On Sat, 16 Feb 2008 11:15:57 -0600, "JohnC"
wrote:


My Harmon Kardon receiver recently started to hum. It does it
regardless
of which input is selected. The hum varies with the volume control. I
opened the unit and tapped on various components with a wood stick. This
had no affect on the hum.

This is an old unit that I have owned new since early 70s. Perhaps this
might not be worth getting serviced by a shop? I'm good with tools but
lack electronics skills. No scope either. What could be the cause of the
hum? Is this something that is fairly easy to replace/repair?

Here is a pic of the circuit board:


I'd try jumping the 3 470uf and the 1000uf capacitors with new good
caps. If this doesn't work, I'd sub the electrolytic caps on that
stand up board. If you have an esr meter, it would speed up the
process. Chuck
http://home.everestkc.net/upperquad/...rmonKardon.jpg

Thanks
John


That's actually a slightly odd set of symptoms. The fact that it is
altered by the volume control, indicates that the hum is getting in
'front end', which is usually down to a grounding issue on the input
signal cable or whatever. However, that being the case, the hum would not
normally be expected to be present on all inputs (and presumably on both
channels ?). OTOH, a power supply fault that is causing hum, will not
usually be affected by the volume control.

I suppose it is possible that one of the electrolytics in the power
supply is getting poor - it is certainly old enough to have electro
trouble - but it is my experience that these older types rarely fail,
especially if they *have* been going for 30 odd years without trouble. It
would, I think though, still be my first move to check them either for
ESR, or better, for ripple present on them. If you don't have a 'scope,
you could get an idea of the ripple level by measuring the voltage across
them, with the meter set to an AC volts range. You should struggle to get
any reading much above zero. After that, I would be looking at a bad
regulator transistor, such as that one at the left hand side, bolted down
under the transformer. I've known rails that have gone high from a short
or leaky regulator, cause such disturbance to preamps.

Arfa


Or a ground loop - is cable TV, outside antenna or a computer connected to
the HK in any way?

Mark Z.


Thank you all for the responses. I apologize for leaving the post hanging.
I have been out of town and unable to post a reply.



Here are some items that I have done along with some additional information:





1. The unit is not hooked up to a cable TV, outside antennae or computer.
It is only hooked up to indoor rabbit ears antennae, tuner, and speakers. I
do not suspect a ground loop.



2. I disconnected everything, antennae, speakers, and tuner. Listening
through headphones the hum is still present.



3. I shorted each RCA input to ground. Hum still present.



4. The hum is inaudible with the volume knob turned all the way down. The
hum is present on both channels. The hum showed up suddenly.



5. I removed the case and bottom and inspected all grey capacitors. There
is no hint of corrosion or a bulge in the end seals.



6. I failed to mention in my original post that for years the volume (pot?)
emitted audible static when the volume was adjusted. Somebody mentioned
that this should probably be replaced. Probably long overdue for
replacement and perhaps the source of the new pesky hum?



John