On Mon, 4 Feb 2008 21:23:17 -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:

http://home.everestkc.net/upperquad/...rmonKardon.jpg

Thanks
John

Varying with the volume says the source of hum is before the volume
control. If you have inputs connected you can quickly eliminate a
ground or ground loop problem by unplugging the inputs, and shorting
the signal to ground connections right at the RCA plugs.

If that gets rid of it you look for a grounding problem between the
amp and signal source.

If that isn't the situation, the gray caps are the likely culprits and
I'd just replace them. They don't cost an arm and leg and it is an
old set and caps will lose some electrolyte over time reducing
capacity and increasing hum.

Inspect the seals on the caps - the end where the plus lead goes into
the caps - and hint of corrosion or a bulge (pimple) in the end seal
is a sign that cap is bad - but they can fail with no visual clues
too.

The large vertical cap is the main filter cap and you should look at
the bottom to see if there's leakage. Generally, the main filter
cap(s) going out will cause hum at all volume settings - but not
always.

The 2,200 uf / 35 volt caps are power supply filters also, from the
looks of it. The 470 by the 2,200's looks like it may be part of a
filter for a regulator so that one is suspect. The two other 470's I
can't tell without a schematic - they are either filters for the pre
amp board or speaker coupling caps. I'd guess coupling caps.

With great care and a little tape so you don't become part of a
circuit, you can just hand hold a cap and bridge the ones in there
with a new one (470/35 or so) the hum will be eliminated or decreased
when they are bridged. Discharge against some metal between checks.
A smaller capacity cap (same or higher voltage) than the one you are
testing is desirable - since it will drag down the supply momentarily
when it first makes contact and that may not be good for the output
stages.

or you can connect it backwards and cause smoke and fire or have it
blow up in your fingers - repair people do it all the time, but you
have to be comfortable with the idea. Dry hands, loose grip, and no
cuts and a 35 volt supply shouldn't shock.

Or just replace those three gray caps on the left and hope for the
best.


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