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Naim NAP 140: Offset voltage problem

G

guy

pfm Member
Hallo guys.
Yesterday I fully recapped (tants and reservoir caps) a NAP 140 and after the bias check I found an offset voltage of about 3.2V on the right channel.
The bias is set to 7mV read on the green resistors on the left and the right channel.

What is wrong, any idea?

Thank you in advance for your help.

Guido.
 
I have replaced caps and found a high offset on one channel , just a faulty feedback cap , swapped it and all was fine. New caps can sometimes be faulty.
 
Just to add, the 10uF input tant has it's +ve end at the signal input, and feedback tant +ve end is at ground.
 
One more thing: you did re-attach both the +ve and -ve board supplies didn't you?

Been there, done that.
 
If you power up the board with only the negative rail connected to the supply, what damage is likely to occur?
 
All the supplies was connected properly.
I will try to:

1. swap the feedback cap with the cap in the 'good' board
2. the same but with the input cap
3. the same but with the BC239C input pair

Guido.
 
There have been several instances reported on here that problems can occur if the negative power rail is applied to the amplifier board before the positive rail from a power supply that has not been fully discharged.

In addition to causing damage to the driver transistor(s) this can result in the voltage rating of the Vbe capacitor (the one across TR5) been exceeded which can cause it to become leaky and result in excessive d.c. offset.

Malcolm
 
Thanks Malcolm,

Thought the VBE might take a beating in this scenario...hadn't occurred to me that the driver transistors would also be liable to be damaged.
 
Yesterday I changed all the tants but unsuccessfully so I tried to change the differential input pair and the offset decreased from 3.2V to less than 10mV.

I don't understand why but one of the bjt of the input pair had the junctions ok but the hfe was very low, a sick bjt!
 
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