Wait a minute - you changed the power transistors, right? I'm guessing 2N3055 type, because it is easily found. Note that the 3020 amp. is designed to use an old type of transistor known as Hometaxial type. Your replacements were probably the modern Epitaxial type but these require different bias settings and they they don't even sound right. It is possible to adjust the bias to correct level for the epitaxial type but it is not a simple process.
Fortunately there are already many threads here about this very common problem with ancient NAD 3020 model and related models that use the same electronics. Essentially, don't assume that every 2N3055 or MJ2955 is the same device. They have changed since RCA stopped making them, many years ago. ST Micro never produced that type but Motorola/ Onsemi did with 2N3055H, I believe. NAD had other manufacturers producing them but no longer.
Google "NAD 3020 power transistors" or similar - see what you find.
Fortunately there are already many threads here about this very common problem with ancient NAD 3020 model and related models that use the same electronics. Essentially, don't assume that every 2N3055 or MJ2955 is the same device. They have changed since RCA stopped making them, many years ago. ST Micro never produced that type but Motorola/ Onsemi did with 2N3055H, I believe. NAD had other manufacturers producing them but no longer.
Google "NAD 3020 power transistors" or similar - see what you find.
Another question regarding crackling - What type of heatsink grease did you apply? Some types intended for computer CPUs are partially conductive - good for making shorts and crackling sounds with power transistors, too 😱
No suspects parts.... transistors are ok
Breaker is ok.... Caps are ok.
No bad solder joints...mistery
All parts was desoldered and tested with electronic parts tester
Breaker is ok.... Caps are ok.
No bad solder joints...mistery
All parts was desoldered and tested with electronic parts tester
If I connect a speaker on right channel,2 power transistors very hot in 2...3 sec.
If I connect headphones,no hot
The 2 speakers is good
If I connect headphones,no hot
The 2 speakers is good
Last edited:
So, you've checked the pre-driver / driver stages (all OK?) and the idle current (bias) is good?
I have to think that bias current is not correct for the transistors to just heat up in seconds. The headphone jack just adds resistance (220 Ohms) to the headphone connection so the load to the power amp stage is reduced.
D606 (BAW62 fast switching diode), check this for failure?
I have to think that bias current is not correct for the transistors to just heat up in seconds. The headphone jack just adds resistance (220 Ohms) to the headphone connection so the load to the power amp stage is reduced.
D606 (BAW62 fast switching diode), check this for failure?
I guess I'm looking at one of the other versions of the "3020". Sorry for confusion.
Sorry q602 12v on E
Q601 0.5v on E
Q602 is dead.
But my part tester not recognise .
I will replace this transitor.
Q601 0.5v on E
Q602 is dead.
But my part tester not recognise .
I will replace this transitor.
Good news
To be clear 602 is designated as 2sc1400 on the schematic you linked. Are you saying it didn’t match schematic, previously replaced or you meant Q604 was the dead transistor? Want to be clear so in the future anyone reading this will understand your findings
To be clear 602 is designated as 2sc1400 on the schematic you linked. Are you saying it didn’t match schematic, previously replaced or you meant Q604 was the dead transistor? Want to be clear so in the future anyone reading this will understand your findings
Excuse me...
Service manual is not good in first post.
Really Q2 is BC549C.
My 3020 is with 4 big caps in center
Service manual is not good in first post.
Really Q2 is BC549C.
My 3020 is with 4 big caps in center
Glad you fixed the circuit.
FYI: I'm looking at a 3020B/3120 service manual (from hifiengine). This one seems to match your amp....just for future reference.