Consistently blown fuses, CB NAP 250

Hello everyone.

Doom and consternation have settled over the Donkey Sanctuary. Carrots are left untouched. Customers un-bitten. Hay is being kicked at in a moody sort of way. Tails, in short, droop.

Last night no sounds were being emitted. John Cage was not playing.

I checked the fuse in the NAP 250 and it was blown. We were watching something with guests so I switched to the internal amp on the SU.

This morning I ordered two replacement fuses from Cymbiosis. In the meantime, I popped into the local electrical shed. He couldn’t exactly duplicate the anti surge 3.15amp fuse that blew, but gave me a 3.15amp ‘fast blow’.

This blew. Fastly.

Then I remembered the spare fuse in the SU.

This fell in with prevailing public opinion and promptly blew too. The NAP 250 made a brief vibrating noise on power up.

I think something is amiss in the NAP’s workings. Has anybody experienced anything similar?

Is it due a trip to Naim? It was last serviced in 2013 by WitchHat, I believe. I bought the unit in 2014 and it’s been faultless.

Any suggestions or advice deeply welcomed.


Not all Naim items have the same fuse IIRC. Until you have tried a fuse known to be appropriate to the 250 you have I don’t think you can be sure that the issue is with the unit. It may be that the original fuse blew as sometimes happens and that the fast-blow fuse was always going to blow as it isn’t the right spec.

Is the SU fuse the same rating as the one in your 250?

Hi - the SU fuse is definitely 3.15A but I’m not totally sure it’s the same as that in the NAP. Cymbiosis lists the ones I’ve ordered as being useable on the HDX, Uniti, Supernait, 200, 250, 300, 500 and the 555PS.

Fair enough - sounds as if it’ll have to go for servicing/repair then.

Hi Svetty,

The spare fuse from the SU would certainly have held if there wasn’t a fault downstream - it is what we would fit as a replacement in a CB 250 at HQ.

The usual reasons for blowing fuses in 250s are:

Short circuit mains switch (unlikely, given the unhappy transformer noise).
Short circuit reservoir cap (unlikely, given the 2013 service).
Short circuit bridge rectifier.
Short circuit output device in the power amp section.
Short circuit transformer windings (hopefully not - Naim no longer have stocks of these, both H&F & Nuvotem).

Hoping you get it back to good health soon!


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If you have use a Naim Powerline for your amp then check it is inserted correctly as if not it may work loose and potentially blow your case fuse.


I’m sure Neil’s advice is definitive.

All I might add is that it’s never worth buying fuses in twos. They are cheap and so buying a packet of six or ten makes much better sense.



Thank you all for the kind and wise responses. David, I’ve ordered a pair of replacement fuses but suspect it requires more invasive surgery.

Is Rocinante suitably stabled and ready to transport NAP to Salisbury? Or more likely your local dealer.

Ha! Indeed. Rocinante’s pep is up after an incident with some ginger and the terminus of his digestive tract.

NAP will be despatched today for fettling and should be re-stabled within a week and a bit.

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A fast blow fuse is the opposite of what you need. Naim use T fuses, which have a momentary time delay to allow the current surge at switch-on. Anyway, I hope you get the 250 sorted out.

Thanks, Richard! Good to know - I’ll try not to make that mistake again

Perhaps your experiences have been worse than mine, but I’ve never had a fuse blow, and if I did, I’d rather have one spare fuse remaining than 5 or 9! But then, like the OP, I don’t have an XPS.

It’s just that I would never buy in ones or twos things that should cost pence each.

The only problem I have had myself is that my ex-demo CDX2 has blown a couple of fuses in its life. When the first one blew in my ownership, I found the spare fuse space was empty and I had to find something (anything!) sufficiently bonza in my shed to keep me going until after the weekend and until the new ones arrived a few days later from the Naim dealer who sold me the ex-demo unit.

It’s like I buy a packet of stamps, not one or two. I want the spare ones nearby, not a few days delivery away.


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