hello to all .. first of all I apologize for my translation
I had a problem with my NAD 3020 first model
R652 as burnt, destroying Q610 / 612/614 and both power trans Q616 / 618 where they were fried.
The left channel is intact.
I will also change Q612 /Q614 to BD139 / BD140 as the original is not easily obtainable. (Changing the pinout)
I have read from several parts and here too the advice / need to isolate the emitters and insert a 0.33 ohm 3w resistor between the emitters and the common connection point.
and also insert a 4.7 ohm 3w resistor between the drivers and the bases
Without this mod there is a risk of thermal runaway due to the newer transistors which are of epitaxial construction. The original 2955/3055 pair was hometaxial.
x to put it into practice, for me it is difficult because I am not very practical, but a great desire to learn
I ask you if this screen of mine is correct to run the mod
I had a problem with my NAD 3020 first model
R652 as burnt, destroying Q610 / 612/614 and both power trans Q616 / 618 where they were fried.
The left channel is intact.
I will also change Q612 /Q614 to BD139 / BD140 as the original is not easily obtainable. (Changing the pinout)
I have read from several parts and here too the advice / need to isolate the emitters and insert a 0.33 ohm 3w resistor between the emitters and the common connection point.
and also insert a 4.7 ohm 3w resistor between the drivers and the bases
Without this mod there is a risk of thermal runaway due to the newer transistors which are of epitaxial construction. The original 2955/3055 pair was hometaxial.
x to put it into practice, for me it is difficult because I am not very practical, but a great desire to learn
I ask you if this screen of mine is correct to run the mod
Attachments
the photomontage is only to illustrate the wiring, I am interested in knowing if the connection on the tracks is correct
If you insert emitter resistors, you'll have to adjust the bias circuit as well, otherwise it will run in class-b with higher crossover distortion.
The NAD3020 service manual is online somewhere I'm sure, where you'll find the schematics.
It seems that the 2N3055 did act different then todays normal BTJs see:
https://www.diyaudio.com/forums/parts/350298-hometaxial-2n3055-mj2955.html#post6145058
I would try to salvage some real old Motorola 2N3055/MJ2955 somewhere and use these, instead of trying to modify the circuit.
Also if you use a newer type which have higher Ft it might affect the stability of the amp (could lead to oscillation) ... the originals had a very low Ft of 0,8 MHz I think.
I have had a NAD3020 for more than 30 years myself, and I used to use it really hard for parties, even took 2 and bridged at some point 🙂
It shot down a few times (the output relay), but never had any other issues. So emitter resistors or not, I do not think is is prone to thermal runaway.
Therefore I'm wondering if something else went wrong with your amp?
One could wonder if all the caps are still ok after all these years?
These amps are history, so really great that you put it into working condition again
KR Baldin
The NAD3020 service manual is online somewhere I'm sure, where you'll find the schematics.
It seems that the 2N3055 did act different then todays normal BTJs see:
https://www.diyaudio.com/forums/parts/350298-hometaxial-2n3055-mj2955.html#post6145058
I would try to salvage some real old Motorola 2N3055/MJ2955 somewhere and use these, instead of trying to modify the circuit.
Also if you use a newer type which have higher Ft it might affect the stability of the amp (could lead to oscillation) ... the originals had a very low Ft of 0,8 MHz I think.
I have had a NAD3020 for more than 30 years myself, and I used to use it really hard for parties, even took 2 and bridged at some point 🙂
It shot down a few times (the output relay), but never had any other issues. So emitter resistors or not, I do not think is is prone to thermal runaway.
Therefore I'm wondering if something else went wrong with your amp?
One could wonder if all the caps are still ok after all these years?
These amps are history, so really great that you put it into working condition again
KR Baldin