NAD C300 schematic and/or help...

Since a long time I have a NAD C300 amplifier and I really liked it. It's simple, does (or must I say... Did) what it has to do and sounds pretty good.
But now, he's dead. It happened about a year ago when a leaking roof spilled some water into the PCB. First I heard a noise, like an overdriven sound, then the red LED lit up, what means the amp goes into 'protect mode' and then it failed completely. The two big (5A) fuses were blown and they immediately blow again when the amp is powered. When the PSU is disconnected, they don't blow so it is a short circuit in the main PCB, not in the PSU itsself.
I checked several Big SS-components for short circuits (still soldered on the PCB) and the two big 10.000µF Capacitors but I couldn't find a short.
Does any one have a schematic of this amplifier. I allready tried several NAD dealers and repair shops but I think they don't like to give away this information.
Without a schematic it's really hard to start looking because a lot of components were used in this amp and the faulty one(s) could be everyware...

Grtz J
 
Hi pir8erxxl,

At 6 pin connector CZ703/CB703 you should have this voltage:

- pin #1 = +37VDC
- pin #2 = GND
- pin #3 = -37VDC
- pin #4 = +32VDC
- pin #5 = -32VDC
- pin #6 = GND

From these DC voltages you should be able to calculate AC voltages at transformer secondaries...and don't forget to count in voltage drop over diode bridges ;-)
 

pir8erxxl

Member
2009-04-01 7:35 am
Hi pir8erxxl,

At 6 pin connector CZ703/CB703 you should have this voltage:

- pin #1 = +37VDC
- pin #2 = GND
- pin #3 = -37VDC
- pin #4 = +32VDC
- pin #5 = -32VDC
- pin #6 = GND

From these DC voltages you should be able to calculate AC voltages at transformer secondaries...and don't forget to count in voltage drop over diode bridges ;-)

Thanks alot!

That is what I am exactly looking for!
 

lord65

Member
2020-06-15 4:53 pm
NAD C300 voltages

Hi everybody. I am trying to teach myself electronics in lockdown and have bought a NAD C300 which "wasn't working on one channel".

I have cleaned the scratchy balance control and replaced the obviously bulging caps in the power supply board.
On headphones it sounds ok but with speakers attached it sounds thin and awful.

I found the service manual and set up the output offset voltage and idling current to the correct values but I noticed that the offset voltage visually drops on the meter while I'm looking at it.

I found this post....

At 6 pin connector CZ703/CB703 you should have this voltage:

- pin #1 = +37VDC
- pin #2 = GND
- pin #3 = -37VDC
- pin #4 = +32VDC
- pin #5 = -32VDC
- pin #6 = GND

but I found this.......

-pin #1 = +46VDC
- pin #2 = GND
- pin #3 = -48VDC
- pin #4 = +33VDC
- pin #5 = -33VDC
- pin #6 = GND

I've checked the 2 bridge rectifiers and the voltage going into them is different from the transformer, 73v (2 orange leads via 0.5 a fuses) and 50v (green and red leads via 5a fuses). Has my transformer died?

What's going on? Please help!
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
I would hazard a guess that the service manual is wrong... it happens.

Do you see IC 701 which is used as a +35v regulator. The input to that is claimed to be the +37v line which is just not going to work, it is not high enough to overcome the headroom the regulator needs. 46v input sounds much more reasonable.

Further 'evidence' is with D715 which is added to 'lose' a bit of the excess voltage because +46v is to high to apply to the 7805 regulator IC703.

So all that suggests to me that the voltages you measure are correct and the manual is in error.

I can't just see the alignment procedure in the manual... its says page 7 but page 7 is a board layout in mine.

DC voltage at the amplifier output should be close to zero and less than -/+100 millivolts. It's normal for it to wander a little.

Bias current I would estimate at between 50 and 100 milliamps which I assume is measured by looking at the volt drop across that 2 ohm resistor (when it is un-shorted).
 
If supply voltages are okay, but still sounds "thin and awful", have you replaced all capacitors?

If the feedback capacitors in the amplifier are dry, so they have low capacitance, it could reduce the gain of the amplifier and also increase the low frequency cut off, potentially giving the sound you describe.
 

lord65

Member
2020-06-15 4:53 pm
Thanks everyone, what a great forum.

Assuming the voltages are correct, why does the output offset voltage drop away when I meter it? Something is discharging perhaps, could be the dry caps?

Do the mosfets drive the headphone output as well as the speakers? It sounds OK on headphones!
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Yes, the headphones are driven from the main amplifier output via series 220 ohm resistors.

By 'output offset voltage' do you mean the DC voltage as measured at the speaker terminals? If so then that will drift a little with temperature. Placing your meter to measure this should have no effect.

Or do you mean something else?