Nap140/Ncc200 problems

[Stomster]

Hi folks,
Admittedly a relative newbie to tinkering with amps but am no stranger to a soldering iron.

After securing some used NCC200 boards from a fellow PFMer, I installed them in my NAP140.

Upon installation, the bias on each board was reading around 26 mv so was adjusted up to 36mv.
Al seemed to be working fine for a couple of hours initially as things warmed up, although upon initial listening I was somewhat underwhelmed, finding the performance very similar to the Nap140 in stock form.

After a couple of hours I switched the amp off by the front switch in order to swap speaker cables from the NACA5 I was using to my Atlas cables.
Switched the amp on and the glass fuse blew immediately.

Disconnected the amp from speakers and any input, replaced the fuse for a 2A slow blow fuse and tired powering up again. This time there was a pop before I quickly turned off the amp (fuse did not blow).

On removing the lid the LM317 voltage regulator has exploded and burnt some tracks on the power PCB.

So far I am at a loss. Was part of the power unit simply on its way out and coincidentally blew shortly after I changed to the Avondale boards?

I cannot help being suspicious that I have done something stupid to create a short even though the only change was to crimp 6mm spades in place of the 2mm ones.

Could increasing the bias with an admittedly cheap meter have caused such an issue?

Any input gratefully received!

 

[Gervais Cote]

Difficult to see on those pictures but are you 100% sure there is a gap between each legs of the LM 317 at the soldering on the board ?

 

[Gervais Cote]

I use this tool quite often to scrape off any excess soldering between board paths :

 

[Stomster]

Thanks Gervais, there is no continuity between the legs of the voltage regulator. That part was never touched.

 

[david ellwood]

Although Avondale amp boards are infamous for exploding this regulator issue is unlikely to be in any way related to the amp boards. The regulator supplies the preamp, most likely you’ve put some volts back into it as a simple short circuit should just force the regulator into protection.

Check your wiring.

 

[a.palfreyman]

Look to the left of your middle picture. I'm guessing as I'm not familiar with the PSU but between where the grey and red wires are soldered to the board you have two overheated and burned tracks and one has broken and lifted so lots of current had passed through. I think these are the ac feed to a rectifier that looks like it feeds the destroyed lm317. I bet one of the diodes in that rectifier gas gone short and dumped ac into the lm317. Can someone more familiar please confirm my suspicions as to what is what?

 

[Pete MB&D]

The gray wire needs looking at its close to burnt tracks and bridge reciter.
Pete

 

[Stomster]

The gray wire needs looking at its close to burnt tracks and bridge reciter.
Pete

Thanks Pete, the grey wire simply feeds the LED on the front of the case.

 

[337alant]

Agree with the above its more likely that the Bridge rectifier has gone short
Check both of them out with a diode tester
replace both and check the power supply caps are not bulging
Replace the LM317 reg if you need the power supply for a pre amp or disconnect it if you dont

Alan

 

[Stomster]

Look to the left of your middle picture. I'm guessing as I'm not familiar with the PSU but between where the grey and red wires are soldered to the board you have two overheated and burned tracks and one has broken and lifted so lots of current had passed through. I think these are the ac feed to a rectifier that looks like it feeds the destroyed lm317. I bet one of the diodes in that rectifier gas gone short and dumped ac into the lm317. Can someone more familiar please confirm my suspicions as to what is what?

Thanks, yes those tracks are fried. There is a component beneath those tracks but I am unsure what it is. Is it possible to simply remove the 24v preamp circuit and be safe again?

Is the consensus here that it is not related to the installation of the amp boards, but part of the preamp power circuit developing a spontaneous fault?

 

[Avon]

Thanks, yes those tracks are fried. There is a component beneath those tracks but I am unsure what it is. Is it possible to simply remove the 24v preamp circuit and be safe again?

Is the consensus here that it is not related to the installation of the amp boards, but part of the preamp power circuit developing a spontaneous fault?

Yes to both.

It looks like the LM317 has developed an internal short and exploded. Remove it and the small bridge rectifier that feeds it (north of the burned tracks on the left but under the board). Those are used only to supply an external pre-amp and they have nothing to do with the power amps. I assume it’s not powering a preamp.

After removing them, also disconnect power from the amplifier boards and check what voltages you have. IIRC, each board should be getting a bit less than + / - 40V DC relative to 0V.

 

[Stomster]

Yes to both.

It looks like the LM317 has developed an internal short and exploded. Remove it and the small bridge rectifier that feeds it (north of the burned tracks on the left but under the board). Those are used only to supply an external pre-amp and they have nothing to do with the power amps. I assume it’s not powering a preamp.

After removing them, also disconnect power from the amplifier boards and check what voltages you have. IIRC, each board should be getting a bit less than + / - 40V DC relative to 0V.

Excellent, many thanks Avon. I will check this out and report back. Is there likely to be damage to the square component (rectifier?) before the small bridge rectifier, ie south of the burned tracks?

 

[Avon]

Is there likely to be damage to the square component (rectifier?) before the small bridge rectifier, ie south of the burned tracks?

No, because they are not connected in series. Both rectifiers on the left just have the same AC feed. You could always check the diodes in both of the remaining big rectifiers with a multimeter if you wanted to be sure.

Let us know how you get on. I think it was just bad luck to have happened at the same time as replacing the boards. A coincidence. Do please check the voltages before reconnecting the boards though.

PS. I don't know what that grey wire does. Does it supply the light at the front and is that still working?

 

[337alant]

I personally would remove the power supply and test every component, easy enough to fix everything on that board and if its from a tired old 140 then why not renew the caps and rectifiers and do away with the pre amp circuit altogether
Avondale Audio LesW also does a replacement for the 110 which would be ideal

Audiophile Linear Power Supply Modules for upgrades and DIY builders – AVONDALE AUDIO

Alan

 

[Gervais Cote]

I personally would remove the power supply and test every component, easy enough to fix everything on that board and if its from a tired old 140 then why not renew the caps and rectifiers and do away with the pre amp circuit altogether
Avondale Audio LesW also does a replacement for the 110 which would be ideal

Audiophile Linear Power Supply Modules for upgrades and DIY builders – AVONDALE AUDIO

Alan

For peace of mind and longevity, I would go ahead with the Avondale power supply as well. Good suggestion.

 

[Avon]

Please could someone explain. Les’s NAP110 Power Supply Upgrade Module is a mono board. How does that improve upon the NAP140’s stereo supply board?

 

[337alant]

Please could someone explain. Les’s NAP110 Power Supply Upgrade Module is a mono board. How does that improve upon the NAP140’s stereo supply board?

The original Nap110 was also a stereo amplifier with a single shared power supply & 100Va transformer
Many people modified 2 of them into mono amplifiers so that each channel had its own dedicated supply
The NAP 140 is also the same 2 channels with a single shared power supply
So lesW board is a replacement for either, with the advantages Les details in his write up.
I have built more than a few of the Avoldale amplifiers and his circuit boards are superb
Alan

 

[Avon]

The NAP 140 is also the same 2 channels with a single shared power supply

The picture above shows the NAP140 has independent power supplies for each channel though. A total of 3 rectifiers and 5 smoothing capacitors. 1 rectifier and 1 capacitor is for the preamp. Only the transformer is shared. I would not replace this with a mono board.

 

[337alant]

The picture above shows the NAP140 has independent power supplies for each channel though. A total of 3 rectifiers and 5 smoothing capacitors. 1 rectifier and 1 capacitor is for the preamp. Only the transformer is shared. I would not replace this with a mono board.

Both amplifiers use a single "Symmetrical" power supply for both channels, the Avondale replacement does exactly the same.

Alan

 

[Avon]

Both amplifiers use a single "Symmetrical" power supply for both channels, the Avondale replacement does exactly the same.

That is clearly not correct, as the picture in post #1 shows it has two symmetrical supplies, one for each channel. Please look at the picture. Avondale’s is different, as it uses a CIC arrangement I think.