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Help! Hum/Hiss in a Harman Kardon 430

Your measurements seem to confirm what I suspected from the beginning.

Cap C4 seems open. You should replace this cap. The exact value is not important, at least 2200 uF, and at least 35V according to the schematic, but 50V in my opinion.

At LP54, you should expect to see a few Volts for the ripple, say 1 to 3 VAC, but not 50VAC

With a new cap, the DC voltage at this point will be much higher. I suspect it will be close to 30 or 35V.

If the minimum voltage at this point is higher than 27 or 28V, regulator Q2 will operate properly (but to the limit). From your measurements, the minimum voltage at LP54 is... OV (the DC Voltage minus half the AC Voltage)

Does it make sense to you?
 
I think the last one (2200/50V) is preferable if they are the only one you can find.

I don't like the axial cap as a replacement for a radial cap, particularly when there is many steel parts all around (heat sink, chassis) in a crowded space like your application. Take care about accidental short.

By the way, check the solder connection for the actual cap, just to rule out this possibility.

What are the diameter and lenght of the actual cap?
 
I'm at work right now so I can't measure, but you have my hand in the picture for reference. I'd say the top of the cap is maybe a bit smaller than the size of a nickel, and the length is probably about 1.5 inches.

I checked the solder connection, it looks good to me. It appears to be glued a bit to the PCB.

I'm a bit nervous about soldering something this complicated, so no guarantees. I have a Weller soldering iron, but it's old and I only use it for small projects and tinning wires.
 
You should invest in a suction pump. You may be able to make nice solder joints with your actual soldering iron. Keep the tip clean with a wet sponge between each solder joint. If you have the "gun" model, it may be too hot for the delicate printed circuit. But it can work if you grind the tip to have a very sharp point, and switch the gun On and Off to maintain a good temperature.

Practice your soldering skill before you dig into the HK-430 with the solder iron.

I am wondering if your voltmeter is a true AC voltmeter... Some voltmeters have a capacitor to block the DC component, and they can discriminate between the DC and the AC component of the applied voltage. Some voltmeters have rectifier(s) and are "graduated" to give an AC reading, but in reality they respond to a direct current. If you measure the voltage from a 1.5V battery, in AC mode, you should have 0V with a true AC voltmeter, but something close to 1.5V or 3V with the "bad" model.

Try to measure again the AC voltages with a small cap, like a 0.1 uF 100V (at least) ceramic cap, inserted between the positive probe and the measuring point.
 
All right, good call. I'll try a battery and see if it works. This isn't a fancy multimeter, trust me... it's the step-up-from-the-bottom Home Depot analog one, not the digital ones we use in Physics Lab.

I didn't even think about the PCB. This thing takes forever to heat up and darned if I do that and melt the PCB. I guess I'd have to heat it up first and wait until it melts the solder on contact, then quickly break up the bead of solder and pop the capacitor off one lead at a time.

I'll post back in a few hours... once again, THANK YOU so much for the help!
 
Bad news. I have a fake AC multimeter, it would seem. Set it to AC, connected a 1.5V battery, and BOING... sure enough, it thinks it's alternating current.

Where do I go from here?
 
Always insert a small cap in series with your probe when measuring AC Voltage with this meter.

A 0.1uF cap will take about one second to charge with a 10 MOhm meter. When you will connect the probe to the testing point, the needle will deflect to the right, then stabilize to a lower value. It is normal to behave like this.

The accuracy of your meter is certainly not on par with a good True RMS meter, but we can live with it.
 
Actually, I will be receiving a real multimeter from a friend at church on Sunday. I'll post back when I get it. It'll make this a lot smoother. I really don't trust that multimeter of mine, honestly.
 
Update: I got a much better (but old) digital Beckman multimeter, but it still can't measure true AC voltage, so I'm making an order from partsexpress right now and I'll tack a couple caps on including some for replacing that cap since they're cheaper on the site and I'm already making the order.
 
Replace C4 on Rectifier A board, and C2 and C15 on Rectifier B board.

It is very important that the voltage at the output of the second voltage regulator be exempt of AC ripple.

The first stage (Q501 for the right channel and Q502 for the left channel) is an input buffer and a power supply ripple mixer... A ripple voltage as low as 0.5 mV on the supply line (on the tone board) will limit the signal to hum ratio to 60 dB, with the volume knob to max position.
 
Ah crap I already put in the order. Oh well, I didn't know the values for those anyway. I'll replace C4, see what happens, then replace C2 and C15 if it doesn't work.

To be honest, you are speaking another language. I only know the bare basics here... and that's saying too much, in reality! haha
 
OK, I replaced C2 today without too much trouble. I forgot to order desoldering braid, but I got it off anyway somehow. These boards are really durable, I've discovered.

Believe it or not, it seems to have made a difference, although I still get the same effect: A few seconds after turning on the power, I hear a "...HUUUMMmmmmm...." it's like a surge in hum and then it stabilizes to a lower level of humming.

Anyway, I'll order more caps and get C4 and C15 replaced.

I'm really unsure about how to test the AC voltage. Do I put the black stripe facing the meter? When doing this I detected a little jump and then nothing for both points.
 
The way you measure the AC voltage is OK. The little jump is the charging current for the capacitor in series with the meter.

On post #20, you reported a DC voltage of 25V at LP54. This DC voltage is too low for proper operation of rectifier B board, because the zener EQA01-25 is a 25V diode (equivalent to NTE 5082A, a 25V 1W zener). It is normal that you get hum in this condition.

If you downloaded the service manual on the link in post #16, on page 5 of this PDF file, you will find a 8.2 Ohm resistor (R9, 8.2 Ohm, 1/2 W) between the Rectifier A board and the FU1 fuse. If this resistor has drifted in value (quite possible considering the rating for the fuse and the power rating for this resistor), it is possible that you have more voltage drop in her than expected. What is the AC voltage drop in this resistor actually?

What is the AC voltage between LP55 on RectA board, and LP56 ?

What is the AC voltage between LP55 and the transformer side of FU1 ?

What is the AC voltage between LP55 and the common point between FU1 and R9 ?

Check this resistor (remove the fuse FU1 for this test). What resistance do you have?

Also, check for oxidation in the fuse holder for the FU1 fuse.
 
ecluser said:
The way you measure the AC voltage is OK. The little jump is the charging current for the capacitor in series with the meter.

On post #20, you reported a DC voltage of 25V at LP54. This DC voltage is too low for proper operation of rectifier B board, because the zener EQA01-25 is a 25V diode (equivalent to NTE 5082A, a 25V 1W zener). It is normal that you get hum in this condition.

If you downloaded the service manual on the link in post #16, on page 5 of this PDF file, you will find a 8.2 Ohm resistor (R9, 8.2 Ohm, 1/2 W) between the Rectifier A board and the FU1 fuse. If this resistor has drifted in value (quite possible considering the rating for the fuse and the power rating for this resistor), it is possible that you have more voltage drop in her than expected. What is the AC voltage drop in this resistor actually?

What is the AC voltage between LP55 on RectA board, and LP56 ?

What is the AC voltage between LP55 and the transformer side of FU1 ?

What is the AC voltage between LP55 and the common point between FU1 and R9 ?

Check this resistor (remove the fuse FU1 for this test). What resistance do you have?

Also, check for oxidation in the fuse holder for the FU1 fuse.
Click to expand...


Honestly, even with two multimeters, I am unable to get a decent reading. Maybe I've blown out the capacitors I have somehow. All of those readings are showing as around 23.0 to 23.3 volts... and it makes no difference whether there's a capacitor in line or not.

The resistor is 7.9 ohms. I'm suspicious of that fuse FU1. It looks rusted on the inside. Gives a reading of 3.2 ohms... eh... I'm just going to replace it for the heck of it in my next order. I'll replace those other caps you mentioned to now that I have the tools. By the way, desoldering pumps are awesome! The one I got for $3.00 is really nice... works extremely well.

Maybe I need to just buy a new multimeter that does true AC. Any recommendations?
 
If you want to buy a new multimeter, try to find one with a Diode check mode, and a hFE scale. They are useful for testing and matching transistors.

For the measurements at the output of the transformer, it is normal that you have the same readings with or without capacitor in series with the meter, because there is no DC voltage at this point.

A 3 Amp fuse should measure much less than 3.2 ohms.

The voltage readings are suspicious. With 23 VAC before the bridge rectifier, you should have close to 28 to 30 VDC at point LP54, but you reported 25V (post #20)

Can you measure the DC voltage at C15 ? It should be 25 V if the zener is good.

Once the amp is ON, and you hear hum, bypass the resistor R9 and the fuse FU1 with a jumper. Does it change the hum level?
 
I'll remeasure LP54 when I get back. That was before I calibrated the meter. I'll measure the DC and the AC voltage there and post back.

I'll see if I can find a speaker to hook up to test the hum when I bypass that fuse. That's an interesting test.

By the way, am I or the amp in any danger while doing this? I've seen sparks coming from the leads and such when I place them too close together.
 
No danger! The resistor is a surge current limiter to protect the transformer's secondary when the cap is charged at start on.

You measured only 0.3V across them, this is a very small current. (about 30 mA)

Bypass both R9 and FU1 with one (1) jumper.

The voltage at C15 should be about 0.6V higher than the voltage at C2. According to the information in the service manual the voltage at C2 is supposed to be 25.4V. This is incompatible with the 25V zener
 
I can't tell with capacitor C15 is. Show me in the diagram.

I retested all of them with the good meter with reference to ground. When I test AC, I put the capacitor in line and set the meter to "AC". Is this right, or should I leave it on DC?

LP56: 13.3 V DC
11.2 V AC

LP55: 13.4 V DC
18 V AC

LP 54: 27.8 V DC
1.1 V AC

LP501: 23.5 V DC
1.3 V AC

Keep in mind the LCD is hard to read. I'm really unsure about the AC voltage at LP 54 as the LCD glitches out when I measure the thing using AC voltage. The DC voltages should definitely be accurate, though.
 
C15 is the capacitor across the D8 zener, on Rectifier B board.

The hum you hear comes from the 1.3 V AC at LP501

What voltages (DC and AC) do you have across the D8 zener?
 
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