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Early NAD 3020 fried channel
Early NAD 3020 fried channel
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Old 20th September 2019, 08:58 AM   #1
cowneko is offline cowneko  France
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Question Early NAD 3020 fried channel

Hi everyone,

I'm restoring a venerable old NAD 3020 series 20 (silver one first serie). I got everything done (full recap, changing transistors on the regulator board witch where fried, installing new 2n3055/mj2955 pairs) I've setted the bias and offset without any trouble and finally put some music through the amp, perfectly fine for the first 3min or so, then BAMM!, magic smoke on the right channel......

Some precisions on the amp and what I've done:

The Amp is a first generation 3020

Click the image to open in full size.

But some resistor values comes from 3020b amp:

R647/648= 180r instead of 1k2
R635/636= 1k2 instead of 2k2
R627/628= 680r instead of 1k2
R629/630= 330r instead of 390r

There's no D605/606 on my amp (just a strap)

So after changing the caps, repairing the regulator board and controlling every semi on both channels I installed new pairs of 2n3055/mj2955 (they where absent).
I then installed 1R 3w resistors in place of the strap on r653/654 to measure and set bias, I also substitued RX1 and RX2 for 2K trimmer resistors.
The bias setting went smoothly and I was able to get 30mV on both sides (I controlled temperatures of all the fets and no one where higher than 36). I let the amp warm for 15min or so to fine tune the bias. I also set the output offset to virtually 0 (0.2mV/0.4mV).
All of those manipulation where done with a light bulb tester in case something went fishy. After confirming the bias, I shorted both 1r resistors as per manual instruction.

So after all that I decided to take a listen to the amp. I plugged it directly to the mains and hooked a source to the aux input and started listening with headphone. No problem at all, sounds nice on both channels, at one time I pushed on the loudness button, tried tone pots, everythings fine.....then after 3 min or so, a little plop then no sound on the right channel, and smoke from the amp.

R652 as burnt, destroying c634 in the process, I tested the semi and Q610/612/614 and both the power trans Q616/618 where fried.

The left channel is intact.

Looks like some kind of oscillation destroyed the right channel, as the runaway started when signal was present.

What are your thoughts on this?
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Old 20th September 2019, 09:42 AM   #2
avtech23 is offline avtech23  Australia
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Early NAD 3020 fried channel
Newer 2n3055 are made differently from the originals and generally require an emitter resistor to prevent oscillation / thermal runaway.
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Old 20th September 2019, 09:44 AM   #3
JonSnell Electronic is online now JonSnell Electronic  United Kingdom
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If C634 is damaged, the amplifier is taking off because incorrect parts have been used. If the other semiconductors have been changed for substituted, that will cause the amplifier to take off and be very unstable.

Never use a resistor instead of the link, always close the solder bridge when 30mA is flowing normally.
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File Type: png Screenshot 2019-09-20 at 10.41.39.png (178.6 KB, 317 views)
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Old 20th September 2019, 09:49 AM   #4
MarsBravo is offline MarsBravo  Netherlands
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There is a sincere difference made in the bias circuits, but it does not explain why one channel is happy and the other not so.
R652 is blown by to much current... is that dc (bias) or ac (signal/oscillation) related?
Replace 2955/3055 with bc547/bc557 (cheap) with both Re of 1k to mimic the end stage.
If they blow, it won't hurt. But you're able to track down what the cause might be.
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Old 20th September 2019, 09:56 AM   #5
ColinA123 is offline ColinA123  United Kingdom
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Quote:
Originally Posted by cowneko View Post
Hi everyone,

All of those manipulation where done with a light bulb tester in case something went fishy. After confirming the bias, I shorted both 1r resistors as per manual instruction.
What are your thoughts on this?

Did you re-check the values after you remover the "light bulb tester"?
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Old 20th September 2019, 11:25 AM   #6
avtech23 is offline avtech23  Australia
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Early NAD 3020 fried channel
Just to follow up (I didn't have a chance to reply fully earlier).

It is common for 3020 series NADs to die shortly after being put back into service with replacement output transistors.

Apparently the NAD designs relied on the internal base-emitter resistance of the original hometaxial construction 2N2955/3055 for thermal stability, but the newer epitaxial parts do not have that resistance and therefore are not thermally stable enough for the design without compensation. That is why they require taming with emitter resistors.

There are plenty of threads with the information to carry out the mod (after the channel is repaired again). Search google for 'diyaudio.com nad 3020 emitter resistor' and you should find plenty of good info

Did you make any variations to the other components whilst restoring the amp?
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Old 20th September 2019, 12:05 PM   #7
JonSnell Electronic is online now JonSnell Electronic  United Kingdom
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With Q610 thermally coupled to the output heat sink, it will track the temperature changes. DO NOT fit the bodge emitter resistor, there is no advantage.
C634 is part of the output stage HF load network and only fails when the amplifier oscillates above 50kHZ, just before it destroys itself. Nothing to do with bias settings.
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Old 20th September 2019, 12:34 PM   #8
cowneko is offline cowneko  France
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Quote:
Originally Posted by JonSnell Electronic View Post
If C634 is damaged, the amplifier is taking off because incorrect parts have been used. If the other semiconductors have been changed for substituted, that will cause the amplifier to take off and be very unstable.

Never use a resistor instead of the link, always close the solder bridge when 30mA is flowing normally.
To be exact, C634 has been damaged by the burning res (they touch each other)

And of course I shorted both 1R resistor prior to inject signal

Can I change C634 with a Polyester 0.1uf instead of a mylar?

Last edited by cowneko; 20th September 2019 at 12:37 PM.
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Old 20th September 2019, 02:01 PM   #9
cowneko is offline cowneko  France
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Quote:
Originally Posted by avtech23 View Post
Just to follow up (I didn't have a chance to reply fully earlier).

It is common for 3020 series NADs to die shortly after being put back into service with replacement output transistors.

Apparently the NAD designs relied on the internal base-emitter resistance of the original hometaxial construction 2N2955/3055 for thermal stability, but the newer epitaxial parts do not have that resistance and therefore are not thermally stable enough for the design without compensation. That is why they require taming with emitter resistors.

There are plenty of threads with the information to carry out the mod (after the channel is repaired again). Search google for 'diyaudio.com nad 3020 emitter resistor' and you should find plenty of good info

Did you make any variations to the other components whilst restoring the amp?
So If I understand well I got to insert a resistor between the base of the power trans and the emitter of the driver (after R651/652 and C633/634).

Will a 4r7 res be ok as emitter res?

I'll also change Q611/612 to BD139/140 as the original one are not easily abtainable...
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Old 20th September 2019, 07:48 PM   #10
sgrossklass is online now sgrossklass  Germany
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Quote:
Originally Posted by cowneko View Post
So If I understand well I got to insert a resistor between the base of the power trans and the emitter of the driver (after R651/652 and C633/634).
Nope. The amplifier is missing these guys, found on just about any other similar construction known to man:
Early NAD 3020 fried channel-emitter-png
Typically 0.22 ohms 5 W.

The ones you mean also come in handy at times, but let's start with the basics.
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