NAD C375Bee DC offset problem

[Hangaard]

NAD C375Bee Power Amp Board.
RV104 adjust the DC offset?
My problem is that I measure 2 to 3 volts on DC offset and it does not change anything that I adjust on RV104
Anyone have an idea of where the problem lies?

link to service manual NAD C375BEE Stereo Integrated Amplifier Manual | HiFi Engine

 

[A Jedi]

Here's the procedure for a C370. It *may* be close enough to help.

 

[Hangaard]

thanks but RV104 does not change DC offset so there must be something wrong somewhere

 

[rayma]

Check the bias pot itself, it could be bad.

Then check all the semis and resistors, and connectors.

 

[JonSnell Electronic]

Does the DC voltage vary on the wiper of RV104?

 

[Hangaard]

no nothing happens when i turn on RV104

 

[JonSnell Electronic]

I will ask again ... does the DC voltage on the WIPER of VR104 vary when you adjust it?

 

[JonSnell Electronic]

There should be 20 or so milivots between pins 1 & 3 of R104., if there is and the wiper adjusts accordingly, look at the front end transistors and check there is sufficient Ce voltage to allow them to work correctly.


This amplifier is from the time when they picked computer designed values that don't mean a great deal, especially if you want to replace an odd value, as you usually have to purchase them 100 or 1000 on a roll and only ever use the one.

 

[Hangaard]

I have not tested it, I will try it tomorrow.

In the diagram there are 4 pcs. 3.9Kohm 3W
On the board it is 2.9Kohm 3w (original) but measures them to 1.83Kohm but mounted in the board. (get relly hot)

I do not have such an esr tester
I'm thinking if it was an idea to recap, well know it will be an advantage to find the fault first but could one or more defective capacitor be to blame for the fault?

 

[JonSnell Electronic]

Well as there are only two capacitors in the output stage and if one was short circuit ... that may cause a serious off set issue. It is a DC coupled amplifier so no capacitors.
It is always best, before spending money on components, to fault find first.
Are both channels the same?

 

[JonSnell Electronic]

You will not get an accurate resistance reading with components still on the board.

 

[Hangaard]

Yes both channels have same problem

 

[MikePP]

In the diagram there are 4 pcs. 3.9Kohm 3W
On the board it is 2.9Kohm 3w (original) but measures them to 1.83Kohm but mounted in the board.

These pairs of identical value resistors are in parallel so they will read 1/2 value (i.e. 1/2 of 3.9K or 1/2 of 2.9k).
Unless the soldering is new on these it's likely NAD just made a rolling change of value.
If they look particularly stressed replace them anyway.

(get really hot)

They are 3W for a reason, and they will get hot


As your fault is both channels , I would suggest you confirm all supplies are present first (on the right side of Page 11 )
NAD amps are notorious for heat stressed (small value mostly 4.7uf to 100uF) electrolytic caps, look for colour change of the wrap for worse case examples, and the smaller values near heatsinks in the PSU (see the 10uF on page 11)

 

[Hangaard]

Has now measured over RV104 (1-3) and there is no voltage between pins 1 and 3 and there is no change in adjustment.
but on Offset test points and AGND I measure after 10 minutes to 0.46V 45 minutes to 0.19V again after approx. 2 hours I measure the whole 10V
after the first 10 minutes and up to 45 minutes, the voltage dropped just as slowly from the 0.46V to the 0.19V

I can adjust bias and ISC sens

The other output board is the problem my forde a test stick shorted something that caused the C143 (10uf 50v) to explode ...
So the problem in it is something I have caused, but if I could make this one work first then I have to take the other one afterwards

 

[Hangaard]

It seems that R138 (10Kohm) and R139 (36Kohm) are defective
RV104 can be adjusted in resistance (without power on) so it seems to work fine