NAD S300 int amp problem

[hifienthusiast]

I have a faulty NAD S300 int amp which goes into protection mode when it is switched on. The LED on the front panel is flashing.

I have found a few people who have reported the same problem with this amp. Can someone tell me what the problem is and how to fix it? Thanks.

[Leolabs]

Do you have any electronic background???

[FastEddy]

Don't throw it away ... but you should know your way around some test equipment to fix it right ... A flashing LED would most likely indicate that the output stages or power supply has problems.

[hifienthusiast]

I do have electronic background. I have repaired a few transistor amps and built a gainclone.

The NAD S300 manual said the flashing LED means it is in protection mode. I know a few people had the same problem with their S300 amps and they were fixed by NAD for about 60 UK pounds. I am thinking of having a go at the repair myself before I send it to NAD.

Let me know if you have any more advice. Thanks

[fjcbs]

Hi hifienthusiast.
I have a NAD S300 wich began to have cracking noise on left channel with volume at any level (even in MUTE) and now it has the power led blinking fast.

Did you find the power supply problem?
Can you help?

I have basic electronics knowledge and have been poking inside with a multimeter and found some diferent voltages in a small transistor on each channel... also the amp module for the left channel shows some 40+ voltages on some points and the right channel is at 0.

Thanks.

[dtm1962]

I have had the fast blinking lights which indicate that it in protection mode as a result of irregular voltages as a result of a component failure or bad solder joint.

Chances are the ZTX653/753 transistors have overheated and are leaking or a solder joint from top of board to bottom has separated. I resoldered all solder connections in the area of the discoloured circuit board surrounding these transistors and voila it worked again.
I have done the NAD recommended reliability modification via changing out the 4 resistors that allow these transistors to run cooler and more reliable.
These transistors are located on the main board and are the transistors that are grouped together with 8 small TO-92's together for each channel. You will notice the circuit board in this area has discoloured as a result of the excess heat. I have successfully repaired it without changing out the transistors so I conclude that the solder connection went bad as a result of the excess heat. Email me dmullins@cogeco.ca if you need more assitance.

Thanks,

[dtm1962]

The channel that measures 40+ volts is the channel at fault as I had the exactly the same problem.... Try resoldering around the transistor where you measured the voltage, then try soldering throughout this area, and finally the transistor replacement.

DO the reliability mod to make it run cooler.... I even added heatsinks on the transistors to draw the heat away from the transistors and circuit board...

Email me for details.. dmullins@cogeco.ca

[fjcbs]

Thanks dtm1962.

I will try the solder repair first, sometime this week.

Also, aparatusonitus provided me with the service manual and now I can find my way a lot easier.

[fjcbs]

thanks again dtm1962.

today after work I got home and redid the solder point where you said and my beloved S300 works again!!!

I will try to get some resistors to do the reliability mod and also some heatsinks as you mentioned.

The funny think is that last saturday I went to buy some Kimber 4TC cable to bicable my JMLab speakers with the current 8TC cable I had and as soon as I got home to set the cables connected the amp died! I don't think it was the new cable because the left channel was already making some unordinary noise...

Well, listening to the repaired amp, it sounds much more alive and "fast". Don't know if it is because of the repair job, the new cable or just placebo for me being capable to repair the darn thing all by myself!

[dtm1962]

Your welcome..... as it is great that we can help each other continue enjoying our beloved hobby!

I bought replacement transistors just in case but .... if it aint broke don't fix it!

Keep an eye on it but I would definitely changed out the resistors (as part of the mod) and add heatsinks for peace of mind!