Hum in NAD 7020

majerjack · 6th December 2010, 04:07 PM

Someone gave me a 25-year-old NAD 7020 receiver that has a loud hum and no signal in one channel's output. When I bypass the output amplifier and take signal off the preamp to a separate amplifier I get both channels, so I know the problem is somewhere in the output amplification stage.

I have the service manual and schematics, and I intend to completely recap the unit. I know that hum of the kind I am hearing is a classic symptom of a bad capacitor, but how likely is it that it could be one of the output transistors that has gone bad?

Replies from anyone who has experience with this model will be greatly appreciated, and if anyone else can give me an educated guess, I would appreciate that also. Thanks to all who respond.

Tekko · 6th December 2010, 04:11 PM

There could be electrolytic capacitors going bad.

gmphadte · 7th December 2010, 08:04 AM

If it was kept for years without powering, then u have to change the power supply filter caps. If u r recaping, then the hum problem, most probably will go in the process.

6th December 2010, 04:07 PM	#1
majerjack diyAudio Member Join Date: Feb 2010	Hum in NAD 7020 Someone gave me a 25-year-old NAD 7020 receiver that has a loud hum and no signal in one channel's output. When I bypass the output amplifier and take signal off the preamp to a separate amplifier I get both channels, so I know the problem is somewhere in the output amplification stage. I have the service manual and schematics, and I intend to completely recap the unit. I know that hum of the kind I am hearing is a classic symptom of a bad capacitor, but how likely is it that it could be one of the output transistors that has gone bad? Replies from anyone who has experience with this model will be greatly appreciated, and if anyone else can give me an educated guess, I would appreciate that also. Thanks to all who respond.

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6th December 2010, 04:11 PM	#2
Tekko diyAudio Member Join Date: Jan 2005	There could be electrolytic capacitors going bad.

7th December 2010, 08:04 AM	#3
gmphadte diyAudio Member Join Date: Jul 2003 Location: Dona paula, Goa	If it was kept for years without powering, then u have to change the power supply filter caps. If u r recaping, then the hum problem, most probably will go in the process.

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