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Help! Hum/Hiss in a Harman Kardon 430

Oh and by the way jumping the fuse and R9 made no difference. So much for that theory. I'll still order some new fast-blow fuses to replace the slow-burn one in there right now that looks to be near the end of its life.
 
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Well, I replaced C4 and C15 to no avail. I'd say the problem is worse if anything. Again, basically, I hit the power, and about five seconds later, HREEEEARRRummmmm........ along with static.

The good news is, I got a better multimeter, seen here on ebay:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWNX:IT&item=170238083764

The bad news is that I don't have any cables or test leads for it right now. hahaha. I'm working on that, though. It should be a much better meter than either of the ones I have right now.
 
Please, measure the voltage at the zener (D8).

In the beginning of this thread, you reported that the hum level was affected by the position of the Volume control. Is it still the situation?
 
First of all, this new meter is amazing!! I cannot believe how fast and accurate it is. I connected a battery, it read 1.6** (not sure what the last two digits were) volts, then put it into AC mode and after only a few seconds, it read 0.000 volts. Nice!!

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OK, so I retested everything with the new, accurate meter:

LP54: 27.9 VDC
0.0 VAC

LP55: 12.71 VDC
11.81 VAC

LP56: 13.1 VDC
12.57 VAC

LP501: 23.7 VDC
0 VAC (note the change)

I tested across the Zener, I believe. Here are pictures, if you could, please verify that C15 is installed correctly and that I am testing the right diode:

IMG_1589.jpg


IMG_1590.jpg


IMG_1591.jpg


The Zener is reading 24.3 VDC, and 0.0 VAC.

As for the volume control, it doesn't seem to affect the hum at all, although the Bass control results in its own hum effect between 8:00 and about 1:00. Hope this helps. The Treble control still makes the hiss increase and decrease, but the hum is constant.
 
MD1032 said:
The hiss increases when I turn up the treble and vice versa. The hum increases when I turn up the bass and vice versa. The hum and hiss both increase when I increase the volume and vice versa.
Click to expand...

I was under the impression that your hum level was affected by the position of the volume control. For this reason, I suspected the regulator board.

With your last measurements, I think your regulator board is good since you have about 3.5V drop in R7, 4V drop in Q2, and the output is 0.6V bellow the zener voltage. Everything is fine here.

For the hiss, it looks normal. The first stage (Q501/ Q502) generate their own noise and let pass the thermal noise generated by the resistors connected to their bases. The resistors are high, the volume control is a 100k resistor, and the bias resistors are 220k and 1M. The higher is the resistance, the higher is the thermal noise in it. Without re-designing this stage, there is nothing to do for this. If you turn the volume control to minimun, the noise level will be at its minimum level because the base of the transistor (Q501 / Q502) will be shunted to ground. At this position, the noise you hear from this stage is generated by the transistor alone. If this noise level is unaceptable to you, you may consider replacing the transistors by lower noise units. This stage degrades the signal to noise ratio of the amp because it generates noise but it doesn't amplify the input signal, it is a voltage follower stage with a voltage gain less but very close to one. Any tansistor you will put there will degrades the signal to noise.

For the hum, it is more complicated. I understand from your last post that your hum level is affected by the position of the Bass control. This implies that it is present in the Tone board, and not only in the power amplifier stage. For this reason, I think the Tone board is the main cause of hum and your main caps (C6, C7, C8 and C9) are probably good.

I recommand that you check all the ground wires (the black ones) inside the receiver, for good connections.

The addition of an external connection between the chassis and the neutral (the round hole in the AC outlet) of the power line may help.

Sometimes, a different routing of the internal wires may help reduce the hum level. From the information I see in the last pages of the service manual for this unit, it is not a "star ground" routing. In the Tone board for instance, there is a ground from one section of the receiver that is connected to the left side of the board, and another ground on the right side of that board is going to a different point inside the receiver. It is usually better to make all the connections to a single ground point, as close as possible to the main power supply ground.
 
I'll recheck all the ground wires. Again, the hum no longer seems to be affected by the volume control, whereas before, it clearly was.

As mentioned, there is a hum caused by the bass potentiometer itself when it is at a certain position (between 9:00 and 1:00 or so), but outside of this area, the other hum is constant and underlying. I think changing the potentiometers would definitely make the amp quieter, but I have serious doubts that it would affect this hum. Also, I would have no idea how to do that anyway or where to get replacements.

I tried connecting a wire from the chassis to the earth ground before but it only made a harsh buzzing sound occur. However, I will try this again.

How would I check to see if the main caps are good? Would the wires running over them cause any interference?

Also, now that I have a good meter, I will check the DC offset of the actual outputs out of curiosity.

Thanks as always for the expertise!!
 
To test the main cap:

Your amp has no swith to disconnect the power amplifier from the pre-amp. But You have a solder gun... Disconnect the input wires at LP402 (or 412 according to the schematic?)and LP406 on the main amp board.

At this moment, the power amp receives no signal from the previous circuits. If you hear hum without input signal, the main caps should be replaced.

There is no Offset adjustement in this receiver, but the circuit is well balanced. If the transistors are matched in the differential stage (Q402 = Q404, and Q401 = Q403) you should have a low offset.

If the AC plug is not polarised (one leg larger than the other), try to reverse the plug.
 
LP402 is mislabeled on that one page, LP412 is the point with the white wire leading from the tone/preamp board.

I disconnected LP406 and LP412. The hum is still there and sounds identical in intensity. What does this imply? If my guess is correct, it would imply that the rectifier C board (the large one with the four big black caps on the other side) has something wrong with it.

Offset is almost non-existent. Maybe a couple mV, nothing big. You're right, it's a good circuit.
 
MD1032 said:
I disconnected LP406 and LP412. The hum is still there and sounds identical in intensity. What does this imply? If my guess is correct, it would imply that the rectifier C board (the large one with the four big black caps on the other side) has something wrong with it.
Click to expand...

No, it has nothing to do with the C board.

When the power amp board is disconnected at his input, nothing is comming from the previous boards.

It is normal the you hear a very small hiss, but it should be almost unoticable.

If you hear hum, it is likely that your main caps have drifted in value (capacitance) and they don't provide enough filterring.

I think you need to replace the main caps.
 
Which are the main caps? The large black ones visible from the top of the amp? The reason I asked about the rectifier C board is because those big black capacitors (C6, C7, C8, and C9) are on the rectifier C board unless I'm incorrect. The second to last page of the service manual, which is the only diagram showing all of the LP's on it, is the only diagram that seems to show this.

Where would I find suitable audio-grade replacements?

Thanks.
 
Sorry, You are correct, they are on the C board. (I had another amp in the head, it is my mistake)

I thik Mouser is a good place to order them.

Check the ground connections on the main-amp board.
 
There is not a great difference in price between the 50V caps, and the 35V caps.

Today's caps are smaller in size than they were many years ago.

Measure the actual cap size, and try to find a 50V cap that fit in the amp. If you can find 6800 uF/50V cap the same size as your actual cap, it would be better than the original.

With 50% more capacitance, the power transformer and the bridge rectifier will survive.
 
By the way, one more question. The currently installed capacitors do not have any directional markings on them as far as I can tell. How will I be able to tell if I am using the correct orientation with the new ones?
 
oops, looks like I ordered the wrong ones! The current ones are four-pin snap-in and the new ones are standard radial hole-through. I don't think I will make a difference, fortunately.

I figured out which end is negative. I also accidentally grounded the positive end... POP!! Fortunately the amp is still all right.
 
OK, good news:

I ripped out the four old caps and put the new ones in. So far, it looks like everything is good. Unfortunately my dad is asleep so I can't exactly crank it up and test the amp, but both channels work so is it safe to assume that everything works 100%? If one capacitor was not completely soldered properly, the amp would not work, correct?

I just hope that I didn't fry any of them while soldering them on. Things got pretty hot.

The hum is significantly reduced in volume. It's definitely still there, but after the initial "HUMmmmm" of it powering on after about 4-5 seconds, it goes down to a lower level than it was at before. I still get the feeling that there's something causing that, but I'm not going to worry too much about it since I'm going back to college in 8 days.

edit: Case closed! The amps works great. I just did a full listening test and it sounds absolutely amazing even with my MP3 player plugged in. It's just unbelievable how good it sounds now. The hum is significantly reduced.

HUGE HUGE thanks to ecluser... he is the man!!!!
 
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Resurrecting this thread because I'm doing the same work on my H/K 430 Twin Powered. Same exact problem. I have to wait for new caps to arrive though.

Does anyone know where to get replacement transistors? I might have found a source for the 2SC1030 transistors here, but I'm not 100% certain about it being correct, or the quality of the part. Are there better quality transistors out there? Or a recommended source?
 
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