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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"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.
 
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
 
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
 
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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.

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?)
9oYVnmR.jpg


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.
DsF8i3M.jpg


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.
 
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.
 
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
 
"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
 
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.
 
was it a case of dyslexic eye balls from stareing at the board for so long.. could be possible you was looking at c634 not c643. I'm going to make a start recapping my 3140. I will look out for the tricky c643 and see if mine is a 47mf rather than the 3300pf 50v mylar.
chances are your pcb is a later revision which means some of the numbers don't match... I had a ohm pa amp the same version 1 diagram and parts list and a v4 board.. parts where virtually in the same position but when the board was printed part numbers got moved so several caps had different part numbers to the original production boards... luckily I had both revisions of boards to check the locations. chances are nad released a revised layout late in production which meant numbers on the boards nolonger match whats on the part list. only thing I'm worried about is the output transistors do I opt for Toshiba 2sc2565/2sa1095 like my original nad 3140 or keep the mj15003/mj15004
 
Hi, I just finished a repair / recap of an early 3140, and may be able to add a few points for any future readers, or repairers:

1) Download the service manual from hifiengine.com - there are multiple NAD service bulletins appended to the end of it, all of which have excellent additional information not always in the usual Serv. Manual.
2) Compare pcb parts (and labeling) to the schematics - my pcb had no provision for a few (missing) resistors around the DC-offset & bias setting pots, that were on the schematic.... adding those, and replacing the pots with Bourns multi-turns helped for final setting up.
3) Check and/or replace electolytic caps especially in the power supply section - it runs quite hot, and the original (85C rated) caps will almost certainly have failed). I replaced with 105C rated caps.
4) Check also the coupling caps C601/602/603/604 - mine were leaking DC and causing no end of spurious noises, especially operating the 'Low Level' switch.
5) Check the R213/214/215 resistors, in the LED circuit - mine were the original, underspecified, 1/4W items, and well on the way to burning - it's in one of the service bulletins.
6) Check also the 22V Zener diode on the LED board - mine had burnt, so was replaced with a new 1N5358BG Zener and a bit of creative lead bending.
7) The earlier point someone made re. increasing the bias slightly - the service manual / service bulletins give 2 slightly different procedures, which set bias current slightly differently. Use the emitter to emitter measurement, and set for 14mV (12-15 range is specified) - I found it was indeed 'better' at 14mV, than the 6mV specified with the 'test point to speaker terminal measurement.
8) Bias & DC-offset adjustments are very thermally sensitive - I found it took a few hours, with covers on, and a real pain, to reach a stable setting, even using the new trimmers. DC-offset was the worst, but easier with a dummy load connected.

If you have 2SC2565 / 2SA1095 outputs fitted, then you probably have the simplified amplifier circuit - it's described in one of the other service bulletins, and involves other circuit changes as well.... again, check the pcb parts / traces versus the schematic. Fortunately, mine was a very early example, with minimal changes having been made (by previous owners), so relatively easy, but time consuming, to follow.

The final set up (bias & DC offset) was still time consuming, even after a full recap & new trimmers, and very sensitive to thermal changes (draughts), but with patience, it's now adjusted to +/-14mV for bias setting (ca. 32mA), and < 5mv DC-offset, and sounding excellent.

Anyway - hope that helps someone.
 
As far I'm aware they are direct replacement. May need the pots replacing. There where mods made in production for stability for idle current. I'll check my manual.
Hi. I'm doing a NAD 3140 restoration and want to replace the trimming resistor pots. When I go to Mauser to source them they don't have the same spec that's called for in the service manual parts list.

Service Manual Parts List:
  • VR601, VR602 - Semifixed 1KB
  • VR603,VR604 - Semifixed 500B (Is this a typo? Should it be "KB" instead of "B"?)
  • VR201, VR202 - Semifixed 200KB

Mauser's resistance specs are in Ohms - nothing listed in KB. And the parts list doesn't specific the wattage.

Do you have any information on the specs for these trimming resistor pots?

Thanks for any help you can provide.
 
I chose to use the Bourn multi-turn trimmers in my 3140. You will need to be a bit creative
and extend the middle leg with some component (resistor/cap) cut-off wire.

K refers to Kilo ohms B refers to linear taper, A is Audio/"log" taper

VR201,2 Power indication, ie, "the light show"
Don't recall changing these
200K
652-3296Y-1-204LF

VR601,2 DC offset
1K
652-3296Y-1-102LF

VR603,4 Idle/Bias
500
652-3296Y-1-501LF