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-   -   NAD 7020i sudden death (https://www.diyaudio.com/forums/solid-state/363517-nad-7020i-sudden-death.html)

zenzaman 24th November 2020 09:34 PM

NAD 7020i sudden death
 
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Hello everybody!

I bought recently a NAD 7020i (was aiming for the 3020 but none was available) that worked just fine until this evening when I tried to compare head-to-head two CD players (same disc in both playing the same song at the same time, switching between sources from the amplifier). All of the sudden some relay inside the amplifier started switching on and off a couple of times and then the sound became horribly distorted and muted. The preamplifier part is working correctly there was no smoke :lol: and nothing seems fried inside. Could I expect something as mundane as old, dried out caps? At a glance, there are two "dried puddles" around the filter (?) caps...

Any suggestions as to where to start looking are highly appreciated.
Thanks!

MAAC0 24th November 2020 10:46 PM

That glue becomes conductive over the years. On the right capacitor it is already brown / black. I would remove it first. Then place a 100W incandescent lamp in series with the amp and begin haunting for the fault.

academia50 24th November 2020 10:51 PM

Of course you can wait, anything is possible and they really look bad, change them to the same value in capacity and voltage.
Don't have the circuit? You can download the older model (7020 without I) from here and check first if those capacitors are the original values, the PSU should be the same for the 20 watt power of both models.
But, I am struck by that bare wire (bridge) that is blacked out by overheating ...... it could be precisely the consequence of the failure of the electrolytic capacitors ...... You need some knowledge of electronics and a tester If not, he would take it to a trusted repairman.


NAD 7020 Stereo Receiver with AM/FM tuner Manual | HiFi Engine

academia50 24th November 2020 10:56 PM

Quote:

Originally Posted by MAAC0 (https://www.diyaudio.com/forums/solid-state/363517-nad-7020i-sudden-death-post6424659.html#post6424659)
That glue becomes conductive over the years. On the right capacitor it is already brown / black. I would remove it first. Then place a 100W incandescent lamp in series with the amp and begin haunting for the fault.

Was that "damn" glue still being used in those years? 1988?
I remember that they were a typical failure of Sansuí, but it was with the models before that date ..... I think, lately my memory is playing tricks on me. I called my wife by the name of my first great love ! :D

academia50 24th November 2020 11:09 PM

1 Attachment(s)
Here ............

zenzaman 25th November 2020 09:55 PM

I think that was just a optical illusion. the wire looks fine, the PCB looks fine as well. no signs of overheating in that area. I guess I'll attempt a recap and see where things lead. I'll probably post impressions and questions as I go along. My multimeter can only measure capacitance up to a couple hundreds uF...

I'm totally new to electronics but I want to learn more. I don't thing replacing some caps require more than patience and an organized and methodical approach.

For the time being I'm knees deep (for my skills) in another amp :))
There's also a little story as a prologue
Kenwood KA-3700

Ian Finch 25th November 2020 11:32 PM

Rather than guess about the photo image or the history of glues, your description of the relay switching on/off a few times at least tells us that there is an electronic fault, probably a serious one. The relay will respond specifically to excess output current at the output of the amplifier, so indications are that one channel of the amplifier likely has a fault which is more than something you can fix with just a new capacitor or two. I wouldn't jump to conclusions yet, but this often means expensive transistor failure.

First, with no input signal, speakers disconnected and volume at minimum setting, does the amplifier come on normally by just switching power on? If so, can you measure any (even as little as 100 mV) steady DC voltage across either pair (i.e. the red and black terminals) of each channel?

Note that if the relay is faulty, you cannot have any audio or DC voltage to measure at the speaker terminals. You would then need to inspect the relay to find the relevant channel power amplifier connection to the relay and test for any DC voltage there. The black (ground) connection is common to both circuits so you can leave that in place.

NB: There is no conventional relay in this amp, the "relay" is a thermal magnetic switch at the ouput of each channel power amplifier, which opens when current heats up a bimetal strip. If you hear a "click" there is a change I'm not aware of.

zenzaman 26th November 2020 04:44 AM

At the time being one channel measures ok (under 10mV) but the other one is open. A few days prior to the incident both channels measured ok. The amp powers on normally but aside working fine on the preamp stage, that's all it does.

academia50 29th November 2020 11:41 PM

Quote:

Originally Posted by zenzaman (https://www.diyaudio.com/forums/solid-state/363517-nad-7020i-sudden-death-post6425804.html#post6425804)
........... My multimeter can only measure capacitance up to a couple hundreds uF...

Look at this, and if you search YT you will find many videos.
Testing on the electrolytes of a PSU usually comes down to two situations, the capacitor is either shorted or "open" (not charging and discharging in both cases).
And if it's at a loss or exploited, you don't need to measure anything, just look at it. They look good.


how to test electrolytic capacitor | xuanx electrolytic capacitors

Ian Finch 30th November 2020 01:49 AM

The problem is at the output stage, not the power supply or its large capacitors, according to the evidence. There are 2 amplifiers and both are powered from the same supply. One still works so the PSU is not where the problem lies. Further, the protection devices (they are not relays and there is no related protection circuit) are thermal-magnetic switches, AKA circuit breakers TB1, TB2 in the 3020i schematic. These only open or close slowly when the output current flowing is more than an amp or so, continuously.

3020 and 7020 power amplifiers are virtually the same and share the same schematic design. These particular little circuit breakers are very old components now. They seldom fail but I believe there are still some copies available from China, if needed. First though, check that there is continuity between the breaker's terminals (unpowered). No amount of guesswork or parts replacement in the amplifier will change anything if its contacts are burnt or it won't close when the output current is normal.

The problem with such a simple device is that the contacts are just plated metal strip - only good for a small number of operations at maximum power (1.5A), before the contacts burn, since they move slowly and are only useful for occasional accidents with shorted speaker leads etc. However, never assume that any fault in an audio amplifier is restricted to its protection device. The relay or rather, circuit breaker, failed or opened for a high current reason - likely a shorted speaker lead or an extended period of high output power.

I have seen this problem with a 3020i model too - many years ago. Be aware that these old NAD models all had power transistors of an early early silicon transistor type (hometaxial) that was crucial to 3020 type amplifier sound quality but now virtually unobtainium. Modifications to the circuit are needed to adapt it to work with modern versions of 2N3055/MJ2955 but rather than take this risk when it became necessary, NAD just ceased production.


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