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NAD 3020A Amplifier Low Volume

Duni

Jan 24, 2022
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P-20220126-124315.jpg"

Here is an image of the interior of the amp
I have some oxydation traces on the bridge rectifier but with 31V AC input it outputs 24V DC which seems normal to me...
 

Harald Kapp

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with 31V AC input it outputs 24V DC which seems normal to me...
Not quite.
31 V AC (assuming RMS) is ~43 V peak. Minus the diode pass voltages (2 × 0.6 V) of the rectifier, the filter capacitor should be charged to ~41 V ... 42 V. Allowing a ripple of 10 % the average output voltage (DC) should be in the ballpark of 38 V ..., 40 V.
Your measurement indicates excessive ripple. This may be due to excessive current drawn by the amplifier or due to insufficient filter capacity. When you replaced the electrolytic capacitors, you did observe capacity, voltage rating and polarity, didn't you?
If you have an oscilloscope, you can easily visualize the ripple visually. Lacking an oscilloscope you can still use your multimeter in AC range (!) across the DC output (across the filter capacitor(s)) to check the AC content (aka ripple) of the voltage.
 

Duni

Jan 24, 2022
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Of course I replaced capacitors with ones that have the exact same capacity and voltage rating.
As for the polarity, I made sure to put them in correctly (it is labelled on the board).
I did the measurements again without any load, AC input is 48V with DC output 64V, and ripple is 12V.
With speakers plugged in, input signal and volume at max, the measurements are exactly the same and ripple is around 10V
I don't know how the measurements can be so different from the previous time but it is probably my fault.
Sorry about that...
 

Alec_t

Jul 7, 2015
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The only cap voltage ratings I've spotted so far in the thread are 35V (post #1) and 25V (post #19). Something doesn't smell right, given the DC output voltage readings.
 

Duni

Jan 24, 2022
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The output from the bridge rectifier is 64V DC but goes into four caps and their output is 32V.
I don't know if this seems really bad, shoud it be lower?
The 25V caps are not located on the same circuit and they don't seem to be part of the power circuit.
upload_2022-2-10_15-12-25.png
 

Alec_t

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That's a useful pic. So the schematic shows ±30.5V, i.e 61V between the two power rails. If that's under the full working load and your measured 64V is under a lesser load then it's about right (32V per cap).
However, there are no balancing resistors shown across the caps to help equalise the cap voltages, so if one cap is leaky or different in capacity from the others the voltage across it might well be much greater than its rated 35V. There is so little margin between 32V and 35V that I'm surprised such a reputable amp manufacturer as NAD didn't use higher voltage rated caps. I certainly would have..... but I'm not a manufacturer trying to cut costs.
 

Harald Kapp

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@bertus : That's a nice find. The complete service manual!
there are no balancing resistors shown across the caps to help equalise the cap voltages
Not necessary acc. to the schematic Bertus found:
upload_2022-2-10_18-51-49.png
We see here that the center between the caps is connected to GND (for @Duni : the little triangle pointing downwards) as is the center of the transformer. Note, however, that this schematic differs from the one in post #26, I don't know which one applies to the actual amp being discussed here :(
 

Duni

Jan 24, 2022
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The two schematics came from the same document, the one one post #26 does have a connection to ground, doesn't it? (on the left of the picture). On the service manual I downloaded I have both of them and it seems to correspond to my board.
 
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