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Naim NAP250 regulator boards

N

nikiangel

Active Member
Hello,
as I don't have any knowledge so far for the regulator boards of the NAP250 CB I just purchased, I will appreciate some help here.

Here is the problem itself:
qwCGpai.jpg

The amp is working, but the resistors in the red boxes are hot when touched and you can see that their coating disintegrates.
All the power transistors are not hot, the amp is sounding OK. Bias is OK, the output voltage without load and without input signal is also OK.
The voltage to the both amp boards are equal, nothing unexpected so far.

It looks like something which affects both power boards at the same time, maybe malfunctioning electrolytic capacitors?
Unfortunately can't send it for a service as no trusted Naim service in my country.
Any advise what to check and where to look?
 
As always the search engine is our friend so I found what are these used for and why burning :)
Going to replace with some high temperature 1W 3,3 kOhm Vishay resistors at the time when I go to replace the electrolytic caps on the regulator boards.
There I will use 47 uF/40V Promisic (Felsic) as I have some spares of these and they are considered high grade audio caps
and 10uF Vishay 100V (I saw somewhere someone recommending these as replacement for the original 10uF caps)unless someone recommends something better here.


And another question, it's pretty obvious but still... do I need to change the reservoir caps aswell. They seems to be holding some charge as the amp continues to play 6 seconds after switched off.
476xNAs.jpg
 
Definitely replace those reservoir caps - they're leaking round the terminals. Your other choice of caps should be sound. The 10uF one is the most important one.
 
I'm a little confused. What's the original value of the discharging resistors (the ones in the red boxes on pic1)? Is it 3.3kOhm or something different?
 
They are 3k3 1W resistors.

The discolouration is actually nothing to worry about since they run hot by design. Yours will be metal film and will still be on value in all probability. The issue is that older amps had carbon composition resistors which can drift in value.

The cracked casing though dictates that they really need to be changed.

I replace them with 3k3 2W metal film resistors which ought to be good for the rest of eternity.

If you mean by the Vishay 10uF cap the blue axial one originally made by phillips and then BC components prior to being taken over by Vishay which looks exactly the same as the original component, then don't use it, it's actually the weakest link in the chain and causes the majority of trouble on regulator boards.

A much better alternative is the Nichicon VX series 10uF 63V axial cap.

You do also definitely need to change those large reservoir caps and any of BHC (now branded Kemet), Kendeil, Safco and Exox-Rifa make suitable substitutes.
 
Thanks colasblue for the advise.
Unfortunately I've already ordered the 10uF Vishay before reading your post. Why do you think Nichicon will be better?
For the reservoirs I will go with Safco.
 
Because it's less likely to fail after about 8 years.

I don't guarantee your replacement is exactly the same as the BC/Vishay series which Naim use and which is prone to early failure, but the point is that in that position the original equipment and standard Naim replacement isn't he best thing at all.

Naim use a 63V cap from the standard AS series like these

http://uk.farnell.com/vishay/mal203038109e3/cap-alu-elec-10uf-63v-axial/dp/1165436

An important feature is that the cap needs to have sufficient ESR to prevent oscillation so it's usually not a good idea to get a higher voltage version which may have a physically larger form and hence lower ESR or any sort of posh Audiophool cap with low ESR.

The standard ones seem to fail quite often and cause the majority of premature 250 & 135 servicing, though since you haven't specified exactly what you've ordered it's not clear how the specs of yours compare.
 
The one I ordered is this:
http://www.mouser.bg/search/ProductDetail.aspx?r=594-2222-138-39109

Do you know what should be the ESR of this cap to work stable so I can measure it on the ones I've ordered.

I feel really frustrated that I didn't know about the ESR requirement :(, just saw a post somewhere here that someone was advising for these Vishay caps.
However, will do my best to order couple of NICHICON TVX1J100MAD and hope they will do.
 
The standard item has ESR of just under 13 ohms

If you'd chosen the 63v one from that series it also would have been about 13 ohms, and in fact I think that's Neil Jadman's preferred cap in that position.

The 100V variant only seems to have 11 ohms which might be slightly iffy.

The Nichicon VX cap also has about 13 ohms, and is the universal recommendation of all UK 3rd party professional service operations, as does the Panasonic NHG series 63v cap which lives in my 135's (I chose it because its a 105 degree cap rather than 85 degrees and that spot can get quite warm)
 
Thanks colasblue,
I wrote to the supplier if it's possible to include these caps (NICHICON TVX1J100MAD) in my order. If not possible, I will try to get them through different channels.
The strange thing is that the genuine ones which are still on the boards, showing ESR>40 Ohm, the capacitance is around 10uF on all of them.
Maybe they are already out of specs.
 
I use the Nichicon 10/100's here. The ESR is still enough to keep the amp stable and the derated voltage should help it live longer.

On the 250.2 version, where Naim simply removed the trips from the regulator and tied together some loose ends there's a low ESR radial cap on the output with a 2R in series.
 
MJS said:
I use the Nichicon 10/100's here. The ESR is still enough to keep the amp stable and the derated voltage should help it live longer.
Click to expand...

Thank You MJS,
probably that was the intention of the person recommending Vishay,
http://www.mouser.bg/search/ProductD...2222-138-39109
it's 100V and 105C expected to live longer.
In this case I may use these Vishay caps and if some obvious problems, to go with the nichicon TVX1J100MAD which is expected to come later next week.
 
One issue is that published specs are not he whole story here.

Published ESR or tan delta is usually measured at around 100Hz but the frequencies where the problems occur are in the KHz ranges from highish audio frequencies into RF.

Either cap will work out of the box, its just a question of how various elements of its unpublished performance change over time.

ESR tends to increase with cap age which is what you've measured, but it doesn't necessarily increase uniformly across the whole frequency spectrum which is where the problem can arise.

It's an area where "tried and trusted" is probably the best policy.

Interestingly the published tan delta on the VX series suggests 10uF 63V and 100V have the same value.
 
Just to be 100% sure,
can someone confirm that Nichicon TVX1J100MAD is the one that is widely recommended to be used here?
If not the right model can someone link the right one so I can order it.
 
I just replaced all the electrolytic caps, zener diodes and the 3.3k resistors on the regulator boards.
Once set, voltage stays stable. The only concern I have is that on 3 of the zeners the voltage is 10.15V and on one of them 9.85.
Like 3 of them have +1.5% of the spec and one -1.5%.
The zeners are with 5% accuracy but still, sequential samples shouldn't they give the same deviation?
Should I be worried at all?
 
Nothing to worry about.

Can you guarantee that they actually are sequential? Did they come on tape? I doubt many suppliers would guarantee that.

It really doesn't matter since they're in spec and you won't have any difficulty setting the correct output voltage.
 
I just changed the zener, the next one from the tape was exactly like the previous 3, so now everything set for piece of mind with the regulator boards.
Thank You colasblue for all the help you provided here and on private. I really appreciate it!
 
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