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Spectral Balance Control repair / replacement

Bill Pithers

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Are you certain the two very small allen-type set-screws are tight on the brass shaft?

With use over the years the knob may slip because the set-screws become loose.

I've had it happen with my AR-LST's on occasion and when they get loose, I simply tighten the two hex-screws with no further difficulty. Don't go too tight as you may strip the hex-screw head and then you'll really be in trouble. Off hand, I don't recall the size but, I know it's 'SAE' and not metric or imperial type. It's more like 1/8 or slightly larger. 

If this doesn't do the job, let's hope that the brass shaft that is used to turn to different settings isn't screwed or broken. Use Deoxit or similar down the shaft. It will be easier if the speaker cabinet is on its back for all repairs. 

If all else fails, the best way to a remedy this is to pull the control from the inside and double check. In the unlikely possibility that the shaft or its mounting is broke or stripped, you'll have to replace or repair. If entire control is damaged it will be a job for Super-Man or RoyC. either will do.  If the entire control spins, do not use it as you'll be twisting and possibly destroying the wire connections. In the event the securing 'nuts' are loose, simply tighten while holding assembly from the back. In either case, anything to do with this control will require removing the woofer.

P.S. for new knobs with the hex-screws included, check out "Vintage-AR' on the auction site.




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Frank, Thanks for your reply. I've double checked the hex grub screws carefully. I have two pairs of LST IIs and have been able to attach the control knob on three of the four speakers after having changed the grille fabric. Just to rule out the potential that the grub screws might be worn on the spectrum control that isn't working properly, I've just received two sets of the screws from Vintage-AR. (Very luck to have the resources they provide.) I checked the nut securing the spectrum control to the cabinet . It appeared tight. I was able to tighten it a bit further but it had no affect on the operation of the control switch. Is it possible to access the control unit by removing the woofer and removing the securing nut? I'm hoping that might allow me to determine whether damage has been done to the control unit. I'm cautious about doing so as I'm hoping to avoid repairing the spectrum control and a woofer! I appreciate your knowledge and advice!


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