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wkjagt 24th March 2007 10:39 PM

humming Harman Kardon 330B
Hello, my first post here. I am kind of novice to the subject, so forgive me if this is a too simple question ;)

I have an old Harman Kardon 330B tuner/amplifier. The right channel is ok, but the left channel is not. I'll give the symptoms and hope someone will know what's wrong.

- On the left channel is always a constant hmmmmm sound (sorry, don't know the English term ;) )
- That sound doesn`t change in volume when the volume of the amplifier is changed and doesn't disappear when the volume is at minimum.
- The sound doesn't dissapear when a different source (phono/aux etc.) is selected.
- The left channel does function, but less loud than the right.
- The sounds keeps playing for a second or so when the amplifier is turned of and fades away. That is not unusual I think, but when that happens, it's without the hmmmm sound. Strange?

I hope someone will recognize this problem. Any help would be greatly appreciated. When this amplifier is repaired, and the world of amplifiers is slowly giving away it's hidden secrets to me, it would me nice to try some modifications for this amplifier. I have heard it's quite a nice machine for that.

XEAGLEKEEPER 24th March 2007 11:24 PM

Look the left channel over real good you could have a bad ground connection on the left channel. Or possibly a dried up bypassing capacitor.cracked PC board.If you find questionable solder connections just reflow the solder connection. If you find corrosion this will cause bad solder connections/broken ground connections.:)
Look the power supply capacitors over may have a leaky one.Allowing AC hum to bleed through.;)

Check and see if you have any DC on the Left output may have a bad output transistor or two.


wkjagt 26th March 2007 06:54 PM

Hi Dave!

Thanks for that! I will have been looking really close and noticed that the two biggest caps (one left and one right) both have some sticky substance under them which is kind of spilling on other components too. Guess that' not supposed to be there ;) Those will be replaced and I'll try to take that substance off, because it might be a conducting substance, shorting other parts too?


XEAGLEKEEPER 26th March 2007 11:11 PM

Sometimes manufacturers apply "Sony Bond" glue to those large components.This tends to trap moisture and promote corrosion on surrounding components or the Large power supply capacitors may actually be leaking electrolyte usually the caps are swelled up or ruptured when this happens.But if its a old piece of equipment they are probably due/overdue replacement. You could go larger on the microfarads but don't go overboard.The minumum voltage should be whatever is presently in there.If you can get 105ºthe better, but 85º will suffice. The fact that the hum dissappears when the power switch is shut off could verywell be those caps.If you have anymore questions -just yell.

:) :) :)


wkjagt 27th March 2007 04:00 AM

Hi Dave,

It is indeed an old piece of equipment. Probably about thirty years old. All the little lights in there have stopped working too, so no way to tell which station I'm listening too ;-) It's supposed to have a nice dark green scale. I'm looking forward to seeing that when I replace those lights.

When I put bigger caps in, would that effect the sound in any way? Or is it just a "safety issue"?

O yes, something else. I have noticed that two of the resistors in there look really bad. Bursted open would be my best description. The shells are mostly gone but partly still stuck to them, which makes recognition of the color codes very difficult (but not impossible.) They are both right next to a big capacitor. But both in completely different places of the amplifier. Coincidence?

JasonLee 27th March 2007 05:24 AM

wajaqt, Think Dave is right,
One or more transistor at out put stage may have failed. It dose not need to be the final out put transistor pair, could be the drivers transistor as well. It's out put stage is OTL & not OCL. So there

XEAGLEKEEPER 27th March 2007 05:42 AM

Just checked out ebay on a 300b it indeed is a fine looking reciever when its working properly.If the caps are not the whole problem.Its probably time for a schematic or service manual.Are you sure the destroyed components are resistors?


JasonLee 27th March 2007 07:50 AM

Sorry, I just post before i complete.. those resister in power amp section will burnt & break into two parts if output / driver transistor shorted. Think it is not easy to get the schematic. But, you can alway trace & draw the schematic of the power amp section. Take a look on the working channel. You will find the value of the burnt resister.
I strongly believe that the hummimg sound dose not come from power supply nor earth loop. It's because DC offset at the pushpull output. Causing the leftt channel drawing high current. That is why left channel Hmmmmm & softer then right channel.

unclejed613 27th March 2007 08:37 AM

could be an output transistor going into strong conduction...... that's characteristic of the MMMMMMMMMMMM sound....... one thing that can cause that is a bad DC feedback cap in the diff amp of the power amp.... you start out with sound, and slowly (over the period of a second to a few minutes, depending on just how bad it is) going to dc on the output, or if the output is coupled through a large cap, no sound except power supply ripple (in which case, you will find full rail voltage on the amp side of the cap, instead of half of the rail voltage).
first, follow the speaker wire back to the amp board. if it first goes to a large cap, then measure the dc voltage on the amp side of the cap. it should be about half of the power supply rail voltage (this type of amp has only one rail voltage, usually positive). if it is full rail voltage, then the amp has an offset problem, most likely caused by a bad DC feedback cap, which will be an electrolytic near or in the input stage. this cap can either be a polarized cap (common) of about 50 to 300uf, and rated for about twice the rail voltage. or a nonpolarized cap rated at about 20% more than the rail voltage.

if the speaker line goes back directly to the output stage emitter resistors, then the amp is direct coupled to the speaker (this type of amp has two rail voltages, one positive, and one negative), and what can go wrong here is DC on the output, again caused by the same feedback cap.

the glue sometimes used on caps is sometimes rubbery, and sometimes hard and brittle. capacitor leakage looks oily or watery, or like spilled beer or cola that has dried. it is an electrolyte, but is not usually very conductive or corrosive.

JasonLee 28th March 2007 08:35 AM

Just done a search.. found this tread : Harman-Kadon HK 330B Schematic needed. here is the link to the schematic in pdf.

Confirm it's a OTL.

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