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Early NAD 3020 fried channel
Early NAD 3020 fried channel
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Old 20th September 2019, 11:18 PM   #11
jaycee is offline jaycee  United Kingdom
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As said above you need to add some resistance to the EMITTERS of the output transistors. 0.22-0.33 ohms will work. I find vitreous enamelled resistors rated for 3W work well for this.

Better to use MJE243/253 for the drivers - modern BD139/140 are not very good at it.
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Old 21st September 2019, 12:23 AM   #12
wg_ski is online now wg_ski  United States
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The 0.2-0.3 ohm emitter resistors will work to improve stability. But so will adding 10 ohm resistors from emitter of driver transistor to base of output. Sometimes called a “base stopper”, although the function here is thermal stability as opposed to the usual function of oscillation suppression. Modding it this way is reported to sound more like the original than doing it with emitter resistors.

Why? Because the original hometaxial transistors didn’t have a “built in emitter resistor”. They had high base-spreading resistance, which is in effect a base stopper. So using a high-ish value base stopper gives you the same mechanism of improving thermal stability as the original amplifier design.
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Old 21st September 2019, 08:13 AM   #13
cowneko is offline cowneko  France
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Quote:
Originally Posted by jaycee View Post
As said above you need to add some resistance to the EMITTERS of the output transistors. 0.22-0.33 ohms will work. I find vitreous enamelled resistors rated for 3W work well for this.

Better to use MJE243/253 for the drivers - modern BD139/140 are not very good at it.
I got some 2sb649A and 2sd669A at hand, those may work well don't you think?
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Old 21st September 2019, 08:32 AM   #14
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Quote:
Originally Posted by cowneko View Post
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?
That was not made clear. Yes a Poly cap is fine.
Being an ex NAD service centre, before I retired and have repaired many 3020s and it is always so much easier to be working on the amplifier than to second guess what may have happened.
I have never fitted emitter resistors as they do not require them if set up correctly. On-Semi 2N3055/MJ2955 are the same specification as the older Motorola semiconductors.
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Old 21st September 2019, 11:14 AM   #15
wg_ski is online now wg_ski  United States
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Originally Posted by cowneko View Post
I got some 2sb649A and 2sd669A at hand, those may work well don't you think?
Certainly, if they are real ones. Many fakes would probably *work*, but depending on what you actually get they could exacerbate the thermal runaway problem if youre not using emitter/base resistors. Putting in those makes the circuit more tolerant of transistor type, in general.
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Old 21st September 2019, 06:19 PM   #16
jaycee is offline jaycee  United Kingdom
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Originally Posted by cowneko View Post
I got some 2sb649A and 2sd669A at hand, those may work well don't you think?
If they're the real deal, I think that's what NAD used in later versions of the circuit
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Old 22nd September 2019, 12:41 AM   #17
cowneko is offline cowneko  France
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Ok, so I rebuilt the entire right channel (with st 3055/2955, b649A/669A as drivers, a new bd139, new 180r metal resistor and 400v 0.1uf poly cap.)
I added base stopper resistors (10r 3w) on every output trans (left and right channels) .

I made bias adjustments with lightbulb, and everything went smoothly (has before), I got steady 30mV on both channels, every transistors between 26 and 30c at the notable exception of Q506 on the preamp right channel witch where 10c hotter than Q505 on the left one.

So, I tried adjusting bias on mains.... No problems, 30mV steady and I let the amp warming for more than 10mins.
So I decided to short the bias res (1R 3W) and listening to music..... I listened for 8 minutes without problems.

Confident that everything was fine now, in turned the amp on again to set the output offset (nothing connected on any inputs) and reached few mV on both sides just before........ Bam!!! Right channel smoked again..... 😭

Same thing as before, both drivers, bd139, 3055/2955, 180r res fried........ And left channel intact...... Something fishy here..... I checked the 4 remaining transistors on the right power section and they all seems fine (bc556A with an hfe of 141, bc559b hfe=309, bd139 hfe=65, 2sc1400 hfe=692 )...... I'm a bit lost here.

Now, I'm extremely motivated getting this amp fixed😉, I hope you'll have some advice/ideas to help me achieve this repair.
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Old 22nd September 2019, 06:42 AM   #18
irext is offline irext  Australia
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Have you got an oscilloscope? I'd be checking for HF oscillation. It sounds like it's taking off.
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Old 22nd September 2019, 07:21 AM   #19
cowneko is offline cowneko  France
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I got one, but where should I start?
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Old 22nd September 2019, 09:25 AM   #20
cowneko is offline cowneko  France
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So I made a discover....

I rebuilt (one more time) the right Channel and this time spent an extensive amount of time monitoring things (on light bulb tester)

I set bias to 30mV on both sides and let the amp run for some times (with dmms still attached to bias points), in the meantime i checked temperature and still get hotter fets on right preamp channel.

Then after 10 or 15 mins, the bias on the right side collapsed all of a sudden (to 0. something), I immediatelly turned off the amp (even if the tester was not lited). then immediatly turn it back on.... the bias was still collapsed.

Turned off again but waiting some times before turning it back on (a minute or so). The bias was back again and rising normally to it's setted value of 30mV. After a few minutes, collapse again.

I then tried unlinking preamp from amp sections.
But the collapse still occured 10 minutes after turning it back on. So it seems that it's not the fault of the preamp stage.

What can cause this sort of bias collapsing?
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