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Advice on volume control repair
Advice on volume control repair
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Old 20th November 2013, 01:53 AM   #1
Richard Davis is offline Richard Davis  United States
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Default Advice on volume control repair

Some years ago I acquired A Curtis-Mathis solid state analog am/fm stereo receiver. What impressed me about it was its great weight. This,, I concluded (wrongly) translated into a heavy power transformer--and that was good enough for me. And it sounded as good as anything I had heard (Marantz 2220, etc. )
When I finally took the case off because of an intermittent VC I discovered that most of that weight was the heavy wooden case. Nevermind it sounded good.
Now, I have some problems. I decided to replace the VC rather than try to clean it. But it seems to possibly be a 180k-200k pot. My actual measurements across the outside terminal, in-circuit, was !85k and 125k respectively. I don't know what explains it but concluded I need a pot in that range. The easy to get Radioshack pot is 100k. I am mentioning this in case someone know where to get what I need or else that the 100k would work. Second, there is a fourth connection of the back of each pot one with a red wire the other with a white. Measurement seems to show this might be a ground but I don't know. Does anyone know what this would likely be?
While gaining entry, I was unable to get the tuning knob off. It was a simple press on. No amount of prying would get it off. Maybe it was JB Welded on. Eventually the shaft broke, so now I need a coupling and extender shaft to reconnect the tuning knob. The mechanism itself works ok. I am mentioning this, again, in case anyone has any advice?

Last edited by Richard Davis; 20th November 2013 at 02:02 AM. Reason: errors
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Old 20th November 2013, 03:28 AM   #2
linuxworks is offline linuxworks  United States
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Location: Santa Clara, CA
why not put a pot there as a control device and embed a digital vol control inside that spacious area?

ob disc: I worked on one design, myself (lcduino and delta-1). its a relay attenuator that uses a simple linear pot as a control element. pick any linear pot you want, no audio goes thru it, it just sends a control voltage to the relay r2r atten circuit.

just an idea
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Old 20th November 2013, 05:48 AM   #3
Ian Finch is offline Ian Finch  Australia
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Location: Coffs Harbour, NSW
A fourth lead on the volume control usually means it is a tapped pot. The tap, at about the middle of rotation, is only at 10-20% resistance position, due to the logarithmic taper of volume controls. This means you can measure what it does, relative to the min. and max. volume terminals of the resistance track. The purpose is to increase loudness (bass boost) as the volume setting is reduced.

Finding a replacement that fits the available area with a matching pin alignment may not be easy because loudness controls were phased out from the 1980s on. You could look at the ALPS RK27 range which is still widely available thanks to Ebay etc.
If only I had listened....
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Old 20th November 2013, 06:49 AM   #4
VaNarn is offline VaNarn  Australia
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Join Date: Dec 2010
Location: Ivanhoe
Advice on volume control repair
One way to replace a tapped volume control and retain the ''loudness'' function is to use a multi position switched resistor type potentiometer of the nearest equivalent ohmic range and then it can be tapped into , at the appropriate point. Precaution is required when soldering as the surface mount resistors that are commonly used,need lead free solder and a temperature controlled iron.
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Old 21st November 2013, 08:45 PM   #5
Richard Davis is offline Richard Davis  United States
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Thank you for all advice so far. I have found that the 100k universal
vc offered by radio shack does have the fourth lug, but is 100k, while mine measures 180k on one and 125k on the other--in-circuit. What if I used the 100k control? Would it means I would not be able turn the volume completely down? Other repercussions?
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