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NAD 7020 repair

ViperZ

ViperZ

Well-Known Member
Hi all,

I have this old trusty 7020 in the shop. A couple of months ago the power switch failed, so I replaced it. It worked fine, then I didn't use it for a few weeks. Now it makes this strange noise. Continuous ticking through both channels on all line level inputs in background (radio still works, relays still click). On phono, the frequency is higher and it's more loud.

Here is a shot of the problem:

Sounds like a cap or resistor failure in the feedback circuit. Has anyone experienced this problem before? Thank you!
 
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Well, found the cause of the two problems in the video... First oscillations were related to leaked out electrolytic capacitors, changed a bunch, which were out of spec. Phono was randomly starting to oscillate on the left channel - recapped it, cleaned all connectors and the switches, works now...

Hopefully, last problem on this unit, which I can't figure out yet. Speakers A work great, Speakers B work great. When the switch is in Speakers A+B position, only Speakers B are on, there is absolutely no signal coming to Speakers A. Looking at the schematic, this A+B combining is just in this speakers selector switch... I soaked the switch with contact cleaner, still nothing... Any suggestions? Should I disassemble the switch? Thanks.
 
Might be a later run factory mod, preventing two sets of speakers connected at the same time. It's sort of a common cause of amplifier destruction. As in, the most common cause.
 
Definitely a bad speakers switch. I really don't want to take it out and take it apart. I soaked it in the contact cleaner. Now if it's on the side, A, B and A+B work great. If I put it down, A and A+B get distorted. Gotta figure out how to get rid of the dislodged crud :(
 
Yeah, had to soak it a few times, then kept working it, then ran all four speakers at high power for a couple of hours. Looks like it's finally working now. Yay.
 
One thing to remember about these old NADs... recap the whole damn thing :( Now the FM Mute function is gone, when the selector is in FM Mute position, FM is playing just fine. Going to recap the FM tuner daughter board tomorrow night. I saw a bulging cap on it.
 
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