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|16th March 2004, 01:12 PM||#1|
Join Date: Apr 2003
Harman/Kardon HK680 service manual/schematic
Anyone has subject? I'm highly interested. I would to tune the bias a bit.
|16th March 2004, 02:17 PM||#2|
Usually it doesn't take a schematic to tune bias if you see where things are.
If this amplifier uses harman/kardon's usual desings, there should be emitter resistors (sometimes collector resistors instead, but same for this adjustment) on the transistors across which you can take a voltage measurement and thus mathematically calculate the idle operating current, also known as the bias or Q point.
There should be a pair of these large ceramic white resistors per channel (so four resistors if you have a stereo amp) or sometimes they use compound resistors where you have just a single white resistor per channel that has three pins instead of two.
I normally measure the voltage drop across both resistors at once instead of across just one resistor. To do this you must be able to see where the resistors join eachother so you know where to test at the other end of each one. This is easy with the compount ones cause there'll be a pin at the center and one at either side. The center pin is the joint, and the side pins are where you take the measurement. A trick is to see if you get double the voltage of one resistor across both. If this happens, you can have some assurance that you're measuring across the resistors properly.
If you don't get all that, just take the voltage measurement across just one, it still works. Remember that the voltage will be a small one. Set the meter for millivolts cause it's usually anwyhere between .010V and .600V for most class AB amplifiers I've seen (.600 is rare).
Now for the fun part. Get a pen and paper and write down your measurement for the first channel and also write down the resistance across which you measured (add both resistances if you measured across two resistors) and then write that down. Usually these resistors are .22ohms or .33 or .47 and not over 1ohm so when you're looking, look for the number written on the big white ceramic ones near the heatsink.
Now that you've got the numbers written down, you can use ohm's law to calculate the current flowing in that channel, and this is the bias current. The formula is Current = Voltage / Resistance. Where voltage is that which you measured and resistance is that which you read on the resistor where you measured the voltage. (And added if it was across two resistors.)
The resulting current number from the formula shows what current was flowing through that channel during your measurement. The number should be small, less than an amp in most cases. So if you had .080V on a .47ohm resistor, you would've got a current of .080/.47 = .17 .17A or 170mA.
You do this for both channels. And with experience it should be a very simple procedure for most amplifiers.
Thing is now that you must find two potentiometers on the board, one for each channel; these adjust the bias. If there are four pots, this is tricky. I usually have my meter on the resistor and gently tweak a pot to see if the reading changes. If not, I make sure I put the pot back where it was and find another. If no pots cause the number to change, you've either got the wrong resistor, or the amplifier is made for static bias and can't be adjusted.
The only trouble here is that you must do all bias measurements and adjustments while the amplifier is on. Do them without speakers connected and make sure the volume control is all the way down.
To set the current, you need a little experience with these amps. Often I set the current to my own desired value based on the powre output of the amplifier, the size of the heatsink, and the output transistors. Also, I base it on whether it improves the sound at all.
If I don't want to change the current, but just get both channels the same, I look at the measurement for both channels, and set both to somewhere within the two numbers. If they are way far off, I'll often take the lesser of the two, or if they are the same, I leave them alone.
I hope this wasn't too in depth. And maybe you already knew all of it and just need a schematic for the bias current numbers.
Anyway, this is the method I use for doing this and it works every time as long as you're careful. Try not to slip and turn the bias all the way up, I've seen amplifiers get a bit unstable in this condition.
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