"JohnC" wrote in message
.. .

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:

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





Someone may have already suggested this, but do look at the output of any
voltage regulators. Could be a dried up capacitor near one of them. I just
looked over at the Harman dealer site - no service data there. I'll see if I
have a manual at work and post back.


Mark Z.