advertisement


Naim NAP135 Random Fan operation

N

nap90

pfm Member
Hello All,

I have a strange phenomenon with one of my 135s... sometimes when only my wife is home the fan of one 135 starts Spinning (loudly!). This only happens when the System is idle. I have only heard this over the phone but never experienced it "live" as it were. She always turns that amp off then. Once, on turning on the next day, the fan came straight on again!

I have never managed to get the fans on even after Extended listening at higher volumes. When I turn the Little poti on the fan board, i have to turn it a Long way to get the fan to come on. The amps are never very warm to touch.

Any ideas what could be going on? I have checked Bias and voltages as per the 135 servicing thread - all ok. Could it be a dodgy cap or resistor on the fan board? Any other ideas?

Matt
 
Is it going on at full speed?
I've never seen these go wrong so I can only speculate that it could be dry joint between the transistors and the circuit board.
 
Are you sure it's not transformer hum? That can randomly kick in on mine (one unit only) and varies in volume. On rare occasions quite audible in the room.
 
The fan control is relatively simple. There are two transistors bolted to the heatsink - one senses temperature and the other drives the fan. An opamp and zener diode work out how fast! I hope you marked where the pot was.
 
Yes I did mark where the pot was - luckily read the 135 servicing thread from Colasblue a few months ago.

Now one thing that strikes me is that this one 135 hums a lot more than the other one.

I was wondering, if maybe the amp does really get hot occasionally (still never witnessed the fan operation myself). Could it be oscillating? how could i check that?
 
You're going to need a scope to see if it's oscillating. It'll be obvious.
Failing that, and something I've not tried, is put an am radio nearby and see if it picks up any interference. The rate at which those PS boards go I'm sure you'd hear something.

If the amp hasn't been serviced in 10 years it's likely it'll be oscillating. And it's the small caps on the power supply board not the huge ones. Many people get that wrong.

Hum is normal I'm afraid and due to the mains quality and the transformer construction.
 
Thanks. By small caps do you mean the pF ones? Will give it a try with an AM radio, if I can find one.

The hum is annoying, was it particular bad on the 135/250? One 135 is much worse than the other. My 180 in the cellar is totally quiet and it is not even on it's own spur like the upstairs system.

You mentioned somewhere that the ncc200 boards in a 180 are amazing- would they make it approach the 135s? If yes, I could upgrade the 180 and swap it for the 135s in the main system- what do you think?
 
The small caps are the 10uF 63v ones on the output of the power supply board. There's a pic on our website of an unstable 250 doing its thing at 1MHz here http://www.witchhatservicing.co.uk/?attachment_id=547

The hum is mechanical in nature and different between transformers. They may have a number on top of them which means they've been selected at the factory and were the quieter of the batch. Holden and Fisher transformers didn't seem to suffer as much as the Nuvotem ones. When we made our short run of amps I deliberately chose a taller aspect ratio for the case so that the transformer wasn't constrained by height and could be better built.

NCC200s in a 180 are amazing. They get a raw power supply without the constraints of the regulator board and their filtered front end means you get the refinement too. In short they'll blow a 135 away. They're a drop-in replacement, or we can do it.
 
You must log in or register to reply here.


advertisement


Back
Top