NAD 3020i DC offset and bias adjust query

[ssi]

Hi


Always wanted to try a NAD 3020 and the "original" is awalys quite pricey so I went for a later 3020i in good condition. All good but the tinkerer in me wanted to set the bias.


You can adjust the DC at the speaker terminals using the provided adjustment pots so I adjusted them down to 3mv from 17mv per channel. Could go further but decided not to as the range is within 30mv of 0v.


Bias adjust is more complicated and obviously not encouraged. You have to remove a short from a 0.1 ohm resistor before adjusting the pots. The short is a massive amount of solder and I cannot reliably get it all off using braid and a sucker. I have decided to leave it as I could cause more problems than I solve. I did not adjust the bias at all and simply re added the solder blob. The amp sounds good as it is!



Am I ok leaving the DC as it is or should I put it back to 17mv as the service manual asks you to set the DC and then the Bias?

 

[JonSnell Electronic]

Keep the offset as low a value as possible. I would check the bias. Use a soldering iron with plenty of flux on the tip. It is easy to remove the link. Set the bias and then replace the 'blob'.

 

[62vauxhall]

I have an el cheapo solder sucker and had better results with it than desoldering braid.

That was until I discovered the flux pen.

Wiping the pen along a length of braid made it draw solder very effectively. The "sucker" has not been used since. One job was also removing a solder bridge in a NAD. While it used up maybe 2" of braid per blob, it worked well.

 

[ssi]

OK just did not understand exactly where the solder bridge was. Found it and it is fairly straight forward. Hard to work out where the join is if you don't know! Bias was around 40mv so reduced it to 30mv at 8ohms. DC ofset is around 0 + - 0.5 mv

 

[jaycee]

Just remember if you ever have to replace the power transistors in this amplifier, it will require modification to use modern 2955/3055 devices

 

[avtech]

will require modification to use modern 2955/3055 devices

Hi Jaycee


I wonder if you can elaborate on what is required?


I changed out a pair of 3055 with NOS but used brand new 2955s..


The amp biased no problem and seems to be happy but I'd be interested to learn what I need to do.


Cheers

 

[jaycee]

For the newer "epitaxial" type transistors (basically anything produced recently) you should add 0.33R emitter resistors to the amplifier to avoid thermal runaway issues

 

[Ian Finch]

Hometaxial TO3s were fitted to early series models but modern versions of 2N3055/MJ2955 are epitaxial. The bias levels required are not the same either. It could be that someone had already been in and replaced the powers transistors (not unusual) and amended the biasing or the amplifier is a more recent version that necessitated the design change. Often, the appearance of T03s with odd brands like Toshiba and National is a sure sign of specially made hometaxials. NAD used el-cheapo components and construction, so original manufacturers like Motorola or RCA would have been unlikely contenders.

 

[avtech]

I believe the NOS was genuine Motorola so I'll probably need to stabilize those.


Just for clarity.. the emitter resistors go on both the 3055 and the 2955? I understand these are for thermal stability.


I've also read talk about putting a 4R7 1/2W resistor into the base as well. I can't find a coherent source as to the purpose, the information is all scattered across the interweb. Any ideas on that one?


I was going to pull this amp apart again and recap it now that I had repaired the damage the previous owner had inflicted (components not soldered to the pads, tracks blown off, outputs blown, AC short circuited to the case..etc..). I might as well get the outputs stable and happy also.

 

[avtech]

Found some information on another thread.


PDF by 'Mightdub'.