About NAIM NAP 140 building

Hi people,
I wanted to mention about a problem in my project with building a NAP140 amp clone.
After putting everything together, I realized that although the sound is clean and satisfactory, one of the transistors (ztx653) and the resistor on its leg (I believe it was 2K2) on both channel boards are heating up seriously. I came to the point that I had to turn it of so it doesnt burn.
Anyone can offer a solution to this problem, or ways to troubleshoot, and/or experienced a similar problem?
Thanks in advance.
Hakan

 

Hi Hakan,

Have you got a suitable heatsink on the transistors? I seem to remember that Naim mount the power transistors on the chassis to keep them cool. As for the 2K2 resistors, you could try using ones with higher power dissipation.

 

You need to refer to the schematic, quite likely is the orientation of the ztx653 needs to be checked,
in doubt refer to the data sheet for that transistor, referencing against the schematic.

 

Hi, the power tubes are perfectly cool. Ztx is a tiny plastic transistor.
thanks for your suggestion though.

 

I followed the schematics, except for a single transistor eqiuvalent, all parts are fidel to what printed circuit says. Orientation of ztx seems to be correct.

 

What voltage do you have on the power rails ? , I have read 34 V DC on each polarity is suggested.
Check orientation of the two diodes at the base of TR6, and setting of VR1

 

Hi, and sorry for the late reply.
I am rather new to the terms, where should I take a reading to measure on the power rails? And which transistor you meant by TR6? how should the VR1 setting be?
I powered it up today again, and again found that these transistors in both channels are quickly heating up, together with the 560R at their legs.
Thanks.

 

I suggest you post a copy of the circuit diagram you're working to. Without that, any suggestions will be pure speculation as what we might think is, say, TR6, may not be on your diagram.

S.

 

Hi Serge. I used one of the aliexpress chinese circuits. dont have the diagram but I have the link:
https://tr.aliexpress.com/item/1005...earchweb0_0,searchweb201602_,searchweb201603_
thanks for your help.

 

Thanks for the link, but I haven't been able to find a circuit diagram. Did you get one with the boards, or a PDF maybe?

S

 

http://pic2.hifidiy.net/2013/05/31/01_1.jpg
you can see it here, and the power tubes I used are 2SC2922s
Thanks very much.

 

Looking at the circuit diagram, TR6 should have 68ohms in the emitter leg, not 2K2. That transistor may get warm, as it could be dissipating as much as 250mW, it should only have around 10mA flowing through it. Check the voltage across TR6's emitter resistor, it should be around 0.7v. That should confirm the current through TR6. Then check the voltage across TR6, Collector to emitter. Multiply that voltage by the current and it'll tell you the amount of power dissipated by TR6, which should be well within its rating of 1W.

You also need to check the bias through the output transistors, TR11 and TR12. Measure the voltage across their 0R22 resistors, and make sure that's correct for that output stage. I don't know what it should be, but at a guess, something like 3.3mV for a bias of 15mA. If the bias recommendation is more than 15mA, then the voltage will be correspondingly higher.

Best of luck.

S.

 

Thanks again for quick reply. But I did not understand, emitter leg of TR6 is already connected to 68 ohms, isnt it?

 

In your first post you wrote

one of the transistors (ztx653) and the resistor on its leg (I believe it was 2K2) on both channel boards are heating up seriously.

Later, there was reference to TR6, which is a ZTX653, and I assumed that the transistor that was heating up excessively was TR6.

If it's not TR6 that is getting too hot, which one is it? I then may be able to work out what's happening.

S.

 

yes, tr6 which in this case is also ztx653 that is overheating, there is a 560R at the emitter of this transistor, (instead of the 680R shown in the circuit), and this resistor is also heating up crazily. It was my mistake to mention 2K2, very sorry about that.

 

According to the circuit diagram you linked to above, TR6 should have 68 ohms in the emitter, not 560R or 680R.

The voltage across this 68 ohms resistor should be around 0.7V. Can you confirm that the two diodes between TR3 and TR6 have around 1.4V across the two together, or 0.7V across each diode?

S.

 

I will check and let you know

 

Hi Bermek, any news?

S.

 

Have not been able to do any measurements, but I sure will post here when I can. I had disassembled the connections of the circuits with the power supply, so I can measure the voltage across the legs of 0.22 ohm, which was less than 0.1mV for some reason I did not understand. So, it will take some time to me to find free time to re-attach the circuits.

 

That 0.1mV across 0.22 ohms translates to a bias current of only 45uA, so hardly anything at all.

As both amps are doing the same thing, it's unlikely that there's been an identical failure on both, so I would look for a wiring error, such as one or both of the diodes between TR3 and TR6 being the wrong way round. If that's the case, then the voltage on the bases of TR3 & 6 will rise and drive a lot of current through TR6, heating it up.

S