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CB 250 amp board question

diybry

diybry

pfm Member
Hi, I could do with a bit of help please...
Tha photo is the amp boards of my nap250cb bought back in 1985.
I've recapped it once, ages ago, and replaced what I thought was the input cap (as far as I can remember) with a mil spec wet tant. I've opened it up to recap again, and notice that the black transistor heat shields are mounted vertically on both my reg boards and amp boards, and the circuit layout differs in some areas from those on pfm such as the 135 recap thread, and 'feedback capacitors for naim power amplifier' and also on accoustica. I've no idea why I didn't notice this previously, and I'm now wondering if I have replaced the correct cap with the wet tant. Could some knowledgeable Fishy please have a look and confirm if I've done the right thing with the wet tant, and What's the pedigree of the boards in my 250?
 
Hmm, no photo...hopefully here it is.
IMG-20230812-124235.jpg
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How do I post a photo?
I'm using post image, and tried both post image for forums and hotline for forums, but no go
 
diybry said:
How do I post a photo?
I'm using post image, and tried both post image for forums and hotline for forums, but no go
Click to expand...

Using Postimage:

Open up My Images - hover over the image you want - click on the "<" shaped icon at the top of the image - right click on 'Hotline for forums' - from the menu that appears click 'copy' - go
to your message on PFM and paste your link directly on the page (don't use the link icon on the heading of the page) - click on 'More Options...' on the bottom right of your message and you should see the image.

Here's an image of an early NAP160:

 
The wet tantalum, next to the green wire on the right, is the feedback bypass capacitor and that’s correct. You don’t need to change that again. The other four tantalums you can change as you wish. The input coupler is next to the red wire on the right.
 
Avon said:
The wet tantalum, next to the green wire on the right, is the feedback bypass capacitor and that’s correct. You don’t need to change that again. The other four tantalums you can change as you wish. The input coupler is next to the red wire on the right.
Click to expand...
Excuse my ignorance but does that mean any wet tantalum capacitors simply last forever ?
 
Greetings Gervais. I don’t believe they do last forever, just as any electrolytic capacitor will not. However, that wet tantalum is in series with a 1k resistor, so as it ages and its resistance (presumably) increases, it’s going to make little difference. I cannot prove that of course. If it starts leaking its electrolyte though, which is sulfuric acid, then that could be disastrous obviously. Remember they cost maybe £100 each?
 
Avon said:
The wet tantalum, next to the green wire on the right, is the feedback bypass capacitor and that’s correct. You don’t need to change that again. The other four tantalums you can change as you wish. The input coupler is next to the red wire on the right.
Click to expand...
Thanks, it's a long time since I put that tant in place.Good to know I got it right! I have RSL boards in my 32.5. Iv'e read somewhere that the input capacitor is no longer required with these installed- is that right?
 
diybry said:
Thanks, it's a long time since I put that tant in place.Good to know I got it right! I have RSL boards in my 32.5. Iv'e read somewhere that the input capacitor is no longer required with these installed- is that right?
Click to expand...
With a DC voltage at the input and no coupling capacitor, the NAP will produce the same DC voltage at the speaker output, so it would have unit gain at DC. It’s much higher for AC voltages of course (27 times).

The reason the RSL boards can be used without power-amp coupling capacitors is that they have film capacitors at their outputs. Unlike (dry) tantalums, they can never go short circuit and supply DC to the power-amp. Plus you have wet tantalums for bypass, keeping the DC gain at bay.

The DC might get there if you connected things up wrong etc., which is unlikely but possible I guess. I’d feel uncomfortable doing that myself, but others have done, so it’s your choice. I’m not sure that any commercially designed power-amps would have DC coupling, presumably because they have to work with any type of preamp, plus it’s a belt and braces approach.
 
Avon said:
With a DC voltage at the input and no coupling capacitor, the NAP will produce the same DC voltage at the speaker output, so it would have unit gain at DC. It’s much higher for AC voltages of course (27 times).

The reason the RSL boards can be used without power-amp coupling capacitors is that they have film capacitors at their outputs. Unlike (dry) tantalums, they can never go short circuit and supply DC to the power-amp. Plus you have wet tantalums for bypass, keeping the DC gain at bay.

The DC might get there if you connected things up wrong etc., which is unlikely but possible I guess. I’d feel uncomfortable doing that myself, but others have done, so it’s your choice. I’m not sure that any commercially designed power-amps would have DC coupling, presumably because they have to work with any type of preamp, plus it’s a belt and braces approach.
Click to expand...
Thanks Avon, I'll leave it well alone.
I'm now moving on to the recapping the reg boards- I've read Colasblue's 135 servicing thread here
https://pinkfishmedia.net/forum/threads/naim-nap-135-servicing-thread.165333/

and the nap 250 reg boards thread here:
https://pinkfishmedia.net/forum/threads/naim-nap250-regulator-boards.196900/
So I need:
4 x 47uf 63v preferably elna silmic 2
Hi fi collective appear to have these here:
https://www.hificollective.co.uk/ca...lna-silmic-electrolytic-capacitor-p-6006.html
Are hi fi collective a reputable source- I'm concerned about fakes.
Or can anybody recommend a suitable alternaive please? a suitable alternative please?
I also need:
4 x 10uf 63 volt min 13ohm ESR
Panasonic NHG or Nichison VX series are recommended, but appear to be unavailable. Everything I look at on mouser/RS appear to be low ESR, or only available in 100's Again, can anybody please recommend an alternative? Thanks again
 
Update: I've sourced the 10uf from Farnell, I appear to have got the last 4! So just the 47uf remain, with hi fi collective appearing to be the only source, unless somebody can suggest an equivalent, please?
 
My component choices were originally from 13 years ago, though the components have lasted well and I don't think I need to recap the amps again anytime soon. Indeed my original 82 has Elna Simic IIs in it which are now over 20 years old and it is not showing its age relative to a recently acquired, serviced and superlinked 82 which came up at a price I couldn't resist.

Nevertheless there are other options.

I believe Witch Hat were using the Vishay 118AHT series axial caps for both the 10uF and 47uf requirements on the regs boards until their unfortunate demise.

Neil James (neiljadman) quite likes the green Nichicon muse bipolar in the 47uF position (which wasn't available to me).

HiFi collective is where I get my Silmic IIs from.

I don't think there's any problem getting Panasonic NHG series - just checked on Farnell and they have over 9600 of the 10 uF 63V in stock! You had me worried that another thing we like had gone unobtainium for a moment.

The thing you won't get at the moment (at any sensible price) is the Kendeil res cap. If I needed to service now I'd certainly be looking at alternatives.
 
Thanks Colasblue, you are as ever a superb source of info.
Ive already ordered the 10uf from Farnell, and have just ordered the 47uf silmic 2 RFS from HiFi collective.
When looking for the 47uf, I found that ESR on currently available caps seems to be very low vs the 13 ohm you recommend on the regulator board thread here :https://pinkfishmedia.net/forum/threads/naim-nap250-regulator-boards.196900/
Here's a novice question... Assuming supply of high ESR caps dries up completely, could this be replicated by placing a resistor is series with a low ESR cap?
Apologies if this is a ridiculous idea!!
 
I've used Banzaimusic in Germany for several of the harder to find / source components - transistors and capacitors including some considered almost unobtainable.
I've always found them very helpful and never had reason to doubt the provenance or quality of any of their products.
I'm happy to recommend them; they also have very helpful English speaking staff.

https://www.banzaimusic.com

Also don't forget Cricklewood Electronics (UK based) - they too often stock some of the more difficult to obtain components
 
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