Hello everybody,
The last months I read through this threads to help my in my repair of a NAD C372.
Since september last year I decided I wanted to learn to repair on a deeper level and so I use this case as a learning study.
What was happened?
The amp went into protection and the left channel was toast
What did I already do?
I went through the entire left amp channel pcb and tested every transistor out of circuit (with a Peak DCA75)
The following were gone and were replaced:
Q316, Q313, Q318, Q338, Q317, Q337 and Q328, the one that caused all this.
Q315 is also replaced.
3IC1 en 3IC2 were also replaced, as were R349 and R329 who are fusible resistors for the 64V line.
VR302 and VR301 were also changed.
At last IC301 was also toast, which I replaced.
All diodes were tested. All resistors were measured, some out of the board to make sure they value is correct.
All capacitors in the entire amp I changed to new ones, including the PSU rail.
So what is the problem now?
I can regulate the DC offset voltage between 0 and 3V als described in the manual.
But the big problem is the bias doesn’t go higher then 3mV.
The weird thing is that on the other channel I can’t even go beyond 2mV.
Here the DC offset opamp is also changed and also the 18V zener diode who was defective.
Since I checked both board completely as described above I can’t find the cause of this.
The only thing that stands out is that the supply voltage is 73V instead of 64V.
Is this the problem?
The amps plays music and works already but this bias problem indicates there is something wrong…
Can you guys help me?
Service manual is in the attachment
The last months I read through this threads to help my in my repair of a NAD C372.
Since september last year I decided I wanted to learn to repair on a deeper level and so I use this case as a learning study.
What was happened?
The amp went into protection and the left channel was toast
What did I already do?
I went through the entire left amp channel pcb and tested every transistor out of circuit (with a Peak DCA75)
The following were gone and were replaced:
Q316, Q313, Q318, Q338, Q317, Q337 and Q328, the one that caused all this.
Q315 is also replaced.
3IC1 en 3IC2 were also replaced, as were R349 and R329 who are fusible resistors for the 64V line.
VR302 and VR301 were also changed.
At last IC301 was also toast, which I replaced.
All diodes were tested. All resistors were measured, some out of the board to make sure they value is correct.
All capacitors in the entire amp I changed to new ones, including the PSU rail.
So what is the problem now?
I can regulate the DC offset voltage between 0 and 3V als described in the manual.
But the big problem is the bias doesn’t go higher then 3mV.
The weird thing is that on the other channel I can’t even go beyond 2mV.
Here the DC offset opamp is also changed and also the 18V zener diode who was defective.
Since I checked both board completely as described above I can’t find the cause of this.
The only thing that stands out is that the supply voltage is 73V instead of 64V.
Is this the problem?
The amps plays music and works already but this bias problem indicates there is something wrong…
Can you guys help me?
Service manual is in the attachment
Update:
The -68V line was only -16V.
This was due to R723, a fusible resistor that was broken.
I put a replacement in and now the left channel is completely functional as it’s supposed to be.
The right channel goes to 3mV bias now, but not yet to 6mV. So I suppose I’ll replace Q415 and Q416.
The -68V line was only -16V.
This was due to R723, a fusible resistor that was broken.
I put a replacement in and now the left channel is completely functional as it’s supposed to be.
The right channel goes to 3mV bias now, but not yet to 6mV. So I suppose I’ll replace Q415 and Q416.