Receiver has a hum - Harmon Kardon 230A

[JohnC]

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/HarmonKardon/HarmonKardon.jpg

Thanks
John

 

[CJT]

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/HarmonKardon/HarmonKardon.jpg

Thanks
John

Those big grey capacitors near the power transformer are likely candidates.

 

[Arfa Daily]

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/HarmonKardon/HarmonKardon.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

 

[Mark D. Zacharias]

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.

 

[default]

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/HarmonKardon/HarmonKardon.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.


--

 

[Arfa Daily]

One point hasn't been brought up.

If one or more of the caps are bad, then it's likely you'll hear hum even
with the volume control all the way down. If there is no hum with the
control the way down, the problem is probably elsewhere.

Regardless, it wouldn't be a bad idea to simply replace all the PS
electrolytics in a 35-year old receiver.

I'm not a great advocate of the 'shotgun' approach with this type of
equipment. A lot of times, a person who has little electronics skill, as
this poster told us he didn't, can cause himself more trouble by just
replacing items 'willy-nilly'. It's very easy to make a wiring or polarity
mistake. And once he has, he will be in even worse trouble than before, as
he now has both the original problem, and the one he's caused himself ...

I did actually say in my first post in this thread, that hum as a result of
a power supply problem - for example a bad cap - will not usually be
affected by the volume control, so by implication, will still be there with
the volume at minimum.

Arfa

 

[T]

One point hasn't been brought up.

If one or more of the caps are bad, then it's likely you'll hear hum even
with the volume control all the way down. If there is no hum with the
control the way down, the problem is probably elsewhere.

Regardless, it wouldn't be a bad idea to simply replace all the PS
electrolytics in a 35-year old receiver.

Reading all this I'd say a basic overhaul is in order. Replace the
electrolytic caps, the voltage regulator and the volume control pot and
it'll probably last another 35 years. Of course by then some vulture
will snap up the FM bands.

 

[Arfa Daily]

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

 

[CJT]

I'm not a great advocate of the 'shotgun' approach with this type of
equipment. A lot of times, a person who has little electronics skill, as
this poster told us he didn't, can cause himself more trouble by just
replacing items 'willy-nilly'. It's very easy to make a wiring or polarity
mistake. And once he has, he will be in even worse trouble than before, as
he now has both the original problem, and the one he's caused himself ...

I did actually say in my first post in this thread, that hum as a result of
a power supply problem - for example a bad cap - will not usually be
affected by the volume control, so by implication, will still be there with
the volume at minimum.

Arfa

AFAICT, he never said there was no hum with the volume down -- just that
it varied with the volume control.

 

[Arfa Daily]

Yes, you did say that. But one might reasonably assume that the PS hum
ahead
of the volume control would add to the hum following the volume control.

As Don has said, it only works as a power supply problem, if the preamps
have their own supply rails, derived from their own transformer winding,
rectifier, and filter cap(s), in which case, a failed cap, or a leaky diode,
will place ripple on the preamp supplies, giving an audible hum which will
completely disappear, when the volume is turned to minimum. If the preamp
rails are derived from the main power amp rails, as is quite often the case,
then any cap or diode failure that produced ripple on the main rails, would
cause the audible hum to be present even with the volume at minimum.
Downstream decoupling on the preamp rails, would likely remove most, if not
all of the ripple from those rails, so when the volume was turned up, there
would be no additional amplified hum contribution from the preamps.

Arfa

 

[Arfa Daily]

AFAICT, he never said there was no hum with the volume down -- just that
it varied with the volume control.

Ah ! Quite right, he did. Very likely the hum does go away completely with
the vol turned right down, but having read it again with the possibility
that it doesn't in mind, that makes a difference to my thinking, and makes
what William said more of a possibility. Regardless, either way, this should
not be a difficult problem to locate, but might be a shade too 'electronic'
for the OP to diagnose on his own, with the limited skills that he tells us
that he has in the field.

Arfa

 

[jakdedert]

On 2/5/08 6:15 PM, in article [email protected], "Arfa

(snip)


That wasn't my point.

My point was that even if there is a faulty DC supply common to both the
amplifier and the preamp (typical low-end), changes in the volume control
setting will still be heard as an amplitude or character sound change in the
hum from the speaker.

My issue is that--all too often--someone drops a problem in here, for
which there is a pretty standard, simple decision-tree diagnosis
process. Then we're left to dangle for days, speculating on possible
solutions, while the OP is totally silent.

Until he comes back, all is speculation.

jak

 

[Arfa Daily]

Well, you could have a ground loop problem with the cable TV system, but
that's not the sort of thing that would _suddenly_ show up.

By the way, if you have such a problem, you can get rid of it by
connecting
two true baluns back-to-back and inserting them in the cable line. There
are
speciality isolation transformers for this problem, but they're pricey,
and
all you gain is a bit less insertion loss.



Is that true? I haven't looked at any schematics recently, but my memory
is
that volume controls are not always at the "end" of the input chain.

Pretty much all the ones I've seen, that's the 'standard' arrangement.

Arfa

 

[Arfa Daily]

On 2/5/08 6:15 PM, in article [email protected],
"Arfa

(snip)


That wasn't my point.

My point was that even if there is a faulty DC supply common to both the
amplifier and the preamp (typical low-end), changes in the volume control
setting will still be heard as an amplitude or character sound change in
the
hum from the speaker.

Maybe. But by no means guaranteed. Low(er) rails derived from the main
output rails, will usually have their own decoupling caps, and if they are
derived via monolithic regs of the 78 / 79xx series, then the typical ripple
rejection of some 80dB that these devices have, will pretty much knock any
ripple on the head, before you even start counting in downstream decoupling
caps. Thus, my point, that you could have a DC fault giving rise to dirty
'raw' rails fed directly to the power amps, whilst still maintaining clean
low rails to the preamps, in which case, the volume control will have little
or no effect on the hum.

Anyway, this is just becoming a worthless discussion of perhaps's, maybe's
and diagnostic experience, as Jak suggests below. I can only contribute
suggestions based on what 30 odd years of repairing this stuff on a daily
basis, has taught me about faults like this.

Arfa

 

[JohnC]

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

 

[JohnC]

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.

I sprayed cleaner in the volume control, hum still present. Something
else.. the hum is audible no matter what input is selected from the input
switch, FM, AM, Phono, tape/aux. But there is a Monitor switch and when
switched from Source to Monitor, the hum ceases.

John C.

 

[Mark D. Zacharias]

I sprayed cleaner in the volume control, hum still present. Something
else.. the hum is audible no matter what input is selected from the input
switch, FM, AM, Phono, tape/aux. But there is a Monitor switch and when
switched from Source to Monitor, the hum ceases.

John C.

That is odd... maybe a wiring or corrosion problem at the rear of the unit.

I looked at work - no service manual on a 230A, just a 230E which is
COMPLETELY different.


Mark Z.

 

[Tim Schwartz]

***Major SNIP***

I sprayed cleaner in the volume control, hum still present. Something
else.. the hum is audible no matter what input is selected from the input
switch, FM, AM, Phono, tape/aux. But there is a Monitor switch and when
switched from Source to Monitor, the hum ceases.

John C.

John,

Have you tried feeding a source into the monitor input and see if the
unit sounds OK? I'm guessing it will. While I don't know the HK-230A,
I have seen receivers that the tape monitor switch feeds the volume
control directly (or nearly so) eliminating the the tone control amps.

I strongly suspect that you have a low level (regulated) power supply
failure that is causing the hum. Looking at the power supplies that
feed the preamp circuits with an oscilloscope is going to be the hot
ticket here.

Tim Schwartz
Bristol Electronics

 

[Wayne Tiffany]

John,

Have you tried feeding a source into the monitor input and see if the unit
sounds OK? I'm guessing it will. While I don't know the HK-230A, I have
seen receivers that the tape monitor switch feeds the volume control
directly (or nearly so) eliminating the the tone control amps.

I strongly suspect that you have a low level (regulated) power supply
failure that is causing the hum. Looking at the power supplies that feed
the preamp circuits with an oscilloscope is going to be the hot ticket
here.

Tim Schwartz
Bristol Electronics

Wouldn't checking ESR on the power supply caps be the first thing for a unit
of that age? I have found that to generally be the issue on the old stuff I
have worked on.

WT

 

[JohnC]

John,

Have you tried feeding a source into the monitor input and see if the unit
sounds OK? I'm guessing it will. While I don't know the HK-230A, I have
seen receivers that the tape monitor switch feeds the volume control
directly (or nearly so) eliminating the the tone control amps.

I strongly suspect that you have a low level (regulated) power supply
failure that is causing the hum. Looking at the power supplies that feed
the preamp circuits with an oscilloscope is going to be the hot ticket
here.

Tim Schwartz
Bristol Electronics

Tim,
Yes, when I plugged a source into the monitor input there is no hum. All
controls, bass, treble, balance, volume work for the monitor source.
Another item of interest- one of the channels begins to cut out after about
half a minute while using the monitor input. I can unplug the RCA jacks and
reinsert and both channels come back, only to have one of the channels drop
out again after about half a minute.

John