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?
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.
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.
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.
Remove all connected components and any grounds including the shield if you're using a 75 ohm antenna. If the hum goes away reconnect each component noting which one causes the hum to return.
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.
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.
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.
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.
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.
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.
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.
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.
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