NAD 3020 drivers

[wuffwaff]

Hi,

I´m trying to repair an old NAD3020. One channnel has lost it´s output transistors (2N3055 /MJ2955) and one driver 2N6551.

The output transistors are easy to get but I can´t find any suppliers for the 2N6551 or 2N6553.

Since it seems to be asimple 60V / 1A device there must be quite some parts that are suitable as a replacement.

Any suggestions what could be used as a driver pair instead of the 2N6551/6554?

William

 

[lineup]

2N6551/6554 are 75 MHz TO-202 transistors, max 10 Watt

This is very much like BD139/140.
They should be a perfect replacement.

2N6551 2N6554 Datasheet PDF

 

[Eva]

BD139 and BD140 should be fine. Dissipation may be an issue because TO-202 have a small built-in heatsink, so bolting some small metal sheets to the backs of BD139/140 is advised.

 

[Tube_Dude]

In the last incarnations of the 3020 with the suffix I (improved ?? ) , they already use the BD139 / 138 as drivers...

 

[Giaime]

Nah, mine is one of the oldest (champagne faceplate with "series 20" logo) and already has the BD139/BD140. My thoughts are that NAD put in what they had in the parts bin

It's a beautiful amp, now it's my main amp (yes, I'm serious).

 

[lineup]

Nah, mine is one of the oldest (champagne faceplate with "series 20" logo) and already has the BD139/BD140. My thoughts are that NAD put in what they had in the parts bin

It's a beautiful amp, now it's my main amp (yes, I'm serious).

Yes, Giame.
I found you tell about your NAD3020 in your homepage.

An externally hosted image should be here but it was not working when we last tested it.

Quote:
Here's my lovely amp, the "highly sought after" NAD3020, the very first series (with the "Series 20" logo).
This is one of the only photos I've been able to find on the Net for the "champagne" version.


In the The Class-A Amplifier Site
there is NAD 3020 schematic

An externally hosted image should be here but it was not working when we last tested it.

 

[wuffwaff]

Hi,

thanks! I will try them and also change the pair from the other channel.

Do the newer ones with BD139/140 have little heatsinks or not?

William

 

[Tube_Dude]

Do the newer ones with BD139/140 have little heatsinks or not?

No, they don't use heatsinks ,certainly because the rails are only
+- 28 volts.

But it's not a bad idea to use a small one... just in case...

 

[wuffwaff]

Hi,

I´ve replaced the transistors and would like to see if the amp is working.
Is there anything that needs to be set like bias or so? I can only see VR5 wich feeds power supply voltage to the input. Is this for offset adjusting?

William

 

[Tube_Dude]

I can only see VR5 wich feeds power supply voltage to the input. Is this for offset adjusting?

Yes, VR5 is for offset adjusting...


The BD 139/140 see from the front and the pins in your direction :

| | |
e c b

 

[wuffwaff]

Hi,

thanks, I already noted the different pin layout on the BD139/140.

If there´s no bias to set I will plug it in and see what happens.......


William

 

[Tube_Dude]

If there´s no bias to set I will plug it in and see what happens.......

Pray a short prayer...

 

[Geoff]

Is there anything that needs to be set like bias or so? I can only see VR5 wich feeds power supply voltage to the input. Is this for offset adjusting?

Email me and I will send you a scan from the original service manual for the setting-up procedure.

 

[Geoff]

If there´s no bias to set I will plug it in and see what happens.......

Yes, there is a bias adjustment! See my last post.

 

[wuffwaff]

Hi,

it seems to be working well now. Lacking the resistors to measure the bias current (they are bridged). I compared the temperature of the new and old transistors. All are almost the same at 28-29°C (wich is quite a bit lower than the 70-80°C I´m used to)

DC offset is below 8mV on both channels and even the power meter is giving the right values.

Thank you all for your help!

William

 

[jjanderson]

I have the same problem an i have replased the bd 139 and bd 140 and also the 2n3055 several times but then the amp plays 5 minutes and the BDs get faulty again!
I have the nad 3020i i dont know if that make a difference.
I have no idea why the drievers get lost every time everything seems to be ok the resistors....
Anyone have a idea?

 

[djk]

Cut each trace going to the base of the outrput transistors, insert a 4R7 1/2W. Adjust bias.

 

[Tube_Dude]

I have the same problem an i have replased the bd 139 and bd 140 and also the 2n3055 several times but then the amp plays 5 minutes and the BDs get faulty again!
I have the nad 3020i i dont know if that make a difference.
I have no idea why the drievers get lost every time everything seems to be ok the resistors....
Anyone have a idea?

Inspect R651 out of circuit, must be 180 Ohms.

Inspect Q609 BD139, that is mounted in the heatsink, if is not burned and if is isolated from the heatsink.

Inspect if the case of the 2N3055/MJ2955 is well isolated electrically from the heatsink.

Good luck ...

 

[lineup]

Inspect R651 out of circuit, must be 180 Ohms.

Inspect Q609 BD139, that is mounted in the heatsink, if is not burned and if is isolated from the heatsink.

Inspect if the case of the 2N3055/MJ2955 is well isolated electrically from the heatsink.

good advices, Tube_Dude
for a classical audio history amplifier
by Bjorn Erik Edvardsen, Norway = BEE

thanks very much
Lineup of lineup audio labs
http://lineupaudio.freehostia.com/

 

[djk]

Because the outputs have no emitter resistors, the drivers go into thermal runaway and take everything with them.

Add the base resistors as I suggested.

(former NAD authorized service)