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-   -   Help needed for reparing Naim Nait 1 (http://www.diyaudio.com/forums/solid-state/44017-help-needed-reparing-naim-nait-1-a.html)

sinski 28th October 2004 08:43 PM

Help needed for reparing Naim Nait 1
 
Hi,
My friend has one Nait 1 - serial nr.10701 , built 1983 !
But the amp is not working anymore

When i am turning the amp on, then there is a low level noise coming out of the ludspeakers - something like "tsrtsrtststr" and the bass loudspeakers are moving very slowly. It is happening already before the preamp relais is turned on. Both channel are identical and there is not music coming out at all. My friend said, that few months ago after the amp was turned on half an hour, than the noise was away and the amp was working.
Does anybody can help me ?
I am thinking that it is something probably in the poweramp stage, because this is appearing immediately after turning the amp on. The big caps dried out ? or something else ? Any ideas ?

Cheers, Tomek

Netlist 28th October 2004 08:49 PM

Tomek,
You have a good chance caps are worn out. Go for the ones in the PSU first, then those located near heat sinks.

/Hugo :)

anatech 28th October 2004 09:01 PM

Hi Tomek,
I am thinking the bypass caps are open. Main filters may be bad too, but you would hear a hum. The feedback cap, if used, should be replaced as well.
-Chris

sinski 28th October 2004 10:22 PM

2 Attachment(s)
Hmmm,
I had some ps caps from recapping of my own nait, old but OK, i exchanged them, but there is no change.
I did some measurement - the tension on both main ps caps is pulsing form 15.5 to 17 Volts and the tension on one preamp ps cap (nait has only one big ps cap for preamp section) is also pulsing in the same range. The feedback caps are tantal, and they are only few electrolite caps in the pa section. And when you look at the picture, one can see that three of pa transistors have different colour ?
Maybe the rectifier is not OK and is not delivering stable dc ? Some mix from dc and ac ?
I have not the schematics, i send a picture for the better orientation.
:)

anatech 28th October 2004 10:43 PM

Hi Tomek,
I see two electrolytic caps per channel. The tantalums are not immune either. In the area to the right there are piles of capacitors (13). There are more behind the preamp filter cap.

The reason the voltage is pulsing is current draw. This will affect both channels. It sounds like the circuit is motorboating but the hiss may indicate HF oscillation. An oscilloscope would be real handy right now.

-Chris

sinski 28th October 2004 10:53 PM

Hi,
I have a "scope". Where should i connect it ?
I will try it tomorrow. On the right side there is mm premp, there is a lot of caps there ...
But i think that it should be something in the pa section left, because the preamp section right is getting power ~3 seconds after turning the amp on, and this "pump" effect is just from the begining. So i will check the small elcos and tants on the left side tomorrow.

:) Thanks for so much attention

anatech 28th October 2004 11:05 PM

Hi Tomek,
Look at both the positive and negative supplies to see what's there. Also, check the speaker output terminals. You may get a large HF signal. Check the "top" or feed terminals of the volume control to see what is coming from the preamp circuit, if anything.
-Chris

sss 28th October 2004 11:16 PM

the circuit looks simple , it shouldnt be a problem to fix it

this is a very old amp and probably got bad solder joints ,i think thats the problem

sinski 29th October 2004 09:22 AM

Yes, i noticed that some solder points are looking different than other and i will try through some suspect areas.
:)


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