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NAD C320 woes
NAD C320 woes
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Old Yesterday, 07:40 PM   #31
Mooly is offline Mooly  United Kingdom
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NAD C320 woes
No problem

The voltage measurement is where those three parts meet, so if you look at the circuit that could be the plus end of C11 or (because its so low in value) either end of the 100 ohm (R14).

Black lead to chassis, red lead to do the measurement. Use a low voltage DC range on the meter.
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Old Yesterday, 08:49 PM   #32
RainTown67 is offline RainTown67
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Originally Posted by Mooly View Post
No problem

The voltage measurement is where those three parts meet, so if you look at the circuit that could be the plus end of C11 or (because its so low in value) either end of the 100 ohm (R14).

Black lead to chassis, red lead to do the measurement. Use a low voltage DC range on the meter.
Thanks Mooly!

I measured at all 3 points and voltage is very low when power is on, +3.3 mV or so, and stays basically constant until and after the safety cuts in.

After I shut power off the meter then jumps to -170 mV or so which then goes down slowly, presume some capacitor is discharging?
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Old Today, 07:04 AM   #33
Mooly is offline Mooly  United Kingdom
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NAD C320 woes
That sounds normal but it had to be checked.

You are going to have to work on this in the faulty state and make very careful voltage measurements. The clue will be in interpreting those readings and seeing where the 'loop' fails. With the input at the measured 3 millivolts or so you will see the feedback voltage rise on R115/116 junction.

The problem could be a slightly leaky semiconductor (transistor or diode) but it probably wont be very obvious. Freezer spray used very very sparingly on suspect devices might help locating the issue.

A check with an oscilloscope is also always a good idea. Although unlikely, there could be some very high frequency instability shifting the DC operating point.
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Old Today, 07:27 AM   #34
madtecchy is online now madtecchy  England
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Hi
The electrolytic s in them amplifiers are really not to be trusted. I have owned a few of these in the past and found the capacitors to dry out with no evidence of leaking electrolyte. In the main psu caps the internals could be heard rattling around inside . I would suggest you change all electrolytics to start with .
Ian
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Old Today, 01:57 PM   #35
RainTown67 is offline RainTown67
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Quote:
Originally Posted by Mooly View Post
That sounds normal but it had to be checked.

You are going to have to work on this in the faulty state and make very careful voltage measurements. The clue will be in interpreting those readings and seeing where the 'loop' fails. With the input at the measured 3 millivolts or so you will see the feedback voltage rise on R115/116 junction.

The problem could be a slightly leaky semiconductor (transistor or diode) but it probably wont be very obvious. Freezer spray used very very sparingly on suspect devices might help locating the issue.

A check with an oscilloscope is also always a good idea. Although unlikely, there could be some very high frequency instability shifting the DC operating point.
Sorry, I have no oscilloscope available (and would not have clue to use it).

The measurement at that junction is shown as 3.3 / 3.4mV, both before the "fail" and after. It barely deviates. This is lowest setting I have for DC on my multimeter so I cannot be more precise.

I am a little lost now. I don't really understand the line of thought here as to what might be wrong, and which test result either confirms or eliminates the theory? Is 3mV indication of an issue?

Earlier I was asked to check R126/R127 for 46V and found neither was very close to 46V, also not that close to each other in fact. Is that not an indicator of an issue?
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Old Today, 02:06 PM   #36
RainTown67 is offline RainTown67
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Quote:
Originally Posted by madtecchy View Post
Hi
The electrolytic s in them amplifiers are really not to be trusted. I have owned a few of these in the past and found the capacitors to dry out with no evidence of leaking electrolyte. In the main psu caps the internals could be heard rattling around inside . I would suggest you change all electrolytics to start with .
Ian
Thanks for replying.

Does the various measurements made / evidence collected indication of an issue with main PSU caps? I am loath to start changing things, with all the risks of an amateur like me making things worse, without evidence to indicate there is specific fault? How to confirm a fault there?

I am also loath to reject ideas from someone who knows more about these things than I do, and has "experience" which is much under-rated diagnostic tool! But replacing "all electrolytic" is not something I would easily undertake on my own.
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Old Today, 07:09 PM   #37
Mooly is offline Mooly  United Kingdom
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NAD C320 woes
The 3.3mv is normal and very very close to the theoretical zero volts. Any voltage at the this point (the input to the amplifier) would be amplified just like the wanted audio signal.

So we had to make sure that the input of the amplifier was at 'zero' when the fault appeared. And it is.

The whole amplifier relies on the action of negative feedback to stabilise the operating point of the output and ensure it is close to zero volts.

There are actually two inputs to the amplifier, the one we have measured at 3.3 mv and this is called the non inverting input. The second input is called the 'inverting input' and is where the feedback is applied. This point is the junction of R15 and R116.

Just like an integrated circuit 'opamp', the amplifier output should assume a voltage that will keep the difference in voltage between these two inputs at zero and that does not seem to be happening when your fault appears.

This R126/127 voltage is actually a measure of the supply voltage... do we know what that is ? The voltages you mentioned of 38 and 41 sound reasonable enough. R126 should be PLUS say 40v and R127 NEGATIVE 40 volts.

What voltage do you measure on the other end of those resistors ?
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