bias sounded great to my freind. just not to high to over heat or drain the pwrsupply tomuch.

good luck
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Old 12th September 2013, 11:52 PM   #4
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Hi
A schematic rather than physical layout is what you need to start assessing. I looked up a manual here: NAD 3140 Service Manual free download, schematics, eeprom, repair info for electronics

It lists c643, and C644 as a 3300pf Mylar.

Regarding the burnt out regulator section, my first thoughts are that the wrong voltage primary has been supplied ie the amp was switched for 120v and 240v was supplied. Another possibility is a liquid like coffee found its way in there.Its good to plan for the worst, and work out first what might have happened. So make sure the amp is switched for its correct Voltage - there should be a selector switch on the rear panel.

with all of the precautions necessary when working on amplifier chassis !! You need to first test fuses WITH POWER OFF AND POWER CORD DISCONNECTED measure resistance across each fuse expect 0.00 Ohms or very close

Next isolate the secondary ie low tension side of the transformer to see if its still alive. TAKING CARE WITH LIVE VOLTAGES reapply power. With a multimeter at hand For a integrated amp you would expect dual rail and 20-25v, 0v, 20-25v secondaries, Be careful you are on the secondary side not the primary when measuring. Next you would trace down to the diodes or bridge rectifier and you would expect to find 30 to 36v DC POS and NEG. If those voltages are present you then isolate faults further down.

It would be possible to rebuild the regulator section on a breadboard, observing regulation heat sinking and generally sticking to the original intent of the design.

Let me know if you need any further help

Cheers / Chris

Last edited by Chris Daly; 12th September 2013 at 11:57 PM.
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Old 13th September 2013, 12:04 AM   #5
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i think there is somthing wrong with the list. the one he points to on the pcb is 47uf 63v on the scematic. i remember some screwups when i did my buddys. lots of retracing the traces etc...

chris, sound advice.
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Old 13th September 2013, 12:27 AM   #6
noexit is offline noexit  United States
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Quote:
Originally Posted by sakellogg View Post
"there was a group of caps in the regulator section that were fried looking, with dark, melted off wrappers. Is there any way to upgrade this section to prevent it from burning out?"

thats not good! sounds like there might be some troubles other than caps.?
did it work OK before you started?

you can go up in voltage as long as they fit. and a higher temp rating will help them last longer. but i have a feeling the heat came from the outside. could be wrong but (in my limited experience) caps bulge, pop the top off, short and burn till open but, Ive never seen one go crispy on the outside from itself and work after.

the 800 Q's in the regulator cir, have they been changed? they might be bad or where at one time. might have caused some heat.

good little setup IMHO. phono stage is allot better than the ones in integrated now-days. watch those jfet's in there, don't zap them. hard to find now-days.
It looked like the caps in the regulator section that had melted coverings were grouped around the Q800s, and they haven't been changed. (These things right?)Click the image to open in full size.

It did work when I got it, but barely. I had to turn the balance way to the left to make the sound balanced, and it varied a bit depending on how long it was on. The sound output out of the better side also seemed to be less that what it would be. Most of the larger caps had leaked out of the bottom. Click the image to open in full size.

I generally a DIY person (decided I needed a kayak, so I built one, for example) but electronics are fairly unfamiliar beyond a kit I had when I was a kid, and an arduino that I've not played around with as much as I hoped. So I don't know much about what I'm doing.
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Old 13th September 2013, 12:42 AM   #7
noexit is offline noexit  United States
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Quote:
Originally Posted by Chris Daly View Post
Hi
A schematic rather than physical layout is what you need to start assessing. I looked up a manual here: NAD 3140 Service Manual free download, schematics, eeprom, repair info for electronics

It lists c643, and C644 as a 3300pf Mylar.

Regarding the burnt out regulator section, my first thoughts are that the wrong voltage primary has been supplied ie the amp was switched for 120v and 240v was supplied. Another possibility is a liquid like coffee found its way in there.Its good to plan for the worst, and work out first what might have happened. So make sure the amp is switched for its correct Voltage - there should be a selector switch on the rear panel.

with all of the precautions necessary when working on amplifier chassis !! You need to first test fuses WITH POWER OFF AND POWER CORD DISCONNECTED measure resistance across each fuse expect 0.00 Ohms or very close

Next isolate the secondary ie low tension side of the transformer to see if its still alive. TAKING CARE WITH LIVE VOLTAGES reapply power. With a multimeter at hand For a integrated amp you would expect dual rail and 20-25v, 0v, 20-25v secondaries, Be careful you are on the secondary side not the primary when measuring. Next you would trace down to the diodes or bridge rectifier and you would expect to find 30 to 36v DC POS and NEG. If those voltages are present you then isolate faults further down.

It would be possible to rebuild the regulator section on a breadboard, observing regulation heat sinking and generally sticking to the original intent of the design.

Let me know if you need any further help

Cheers / Chris
I hadn't paid much attention to the schematic as I was going off the parts list and the physical locations on the board as I replaced them. Looking now on the schematic, C643 is a 47uf 63v. I'm pretty sure that was 100v on my board.

I would think it would be pretty hard to put 240 through this since I'm in the US, and 240 is uncommon, and a completely different socket. Also, bsaed on the service manual, it looks like it's hard wired for 120, and to switch it to 240, you have to rewire the transformers.
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Old 13th September 2013, 01:17 AM   #8
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Hi noexit
This sort of damage to capacitors occurs when they are reversed polarized in fact worse as they can, and do explode. So it may be the caps have been put in wrongly. Its unusual but not impossible for caps to just leak like this, particularly when subjected to heat ranges outside of their rating. perhaps the caps were touching the heat sink.

i would obtain some solder wick which is essential for successfully removing parts. you ideally need a temperature controlled soldering iron, or be quick with a 15w iron. Solderwick works to remove excess solder at about 320 degrees Celsius, whereas parts should go in at about 280, ( ic's at 260 )

Once you have all the old parts out , obtain replacements, ensuring the voltage on caps if they were original parts are the same or up to 50% higher in voltage rating. Thesedays capacitors are smaller so be aware they may not look the same. Be very careful to observe polarity when replacing caps.

Hopefully you can salvage the board. some common pinouts are 78xx regulators looking from the front are In Gnd out, 79xx Gnd in Out, you will need to Google or other search engine each transistor pinout as they can differ, if not using exact original parts. Generally NPN are used for positive voltages and PNP for Negative voltages.

It could be a great project to show you the ins and outs of amplifiers, board layout etc
Be very careful with AC mains voltages.

Cheers / Chris

Hope this helps
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Old 13th September 2013, 02:52 AM   #9
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"I had to turn the balance way to the left to make the sound balanced"

had the same trouble. clean the switches on the back. signal goes through them. un solder and clean as good as you can. i took them apart and cleaned the sliders and re-greased the gears.

"Q800s, and they haven't been changed. (These things right?)"

yes the bigger of the transistors labeled q8** on the print and board. i remember 2 getting pretty hot.


"I'm pretty sure that was 100v on my board"

thats a good thing. last longer. looks to me like they got old and leaky. big thing is to clean all that goop off! take lots of pics, tag the wires, take the boards out and clean real good! that goop will ruin the boards!


"would obtain some solder wick which is essential for successfully removing parts. you ideally need a temperature controlled soldering iron, or be quick with a 15w iron. Solderwick works to remove excess solder'

again sound advice. i might add a solder sucker will work for this type of board well. be careful those traces will lift easy. so don't overheat them.

"I generally a DIY person (decided I needed a kayak, so I built one, for example) but electronics are fairly unfamiliar beyond a kit "

i bet building a kayak was unfamiliar too,at first. cool as heck by the way...

"It could be a great project to show you the ins and outs of amplifiers, board layout etc
Be very careful with AC mains voltages."

yes! hard to enjoy the fix if you are dead.
please don't hurt yourself, anything i can do to help please ask.
rewarding 1st project
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Old 8th February 2014, 11:04 PM   #10
mystyca is offline mystyca  United Kingdom
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hey.. I don't know if you solved your problem. but I may have the answer to the differences in the parts..
I was working on a nad 3140 but was forced to leave it as I had no time.
I had a service manual complete with a whole bunch of service bulletins which meant over time the circuitry was made simpler and certain mods where made like larger resistors in the section which goes to the leds on the front panel (power/soft clipping etc). there are a number of bulletins which explain various problems found after initial units where shipped. some mods where made to make the biasing more stable. that sort of thing. if you need these service bulletins I will have a look for them. I may still have the pdf file.. was about 28 pages. the service bulletins kind of got me sidetracked as most the circuitry on my nad was original and was trying to apply some of the alterations. I may pull the amp out and have another go. I have two but don't know what happened to my original one during a house move as all my stuff got scattered between rooms in my mum and dads and aunties.
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