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Old 6th October 2011, 07:24 AM   #1
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Default Slide Switch & Potentiometer Replacement

Hi all. I'm trying to replace the broken power on-off slide switch and the balance control of my vintage pre-amp. The slide switch is 2-lug 3A AC and the balace control is dual 750K potentiometer.

I've not been able to find slide switches with only 2 lugs. Only those with 6 lugs are available. How am I going to wire the switch? Are the lugs in the middle common? Should I just wire one wire to a common lug and the other wire to either lug to the right or left on the same row and leave the remaining lugs untouched?

As for the balance control, can I replace it with a dual 500K potentiometer? Is this change to a lower ohm rating potentiometer gonna affect sonic qualifty?

Thanks for your help.
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Old 6th October 2011, 07:37 AM   #2
Mooly is online now Mooly  United Kingdom
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To identify switch connections just use a dvm on ohms range. Usually the "middle" connection is common (as you call it) and connects to the outer connections depending on switch position.

Don't guess... measure !

I can't see using a 500k instead of 750k will affect much. They are high values though. Is this a valve preamp ? Circuit diagram might help.
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Old 6th October 2011, 06:50 PM   #3
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Quote:
Originally Posted by Mooly View Post
To identify switch connections just use a dvm on ohms range. Usually the "middle" connection is common (as you call it) and connects to the outer connections depending on switch position.

Don't guess... measure !

I can't see using a 500k instead of 750k will affect much. They are high values though. Is this a valve preamp ? Circuit diagram might help.
Since I'm not good at electronics, please allow me to be inquisitive. Is "dvm" means digital volt meter? To use dvm to measure means putting the black probe on the common lug and putting the red probe to other lugs and to watch if there is dvm reading when the switch is switched? I thought that for an on-off switch, 2 lugs would do the job, why the 6 lugs?

Yes, it is a valve pre-amp. I have to search if there's diagram.
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Old 6th October 2011, 07:12 PM   #4
Mooly is online now Mooly  United Kingdom
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Yes, DVM is digital voltmeter.

To identify connections first make sure (if it's the switch in the unit) that the power is OFF.

Now put the meter on a low ohms range (that you would use to measure say a 10 ohm resistor) and check that with the meter probes shorted that the reading is around 0.00
That's a dead short. So far so good...

To identify which connections in the switch operate it doesn't matter which lead goes where. So probe the switch and see what connects to what in each position.

Why 6 "lugs"...

Well as you say two would do for a bare minimum. That would allow switching of just one side of the incoming mains which is acceptable and done in many products.

A 6 lug switch allows both live and neutral to be switched totally isolating the supply.

Why 6 lugs instead of 4 then ? Because that's how most switches are made. You could use a 6 "lugger" to switch the output of an amp between two pairs of speakers.
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Old 6th October 2011, 09:13 PM   #5
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After some research on the net, it appears that the switch is a DPDT (double pole double throw) that has 2 rows of 3 terminals. So, if this is going to replace the 2-terminal power on-off switch, am I going to just use one row, and use the centre terminal and just one of the outside terminals for connection of supply wires? Do I have to do anything to the remaining 4 terminals, like shorting some of them (as just been told by friends) to allow more current? Thanks.
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Old 6th October 2011, 11:54 PM   #6
Enzo is offline Enzo  United States
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Ignore the remaining unused terminals.

SInce you don;t understand the circuitry of the switch, trying to get fancy wiring terminals together and such offers more opportunity to screw it up than any benefit it might provide.

Yes, wiring two sets of contacts together will increase the current capability of the switch, but so what? You don't NEED more current capability.


All a switch is, is a way to connect two or more things together. They can make a simple 2 terminal switch, called SPST, and they can make three termianl ones - SPDT, and even 6-legged DPDT. But electrically it doesnl;t matter if you leave extra parts unused. SO instead of making and stocking three or four different types, they stock the DPDT, which covers all the other needs as well.


Imagine a taxi cab in the city. They COULD make one passenger cabs, and two passenger cabs, and six passenger cabs. But then they'd have to worry which ones were where when fares hail them. If they just always use 6 passenger cabs, they can still pick up single passengers without a problem.
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Old 7th October 2011, 06:41 AM   #7
Mooly is online now Mooly  United Kingdom
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It's as Enzo says...

Use just one row but be aware that the "unused" end terminal (on that row only) will be live when the switch is off.
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