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Subwoofer: wacky volume = replace variable resistor?

discostu

Jan 23, 2014
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OK. Here goes. I'm hoping someone may be able to help me out. I am an electronics neophyte so be gentle. I know enough to get myself in trouble but that's about it.

The Problem

I have a Harman Kardon Subwoofer (model HKSUB-12). The sub is great and works fine except for one problem. I am unable to adjust the volume. It, for the most part, is at full volume no matter what level I set. The volume knob will work periodically through. It will sometimes go to the level for a brief period of time and then jump back up to full volume at will.

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My lovely wife tells me to buy a new sub. I'd like to take a shot a trying to fix it first.

The Investigation

My "gut" tells me this is a volume control issue based on the subs behaviour. I think if I try replacing the volume control it might resolve the problem (this may be completely wrong but I think it is worth a shot).

I've managed to find the Service Manual for the sub and I've started to poke around.

The volume control sits on a circuit board.

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This is the circuit board

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The volume control is labelled VR201. The parts list is here show it as a Variable Res. 50KA

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Here is Where I Need Help

In my simple way of thinking of things it should be as simple as getting this part and replacing it on the circuit board (I have tinkered with soldering many moons ago).

I don't know what exactly I should be getting or looking for. What would be a suitable replacement part? Like I stated above I'm willing to attempt this repair on my own but I'm not sure what to do next. A Google search of 50K Variable Resistors is a bit overwhelming.

Edit: I should add that I have tried contact Harman Kardon to see if I can get the part from them but no go. This sub is no longer manufactured.

Anyone have any advice?

More Info

Circuit Diagram
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Block Diagram
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Service Manual

Thanks :)
 
Last edited:

duke37

Jan 9, 2011
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The volume control VR201 is a variable resistor which will give full volume if the wiper gets disconnected.
You can try washing it with switch cleaner, operating the control a few times. You will need to get the cleaner on to the rubbing track. If this does not affect things, get a new control or fit a fixed resistor at a suitable volume level.
The problem is finding a replacement which will fit. H-K may use a similar control in other models.
 

discostu

Jan 23, 2014
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Jan 23, 2014
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Thanks Duke. I'll try cleaning the switch. Will isopropyl alcohol work or do I need something specific? How about white silicon?
 

duke37

Jan 9, 2011
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Never heard of white silicon.

There are various makes of switch cleaner but I do not know the best. Servisol is often recommended. The squirt can I have came from a general shop selling all kinds of things. Some cleaners consist of a solvent and some also contain a small amount of lubricant.

Try the IPA.
 

discostu

Jan 23, 2014
5
Joined
Jan 23, 2014
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We'll I got home from work and decided to pull apart the unit to see if a) I can even access the control and b) if there is a hole I can spray cleaner in to. I got it apart and confirm that yes, it is accessible and the is a small hole in the top I can get the cleaner in. I put the unit back together with the thought of picking up some switch cleaner this weekend. Plugged the unit back in and turned it on to test that I didn't muck anything up in the meantime. Well, the control is now working properly. Not sure what happened but I'll leave things as they are unless it fails again.

Thank you Duke for putting me down this path.

BTW - I had heard that white silicone lubricant is good for electronics (whether that is correct or not I couldn't tell you). http://www.amazon.com/CRC-Extreme-Silicone-Lubricant-Aerosol/dp/B008Y1Q4DY

No IPA at the moment but I am enjoying a nice gin and tonic. ;)
 

duke37

Jan 9, 2011
5,364
Joined
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Thanks Duke. I'll try cleaning the switch. Will isopropyl alcohol work or do I need something specific? How about white silicon?

There is a diference between silicon and silicone.
Silicon is a hard semiconductor, used as hard cubes in aluminium pistons as the rubbing component.
Silicone is a compound often where silicon has replaced carbon. The spray you refer to seems to be a replacement to oil or wax to act as a water repellant. I would not put this near your control.

Good that it is now working. One possibility is that the control is OK but that there is a poor connection to the printed circuit board.
 
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