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Fixing a bad balance control on an old Harman Kardon integrated amp

post #1 of 4
Thread Starter 

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!


Edited by fiddler - 3/15/14 at 2:19am
post #2 of 4

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

post #3 of 4
Thread Starter 

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?

post #4 of 4

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

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