NAD 3140 Regulator and Power section questions: Has anyone seen this?

Iamstubbb

Iamstubbb

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Hi everyone, I have an NAD 3140 that's looks as though it's been through some rough treatment. The unit turns on and sounds OK through some benchtop speakers, but there are a lot of missing screws for the chassis and faceplate and some odd things. It's definitely been worked on and looks like it was given up on or just nor finished. I had some issues with the poser meter LED driver, but I may have that bit solved. However, the regulator section looks entirely different than what I have seen with the amp only version, 2140. There appear to be missing or deleted caps and alternative transistors in there. I can't tell if that is a production change or if this is just a half-done job abandoned. I'm surprised this thing is working as I have found several bad solder points in that section. Everything is very baked and there's a blue transistor in there where a BD140 should be.

My approach will be to compare to a working 2140 I have (An early production model without the soft-clipping or amplifier simplification changes) and look for any changes on the board or in my copy of a service manual. I'll repopulate the empty spots as well. But I am curious is anyone has seen anything like this?

For the power section., I'm looking for suitable caps. The ones installed are all measuring way below spec. For my 2140 I used Rubycon 4700/50 PK's, but for this, I have Nichicon LS 4700/63's. Can't find the recommended PW's. The Rubycons look like a pretty good choice. Any experience with the LS's?

Any thoughts or input welcomed.

20260215_161650.jpg
 
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I had most 10uF caps drift and have high ESR in a c270.

No idea if this one uses similar caps but those would be the ones I'd replace.
 
Your best reference for the circuit / production changes is one of the free service manuals. I forget whether it's the 'hifiengine' or 'electrotanya' copy (possibly both ?), but one of them at least includes a long section of service bulletins. They're all appended at the end of the manual, and discuss multiple NAD circuit changes and alterations.

The regulated PS section layout is a thermal nightmare, so you can expect to change all the electro- caps there, as well as checking for dry joints and upgrading the pass transistor(s) heat sinking as much as possible, in what's an already very crowded (and prone to overheating) section of the PCB.

Leesonic already documented various suggestions specifically for the 3140, just search the forum for Lee's threads.
 
Also when looking for replacement caps, be sure to order 105oC. Things get hot and high temp caps can help.
 
I'd take a good look at the back side of the PCB. See if those missing caps were ever soldered there. Those solder holes look too clean and uniform. I think a production change is most likely.
 
Dave1027 said:
I'd take a good look at the back side of the PCB. See if those missing caps were ever soldered there. Those solder holes look too clean and uniform. I think a production change is most likely.
Click to expand...
A couple do look "native" , but some also look like there was some work done. two were only soldered on one leg. This thing is wacky. I think those spots were populated with 47uF/10V on my 2140. No bulletin on that in the Electrotanya 3140 full manual. (thanks @Goldie99). When I figure it out I'll show my work.
 
Here are the missing caps. All 10uF/35-65V so I think this is just a half-done job. I am surprised it turned on and made sound at all.

3140 regulator schematic.jpg
 
This was quite a mess and took some cleaning up while working on it. Here's at the beginning of the work and you can see how scorched the board is around Q808 and R423. I moved the volume/balance pot for access to the diodes D808 and 809. There is also a lot of heat damage on the lower right around that BD139. The pads under that area were lifted and hard to get clean, even though I used a braid rather than my desoldering gun. I had to bridge a few connections. I should have put that resistor below it up on legs and raised R803 down there around the bridge diode. I might go back and do that.
20260217_222757.jpg

Here it is when finished. I used a mix of Panasonic FC and FR 105-degree caps and a couple WIMA's for the 1nF gumdrop and 0.1 uF ceramics. No explosions or sparks when I turned it on. Before, there was a transient glow through the DBT and some noise, but that is all gone. It warmed up without overheating and seemed stable. I drove it through some small JBL computer speakers using the headphone connection. The channels are imbalanced, with the left being heavy bass, low treble, and the right channel the opposite. I didn't notice that before, but I am also getting a lot less scratch and distortion. The speaker EQ switch has a devil in it so I'll have to give that a thorough cleaning as suggested by Leesonic. Overall things seem to be progressing, so I'll work on the power section next and try to get it into alignment. The main output transistors and the thermal paste look discolored. I'll break that section down and clean and see if there are any other issues.

20260218_014836.jpg

Oh yeah, here is that mysterious blue transistor in the place of the BD 140 at Q808. It's an NEC SB536, which is a slightly beefier 18W rather than 12W but similar otherwise.

20260218_014911 cropped.jpg
 
avionic said:
Your small TO-220 heatsinks probably don't need the sil-pads. Collector tabs are all isolated from the circuit.
Click to expand...
Yeah, and they look goofy too. I thought about that as I was finishing mounting the last one. Sort of on autopilot because I use those on the drivers mounted on the big heatsink. I'm a long way from being an expert and it shows! Hopefully it won't hurt. :rolleyes:
 
Iamstubbb said:
This was quite a mess and took some cleaning up while working on it. Here's at the beginning of the work and you can see how scorched the board is around Q808 and R423. I moved the volume/balance pot for access to the diodes D808 and 809. There is also a lot of heat damage on the lower right around that BD139. The pads under that area were lifted and hard to get clean, even though I used a braid rather than my desoldering gun. I had to bridge a few connections. I should have put that resistor below it up on legs and raised R803 down there around the bridge diode. I might go back and do that.
View attachment 3702549

Here it is when finished. I used a mix of Panasonic FC and FR 105-degree caps and a couple WIMA's for the 1nF gumdrop and 0.1 uF ceramics. No explosions or sparks when I turned it on. Before, there was a transient glow through the DBT and some noise, but that is all gone. It warmed up without overheating and seemed stable. I drove it through some small JBL computer speakers using the headphone connection. The channels are imbalanced, with the left being heavy bass, low treble, and the right channel the opposite. I didn't notice that before, but I am also getting a lot less scratch and distortion. The speaker EQ switch has a devil in it so I'll have to give that a thorough cleaning as suggested by Leesonic. Overall things seem to be progressing, so I'll work on the power section next and try to get it into alignment. The main output transistors and the thermal paste look discolored. I'll break that section down and clean and see if there are any other issues.

View attachment 3702550

Oh yeah, here is that mysterious blue transistor in the place of the BD 140 at Q808. It's an NEC SB536, which is a slightly beefier 18W rather than 12W but similar otherwise.

View attachment 3702553
Click to expand...
Hey! Just read that you had transients on your DBT...I'm slowly finishing my 3140 and from I can hear...the Power Switch buzzes. The interesting thing is that I can leave it on, cut the power, and then it would randomly buzz, or just buzzes less when I get the power back on (without physically cycling the switch)

My question is going to the fact that.... can those spike suppressors be blown? Or is it really another case of bad Power sw?

BTW, I also have the nasty NEC B536 in the same position as yours.
 
Iamstubbb said:
The speaker EQ switch has a devil in it so I'll have to give that a thorough cleaning as suggested by Leesonic.
Click to expand...
I've gone through three 3140's, and all of them had rotten spkrEQ switches, beyond repair. Had to bypass the circuitry. Let's hope yours is in better shape.
 
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