Hum from all inputs HK Nocturne Seven Twenty

[omegat3]

I have a (found) Harman Kardon Nocturne Seven Twenty that powers up but has quite the hum on all inputs. I opened it up for a quick look to see if anything had swelled or was leaking, but I didn't see anything obvious. I am not an electrical engineer by any stretch of the imagination, but have repaired iPhones, ps3, etc, and am willing to try if someone could point me in the right direction of what to look for, test, and possibly replace.

If nothing else, a recommendation for someone who can take a look at it in the Cleveland, OH area would be great.

Thank you in advance!

 

[dlucy]

A good start is to register yourself (for free) on hifiengine.com, look up the HK 720, and download the service manual for it. That particular one isn't very comprehensive, but it does have a schematic... which you'll find useful.

 

[dlucy]

That HK is old enough that most or all of the electrolytic caps are going bad or have gone bad. This could certainly lead to hum, but it is not the only cause.

There is a thread on a similar-era, similar-design, same-vintage HK receiver that might have good info for you: https://www.audiokarma.org/forums/i...-old-harmon-kardon.215485/page-2#post-2584755

You could start with just replacing the electrolytic caps in the power supply or rectifier section. That would give you some experience reading a schematic (of this vintage), making a list of needed caps, searching at Mouser.Com or DigKey.com for suitable replacements, asking us for advice on what to do when a needed cap is not available or the replacement costs more than $2, desoldering the old caps, soldering in the new caps, and listening to / measuring the result.