[fiddler]

Hey, it's been a very long time since I've visited these forums...
 
I was wondering if a technically minded can help me with this problem - I've recently purchased an old HK PM 640 integrated amp and one side of the balance control is defective. Playing with the pot can result in getting the signal to come through the left channel, but very sporadically. Even when there is a signal some ugly popping noises can be heard. The design involves two separate pots for left and right that are controlled by one knob, and the right side seems to work fine.
 
edit: I had linked to the service manual of another iteration of the amp, oops! The correct one is here
http://mail.servicemanuals.pro/servicemanuals/harman_kardon/service_manual/audio/amplifier/pm640/download.html
 
Is this a simple matter of cleaning and/or replacing the balance pot, or could it be a more severe issue?
 
Appreciate any help!

 

[wakibaki]

Make sure before you do anything else that this is not just a case of cracked tracking or solder. PCB mounted knobs can be subjected to quite severe mechanical stresses and are a natural focal point for gross breakages. Inspect the PCB where the pot pins are soldered in minute detail, reflow joints as necessary. A jewellers loupe and bright LED torch are a big help. Sometimes flexing a board will reveal hidden cracks, sometimes cracks remain but are undetected until the board is checked for continuity.
 
Otherwise, replacing the pot is going to fix this (unless there's another hidden fault). I notice however that in the BOM the part number has a suffix (M, N). Now, I'm not 100% sure about this, but I think it is a 50k balance-specific type pot, possibly with a centre detent? The individual pots may have slopes in opposition, so you need to look into this, because if it is a specialist part it must be replaced with a specialist part.
 
You can try to fix the pot, baby some life out of it with switch cleaner spray. Otherwise, of course, you can just replace it with a fixed resistor network, 4*25k resistors, hard to get that wrong.
 
w

 

[fiddler]

Thanks for the response.
 
As a test I've shorted out the pot's inputs and outputs, which results in sound coming out cleanly from both channels. I then realized this is probably bad because there's a functioning 50K resistor to ground in one channel, and effectively no resistor in the other. I can't detect any sonic difference in the channels with my ears, but I suppose this is doing funny things to the input impedance of the amp? I think I'll open it up again and install 2x 50k resistors. I think 4x 25k is wrong because from my research, a MN taper seems to be 100%/100% for both wipers in the middle, based on this page>
http://www.alpha-products.com/cd_catalog/new_catalogs/Pots/tapers.htm
Does that sound right?

 

[wakibaki]

Yes, that sounds right. I just guessed that the 2 pots would be regular linear pots with opposite rotations, so halfway for each would be 25k->25k. These appear to deliver full output on one channel throughout the range of adjustment of the other channel, which actually makes more sense than an arrangement that would throw away gain.
 
w

 

[smartem]

Hi fidler,
 
did you solve the problem with your amplifier?
 
It was due to foulty potentiometer?
I have the same problem with HK 670, I gess also it is the problem with old potentiometer.
 
Can you tell me which model of new pots you bought?  Where you bought it?
 
I am looking for new pots also, and your type is the same like in my amplifier.
 
Can you put some picture of your model, just to have a look, becouse I still didn't open my amplifier.
I didn't do this things before but I think I can do it. Now is most important to find the right model .

 

[fiddler]

Sorry for the very late response. I just saw this. I just bypassed the pot with resistors in the way that was discussed in the last posts. I hope you found a way to sort our your problem.