NAD 3020 Recap

[davidyjl]

I just replaced all electrolytic capacitors in my NAD 3020. When I turn on the power .R644 was burnt. I replaced new R644 .It was burnt again.Anybody has idea to solve this problem. Thanks

 

[sgrossklass]

A total recap tends to be a good opportunity to:
1. introduce solder bridges
2. solder caps in backwards, especially so when markings on silkscreen are wrong and you're not paying attention to polarity upon unsoldering

So this is the original plain vanilla 3020, right? Then I would check C626. Its - end must go to R638, + to R636. No solder bridges, obviously.

Oh, and you may want to ask a mod to move this to Solid State, the trusty 3020 obviously isn't a chip amp.

 

[davidyjl]

Thanks for your instruction. I checked all caps I soldered. They are all right. and there are no solder bridge. Is there something that can cause R644 burnt?

 

[Mooly]

Oh, and you may want to ask a mod to move this to Solid State, the trusty 3020 obviously isn't a chip amp.

Done

 

[davidyjl]

Thanks for your instructions. I checked all caps that I replaced according to the Schematics. They are all right. and there are no obvious solder bridge. Is there something else that cause R644 burnt?

 

[Mooly]

If it was working before and isn't now then it has to be something you have done.

Does R644 burn with no speakers connected ? If it does then you need to follow a methodical procedure... use a bulb tester, link the vbe multiplier out to force the output stage to zero bias conditions and see what the measured voltages show.

Its quite likely Q606/8 could be damaged by all this (and possibly the drivers too. You need to find the cause.

 

[davidyjl]

Yes .It was working before I replaced capacitors.I unsoldered R644 . Then I measured voltage as shown in the schematics.all the parts I measured are good.
View image: nad 3020

 

[Mooly]

That resistor is the feed to the driver stages. To fault find on this means you using a bulb tester (to prevent destructive damage) and careful and methodical fault finding. You are probably looking at damaged driver transistors and possibly failed output devices as well but the underlying cause has to be found first.