Nikko NR-1015 Makes a popping noise when powering off?

[labjr]

The owner brought in a Nikko NR-1015 receiver which makes a popping noise when powering off. When powered down, a negative voltage on the output almost instantly jumps up to over -5V. Then goes away afer a few seconds as the main filter caps are depleted. The protection relay cuts out pretty quickly. Seems about normal to me but evidently not fast enough to stop the pop from getting to the speakers. Though, I can still hear it faintly. When powered, on the circuit seems to reach equilibrium before the protection relay turns on.

Most of it goes away if I pull the pre-out, main-in jumpers. And I can measure the same behavior at the pre-out. So it's coming from the preamp circuit. And it happens on both channels. So not sure what's causing it. Or if this was normal by design as I don't see any case histories. I actually don't see much about this model at all.

The full service manual is in the AK data base.

https://akdatabase.org/AKview/albums/userpics/10007/Nikko%20NR-1015%20Service%20Manual.pdf

 

[painttoad]

both of these are on the shoot on site list( same in other channel)



as are these in the amps



you'll want to investigate exactly what transistors are in the unit. the schematic is just a guide as design changes happen and don't get printed and there is also no way to know if someone else has been inside and made changes until you look inside.

Top Ten Worst Transistors - noisy, failure-prone, whatever... and replacements

I've followed AK advice for several vintage solid state receivers now and the difference in sound, from recapping and from replacing "known-to-be-noisy" small signal transistors to be amazing. While following along with others' threads and trying to help out with "consider replacing these...

audiokarma.org

The protection relay cuts out pretty quickly.

then i'd be looking at the protect circuit. look for transistors on that list. check diodes and caps.

i didn't know nikko used powerpacks in this unit until now.

 

[labjr]

Thanks for your reply. The offset voltage is coming from the tone board. I changed all the electrolytics on the tone board. Still the same. I don't have the parts for Q506-508 and don't know where to find them? I could change Q502-504 but why would both channels have the same problem with DC offset on power down?

From what I've measured, the negative rail drains slower than the positive rail. Can that cause the DC offset when turning off?