Tapped Potentiometers

[Michael Maurice]

I've been finding that many potentiometers (volume and tone controls) are not able to be revived with switch cleaner.

Many of these have taps on them, but these are not available any more.

Is there any way of getting around this problem?

[Ambientnoise]

Hi Michael, I had this problem on a Philips tape recorder where the tap on the volume pot is used as part of the loudness circuit. I contacted this chap in the USA, Mark Oppat.

moppat@comcast.net

He could make one up but the cost would have been around $100 so I fitted a standard pot and an RC network around it to give a volume sensitive base boost. Not as good as the original, which also dealt with treble boost but fine in practice. I used LTspice to simulate the network.

Ken

[nigelr2000]

Hi Michael, I too have had this problem, have you stripped the control down? I have found from experience that the slider to centre pin moving contact corrodes and is untouched by switch cleaner however if you take the control apart and clean the contact ring with a fibre pen all is well again, the only ones I have not been able to revive are ones that have some mechanical damage. The worst offenders seem to be the thread-less ones used in later fidelitys held in place by two bendable tabs. Pictures enclosed of a random pot selected from the drawer, you can see where the centre contacts and this is where it is always dirty. Just carefully bend the 4 tabs back and lift the control apart, tandem ones are a little more hassle but you will find the shaft normally pulls out allowing you to strip each one seperately, also ones with switches are a little more complex to do but still not impossible. It is rare not to resurrect noisy or even open circuit controls. Give it a go as you have nothing to lose as it's not working anyway

Nigel

[richrussell]

If I get a pot that goes noisy again after two attempts at cleaning with switch cleaner, I tend to dismantle it if possible. Not all can be taken apart easily (I've had some that appear to have been spot welded), but for the ones with tabs as Nigel said, you've got nothing to lose as you were going to replace it anyway!

Often once you've got the can off you can see what the problem is straight away and clean it up. Usually it's just dust and dirt buildup, and sometimes a little careful bending to improve contact can work wonders.

Also if the track is worn, you can sometimes bend the wiper slightly so it contacts in a different place, giving it years more service before it wears another groove.

[dragonser]

Hi,
there was a post, a few weeks ago, here in the wanted section that mentioned a uk company that might be able to make up custom volume and tone controls.
I think the original poster was looking for dual concentric controls which they can provide as well.
http://www.blore-ed.com/products/ind...?title=Type_45

Of course I don't know how much it would cost to just have one control with tap made up ?



regards Peter B

[Michael Maurice]

I know Blore-ed but he cant make up tapped potentiometers.

[David G4EBT]

Blore Edwards and Omeg are very helpful firms to deal with, but if you only want a one-off, it will come at a price that unless it's for a valuable or cherished radio, will be out of the question. I restored a Stella ST105U which uses a tapped pot, on which the switch was open circuit. I made enquiries with both firms at the time. Blore Edwards couldn't help. OMEG could, but(not unreasonably) had a minimum order quantity and minimum order value which meant it was a non-starter. In the end, I was able to dismantle the switch on the pot and clean the contacts and reassemble it. Had I not been able to, I'd have simply have fitted a 470K log pot and taken the tapped point to the 'hot' end of the pot. I don't think it would have made any discernible difference to the performance of that or any other radio.

However, I did ponder on one possible 'workaround' which might provide a solution.

In the case of the Stella, the pot .5 meg log pot was tapped at 10% down from the 'hot' end - so the tapping point was at .05 meg, and the lower end of the pot was .45 meg. It crossed my mind that by substituting the pot for a 1 meg one, then putting two resistors totalling 1 meg is parallel, the pot would then effectively be a half meg pot. If the value of those two series resistors were 90%/10%, (90k+100k) that would provide a tapping point. (The nearest preferred value to 900K would in fact be 910k).

I've attached a sketch to show what I mean. Can't see why it wouldn't work - it's not like we're trying to put a rocket into space is it?

On a similar tack (non-availability of 'special' pots), I restored a Portadyne Princess, and a Noble, which are identical. Very simple TRF sets in which the volume control doubles as the reaction control. They work surprisingly well for such simple sets. The only info I could find was a circuit in R&TV which doesn't say anything about the volume control. I guess most people would assume that in the absence of any information to the contrary, would assume that it's a log pot, but it isn't - it's anti-log ('inverse' log), which I confirmed by drawing a plot of resistance v degrees of rotation. I enquired of Blore Edwards would couldn't supply one. Omeg (who supply pots to the likes of RS, Maplin et al) could have made them up, but I'd have to order a minimum of ten. They agreed to wave that but couldn't waive the minimum order value, which made it hopelessly uneconomical on sets of little value. Again, in the end, by furtling about with the switches I managed to effect a repair.

[vidjoman]

I don't think the workaround will work. It is just adding a fixed tone correction across the pot. The adjustment of the tone as the pot is altered will not be noticeable, if at all.
I have repaired many pots from jukeboxes and many have two taps, and are ganged in a stereo machine. I can only think of a couple of occasions, when the original pot is missing, that I have had to resort to fitting something else. I have used a ganged pot in a mono amp with one track as the volume control and the other to alter the frequency correction. Took some playing with to get it to sound good.
The normal operation of a "loudness" control is the enhance the bass, and sometimes the treble to compensate for our human inability to hear the top & bottom frequencies at low volume. It can also help if the speaker is of poor quality and needs a bit of a boost.

The only circumstance that I can think of where you have to have the tap is the older Baxandall tone controls where the treble pot has a centre ground. Even that can be 'got roundable' by rewiring to use the more modern version.

(edit... or is it the bass?)

[Pamphonica]

I have also been talking to Blore-Ed about tapped pots. They do list them but they currently have a problem with the die(s) for making the track sections. If you do contact them, it will add to the pressure to get these back in production. And, by the way, their custom pots are not that expensive. I think I paid about £5 each for some custom log-antilog dual pots made to my spec. I was very happy to pay that.

[Lucien Nunes]

A possibility - I haven't tried it but have used similar methods of connection on other devices:

1. Dismantle a new pot and remove any grease from track and reverse of same.

2. Drill a tiny hole (e.g. 1/32") through the paxolin, adjacent to the track at the tapping point.

3. Push the bared end of a carefully cleaned wire through the hole, and secure the insulated tail of wire to the back of the paxolin with epoxy.

4. Cut the wire flush, or as low to the paxolin as needed to clear the rotor as it passes.

5. Connect the wire to the very edge of the track using conductive silver paint, ensuring the paint is clear of the wiper path.

5. Cure well and reassemble.

[bikerhifinut]

I don't know if it's the same, but I noticed on an ALPS motorised pot ( Think the part no is something like RK127) that it had a tapping on it for a "Loudness" control. It was as part of a remote volume kit I got from china. I just ignored the tapping and used the pot as normal. I think there would be a non motorised version, there usually is. They aint cheap compared to the omeg or Alpha pots flogged by dear old Maplins but i guess would be less than getting a "special" made up?

A.

[bikerhifinut]

A quick trawl cane up with this, there seems to be others out there.
http://www.ebay.com/itm/1-X-MADE-IN-...item4610ca0ee4

Hope that may help someone.

I know it's a 50k but I'd imagine they would be available in other values. Depending on the application I imagine one can work around the different values. In a preamp section of a Valve amp it's probably not that critical, depending on the source output resistance looking in to the pot.

A.

[bikerhifinut]

Had another idea.
I have a switched attenuator consisting of a stepped ladder and depending on how its wired you could simply wire one of these with a tapping? It would have to be a Shunt type wiring I guess where the resistors are gradually introduced in series as opposed to the more sophisticated ones that switch in individual potential divider networks.

A.

Originally Posted by bikerhifinut

I know it's a 50k but I'd imagine they would be available in other values

But not much higher, these are made for semiconductor stuff.

[bikerhifinut]

The highest Value I have seen for a tapped Alps for sale so far has been 250k.
Alps have discontinued a lot of their traditional range as may be expected and current manufacture is up to 100k. So for higher values its NOS or Chinese copies?
I guess it depends on the application, I generally use 50k or 100k pots on any new valve stuff. But I have no sources that need a higher input resistance.
I can see the issue with refurbishing older equipment, and perhaps to save an item from the skip one might have to compromise or redesign a part of a circuit if there was a danger of an adverse interaction with a lower value pot. That probably wouldn't be a trivial thing.
Anyway, it does look like there are pots available with a "loudness" tapping up to 250k if that is any help to anyone.

A.