"Don Bowey" wrote in message
...
On 2/5/08 1:23 AM, in article ,
"Arfa
Daily" wrote:


"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.

Not necessarily, and certainly not a front end hum entry problem. If it
is
a general power supply filtering problem (most likely) the hum sound can
readily be altered by varying the volume control due to hum getting into
the
preamp.

Well, a matter of semantics here. This problem is obviously not an input
cabling problem, as it is not confined to one input. I was merely using that
as an illustration of what hum that is varied by the volume control is most
usually as a result of, and why in this case, it was slightly odd that it
was on all inputs and channels.

When I say "front end", I am referring to anything 'North' of the volume
control, which in an amp of this age, is a mechanical control located
between the 'front end' which includes all the preamp, input selection and
tone control sections, and the power amps. Thus, if it is altered by the
volume control, then the hum is getting in 'front end' as opposed to being
on the main supply rails which feed the output stages. Depending on whether
the preamp rails are produced from their own secondary winding on the
transformer, with their own rectifier and filter caps, or are regulated down
from the main output stage rails, will determine how "most likely" the
problem is one of caps. The '330, for instance, uses only a single winding,
rectifier, and main smoothing, from which other rails are then derived. I
don't know what the situation is on the '230. Certainly, on the photo, I can
only see one winding coming round to where all the above-chassis caps are,
but that's not to say that there are not other windings going off to
circuitry under the chassis, where we can't see. If it is a multi-winding
transformer, then I agree that there is a distinct possibility that there
may be hum just on the supply rails to the preamps.


Arfa