Unusual potentiometer

[ccurtis]

Can someone explain the potentiometer, R55, in the figure below. I only know about three terminal pots. It is ganged with an identical pot for the other audio channel and is the volume control for an OLD Grundig radio. I want to simulate the tone circuit but can't because I don't know the internal structure of a 5 terminal pot.

Thanks

 

[rjenkinsgb]

It's probably some version of a "Loudness" volume control - the commoner ones have a single fixed tap on the track as somewhere around 40% or so.

Some examples:

ALPS RH2702 A250K

New Alps 100k ohm Stereo Potentiometer w/ Loudness Tap Receiver Amplifier Volume | eBay

It is a 100k ohm pot but should be fine for replacing 250k ohm pots and would probably be fine replacing 500k ohm pots as well. The shaft is a 1/4" round shaft with a flathead notch at the end. This will mount in a 5/16" or 3/8" hole, and includes the mounting nut and washer.

www.ebay.co.uk

You should be able to simulate the effect using a fixed resistor divider to provide the taps, in parallel with a standard potentiometer for the volume output?

The effect is supposed to be to reduce bass (and sometimes treble) a bit less than midrange with the volume below a certain point, to compensate for hearing being more sensitive to midrange at low volumes.

 

[Nigel Goodwin]

As suggested, they are taps are for 'loudness' adjustment - but usually there's only one tap on the pot.

'rjenkinsgb' is unfortunately mistaken about simulating it using external fixed resistors, as it requires the slider of the pot to pickup filtered signals as it goes up and down, the external resistors don't provide this.

It's LONG been a problem in audio repairs, basically the two options are:

1) Source the correct potentiometer - usually impossible.

2) Ignore the tap(s) and don't bother with loudness compensation.

Bear in mind, most audio systems don't provide loudness compensation, they just use a plain pot - you can do the same, and you'll probably not even notice any difference.

You should also bear in mind that for top end HiFi it's a definite 'no no', as you're modifying (distorting) the original signal.

Having done repairs professionally for 40 years, in the many cases where the pots weren't available we simply fitted a normal pot - and didn't even bother telling the customer. Not a single one ever came back and complained it sounded any different

 

[ccurtis]

Thanks, guys. That very helpful information! I had NO idea.

So, Nigel, would you have soldered the wires that went to the two taps, each connected to the nearest respective endpoints of the 3-terminal pot? Or just leave the tap connections disconnected? Or a combination of those two, depending on which tap? On this radio, the thing actually stills work, but I'm just curious.

 

[Nigel Goodwin]

Just leave them disconnected. making it like most other volume controls. You could take out the top resistor or capacitor if wanted, but it would probably make no difference.

 

[Tony Stewart]

 

[Nigel Goodwin]

View attachment 149487

And what's the relevance of that?.

 

[ccurtis]

Is the volume control built using a single resistance element with fixed taps to the resistance element near each end, or are there separate resistance elements for the taps and a shared wiper. Of course, I'm only talking about one of the two gangs, here.