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Harman Kardon 330c Power Amp Idling Adjustment

JMiP

JMiP

AK Subscriber
Subscriber
I have attached a screenshot of the service manual for the HK 330c where it describes how to set the idling adjustment. I understand how to do the first four parts, but the last one, "Connect 8 ohm (50W) resistor across left and right speaker terminals," I don't know how to do. Can someone please describe what I need, equipment wise, to get this done? I have a Meterman 37XR (that I got a good deal on) with the grabber leads and a small screwdriver to make the adjustments (if needed). What do I need for the step in question? Something like this? How does that hook up to my speaker outs on the back of the amp? Do I need two of them hooked up to the receiver at the same time? Is the receiver on or off during this process? If turned off, do I need to unplug it as well?

Also, is there a way to adjust the DC offset on the HK 330c? I assume not since it's not even mentioned in the service manual.

Thanks in advance.
 

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You are right, the manual asks for that resistor you linked, connected at the speaker terminals, in place of the speakers. But I think you don't need a 50W resistor to just adjust the bias current. You need a high power resistor to test the amplifier output, at high output levels. You'll have almost zero volt there, so a small resistor can do the job.

The manual says to connect 2 resistors, one at each channel , at the same time. You can connect them with alligator clips, or any kind of connector that your speaker terminal can accept.

To just adjust the bias, I'd use a 5 Watt if I have one at hand. Or even less. 1 or 2 watts can be used.

If you post the schematic, at least the power amp part, we can tell you about the DC offset. BTW how high is it now?
 
If you post the schematic, at least the power amp part, we can tell you about the DC offset. BTW how high is it now?​
My last test (about a month, month and a half ago) showed about 19 mV in one channel and about 21 mV in the other channel.
Is it cool to upload the entire service manual or is that frowned upon here? Copyright and all that legal stuff even though the manual was published about 40 years ago. I just don't want to break the rules.

To just adjust the bias, I'd use a 5 Watt if I have one at hand. Or even less. 1 or 2 watts can be used.​
So, get the resistors that I linked to, but make them 5 W, 8 ohm?
 
19mV and 21mV, I'd say you are within specs of many service manuals around, so I wouldn't worry about that.

I suggested a smaller resistor, just in case you have some parts at hand, you could even put 2 x 4 ohm in series, or even a 10 ohm resistor.

If you are going to buy, those 50 Watt will serve as dummy load for many other projects, so go ahead and get them, no problem with that. I personally don's have anything larger than 15 Watt in my bench, for large amplifiers, I use four 10 ohm resistors connected in series-parallel, 15 Watt each, making 10 ohm load - 60 Watt dissipation, attached to a heatsink, or I use just 2, to have a 5 ohm - 30Watt load.

I don't know if those manuals are still copy righted. I usually share or post a link to those that are already on the web, and I don't make public those available from the service manuals vendors, to support that people who keeps record and files of all this vintage papers.
 
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So, I'll just get the resistors that the service manual calls for and merely hook them as if they were speakers, right? One last question: is the receiver plugged in and turned on when I set the bias or do I shut it off and unplug the unit?
 
I remember looking once the schematic for another topic, the receiver amp is something alike the HK citation 12 I have.

Bias current setting you do not need a resistor connected as output is supposed to be zero when adjusting anyway.

DC offset of 20 mV is normal, it is not adjustable and the amount is depending on the input pair transistors base current through the resistors connected to it( higher HFE is lower offset).
 
I agree, I never use any load at the speakers terminals, but let's trust the HK engineers, if they instruct to do that.

I suspect that setup is for adjusting bias and later do some full power tests, if not, It's a non sense to use a 50W resistor without signal.

To adjust bias, set everything with the unit unit unplugged, resistors and measuring device. Then turn ON and read the voltage. Adjust if needed and let stabilize for 30 minutes while monitoring the voltage. It's normal in some amps to see a raise in the first 10 minutes and go down again. So be sure the amp is stabilized and voltageis within specs. You can move the trimpots with the unit powered on. Just do it slowly.
 
OK, thanks. Hopefully I'll get the time time this weekend to do this.

I just cleaned the pots and the input selector and records and CDs are sounding pretty sweet right now.
 
This thread picked my curiosity, so I've tried to adjust the bias in an amp, and connecting a load makes a difference (it's a capacitor coupled amplifier)... Even connecting just the headphones, shifts the voltage across the emitter resistors.

If I set 20mV across the emitter resistors without a load, connecting the headphones moves it to 14mV, and connecting a 10 ohm load, shifts the voltage to 10-11 mV across the same resistors, without touching anything. Disconnecting the load, the bias is back to 20mV.

So, if you manual says to adjust with a load connected, do it that way.
 
Finally got around to doing this last night. Each one was off by about 30%. I got them adjusted and let each sit with the multimeter attached for about 15 minutes. Each one drifted for about 10 minutes, but eventually would stop and stay within the 10% tolerance range.

What, exactly, did I do to improve the sound? Anything?
 
elnaldo said:
I don't think so, specially if it was a bit high. What you did is to set the transistors in class "AB"
http://www.learnabout-electronics.org/Amplifiers/amplifiers55.php so they stay "ON" all the time, but at a minimum needed to to avoid crossover distortion and at the same time avoid overheating. If it's too high, the amp will run hot. If it's too low, the transistors will turn OFF and you'll have crossover distortion.
Click to expand...

Right channel was too high and the left channel was too low. Or both was too low. Dang it, I need to write this stuff down.
 
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