Anti-log Pot track puzzler?

[Yorkie]

Awful day yesterday so I was unable to get both cars out of the garage and get on with making a couple of backs for my Portadyne and Noble radios with my router jig, which creates a lot of dust. My workshop is too small for such antics - just 10' x 8', but fine for tinkering with radios and woodturning, though they're rather incompatible activities.

Hence, I made a start on the Noble, and as with the Portadyne Princess, as well as the dropper being OC, the mains switch on the pot was open circuit too. I dismantled it hoping to be able to clean the switch and make it operational, but sadly, no such luck. As with the Princess, it's a 50K anti-log track, so finding a replacement is well nigh impossible. I do have a 16mm unswitched anti-log pot, and will try to dismantle it and remove the track as a donor for a switched 16mm diam pot, but I'm not hopeful as they tend to be plastic bodied and the pot I have is metal-bodied. I might ring OMEG - they seem to be a friendly little UK based firm - they'll make any pot to order but the MOQ is 10 items, and the MOV is £25.00 plus VAT & P&P, but they may know of a distributor of 50k anti-log pots. They tend to be used in hi-fi gear, but are invariably unswitched. (OMEG supply firms such as Rapid, CPC and Maplin).

The website I've mentioned before 'Secret Life of Pots' shows what seems to be a simple way of converting a linear pot to an antilog (AKA inverse log, reverse log). Rather too good to be true for my feeble mind - I've spent an hour furtling about as per the diagrams, fumbling around with various resistors, with no outcome other than to get the pot to work as a linear law.

I've graphed the results of several pots. Firstly, the Noble one, which had gone high from 50 K to 68k, and only had a rotation of 240 degress rather than the more usual 270 degrees. Secondly, the plot of a new antilog pot (with no switch), thirdly, a linear pot, (just a straight line of course), and finally a log pot, to illustrate how it diffrers from an antilog pot. Basically, a mirror image of each other.

Secret Life of Pots:

http://www.geofex.com/article_folders/po...tscret.htm

I might try a switched linear pot, which - though not ideal, would certainly perform better than a log pot.

How we suffer for our art - barking mad, the lot of us! Fancy, I could have been doing something that countless thousands of others think is sane - be camped out overnight getting wet through and cold on the Thames Embankment, wearing a flimsy poncho to watch boats float by in the pouring rain, while waving a union flag and singing 'Land of Hope and Glory'. Maybe I'm not so barking mad after all - it's all subjective I suppose. I'm obviously 'potty' though

Anyone know of a source of 50k anti-log switched pots please - I've trawled 'tinternet and come up with nothing.

At least I can solve the dropper problem by doing as before and mounting two 50W metal clad resistors on a heat sink that I've set about making, and can get on with other work on the little Noble.

[Bushbaby]

Hi David,
What make of pot is it??

Angie wanted to see the Dunkirk boats so we recorded the great 'event'. Super to whizz through and just pick the bits that you want to watch.
My mothers 89 and I would like to think of her standing up in the damp for hours on end looking cold as HRH did yesterday. I bet she would have much have preferred to be next to a warn fire with a cuppa and the Gorgi's. I take my hat off to her as I wouldn't have wave wanted to be on a boat on the river yesterday - I bet it was cold!

Rob

[Yorkie]

Hi David,
What make of pot is it??

Rob

The original is Egen Rob.

Had it not have been for the faint markings saying it was antli-log, I'd have asumed that it was a log pot like a normal volume control but it doubles as the reaction control, though you wouldn't know it as you don't have to twiddle it till the set just oscillates and then back it off, as was the case with a standard TRF. I'm sure you're familiar with the operation of the control as I think you have a little Champion TRF set. There was probably more than meets the eye to the design and theory of these sets, which were obviously designed to be similar in operation from the user's point of view as a superhet, but without the added complexity of an IF stage.

Back in the 1950s in PW, similar such TRF sets were advertised by surplus stores in kit form - no doubt end of line stocks when firms stopped making them.

Never mind about the Jubilee malarkey - you've got enough on your plate with the Olympics!

[Joe]

I don't know how much space you've got to play with but I've got this switch which goes in front of the pot and is operated by the shaft.

   

To be honest I think I'd take a leaf out of Mark's Grave's book and put a torpedo switch in the flex.

- Joe

[Skywave]

First, let me say that I do realise that what I am about to suggest is not exactly what you are looking for, but it may be of interest to others faced with a similar problem of producing a rotary, variable resistor that has a law that is not readily available. However, if you get really desperate, it might just be worth considering.

With a 10-way, single-pole rotary switch, you can choose the resistors that sit between each fixed contact so that you can achieve an approximation to any law. The downside is, of course, that the resistance is not continuously variable: it changes in steps. However, if you do have something rare and unusual - such as a 20-way rotary switch - you obviously get a finer resolution . . . and so on. Moreover, with a double-wafer switch, you are then equipped for 'stereo' or an exotic control where the 'law' of each 'track' can be quite different from each other. A test equipment requirement springs to mind at that point.

Just a thought.

Al.

[pwdrive]

I have fitted taper resistors to pots, they do have an effect but it's far from ideal, as has been said reverse log pots are not easy to source these days, I think Farnell do a small 100k one last time I looked. If you can't get one it might be worth considering Al's solutiopn.

Good luck whichever way David

[Yorkie]

Thanks for the helpful advice chaps - it gives me several options to think about!

[Mark R]

To be honest I think I'd take a leaf out of Mark's Grave's book and put a torpedo switch in the flex.

- Joe

Joe
That torpedo switch is an original feature! It is the main switch as there is not one one on the pots.

cheers Mark

[pwdrive]

Awful day yesterday so I was unable to get both cars out of the garage and get on with making a couple of backs for my Portadyne and Noble radios with my router jig, which creates a lot of dust. My workshop is too small for such antics - just 10' x 8', but fine for tinkering with radios and woodturning, though they're rather incompatible activities.

Hence, I made a start on the Noble, and as with the Portadyne Princess, as well as the dropper being OC, the mains switch on the pot was open circuit too. I dismantled it hoping to be able to clean the switch and make it operational, but sadly, no such luck. As with the Princess, it's a 50K anti-log track, so finding a replacement is well nigh impossible. I do have a 16mm unswitched anti-log pot, and will try to dismantle it and remove the track as a donor for a switched 16mm diam pot, but I'm not hopeful as they tend to be plastic bodied and the pot I have is metal-bodied. I might ring OMEG - they seem to be a friendly little UK based firm - they'll make any pot to order but the MOQ is 10 items, and the MOV is £25.00 plus VAT & P&P, but they may know of a distributor of 50k anti-log pots. They tend to be used in hi-fi gear, but are invariably unswitched. (OMEG supply firms such as Rapid, CPC and Maplin).

The website I've mentioned before 'Secret Life of Pots' shows what seems to be a simple way of converting a linear pot to an antilog (AKA inverse log, reverse log). Rather too good to be true for my feeble mind - I've spent an hour furtling about as per the diagrams, fumbling around with various resistors, with no outcome other than to get the pot to work as a linear law.

I've graphed the results of several pots. Firstly, the Noble one, which had gone high from 50 K to 68k, and only had a rotation of 240 degress rather than the more usual 270 degrees. Secondly, the plot of a new antilog pot (with no switch), thirdly, a linear pot, (just a straight line of course), and finally a log pot, to illustrate how it diffrers from an antilog pot. Basically, a mirror image of each other.

Secret Life of Pots:

http://www.geofex.com/article_folders/po...tscret.htm

I might try a switched linear pot, which - though not ideal, would certainly perform better than a log pot.

How we suffer for our art - barking mad, the lot of us! Fancy, I could have been doing something that countless thousands of others think is sane - be camped out overnight getting wet through and cold on the Thames Embankment, wearing a flimsy poncho to watch boats float by in the pouring rain, while waving a union flag and singing 'Land of Hope and Glory'. Maybe I'm not so barking mad after all - it's all subjective I suppose. I'm obviously 'potty' though

Anyone know of a source of 50k anti-log switched pots please - I've trawled 'tinternet and come up with nothing.

At least I can solve the dropper problem by doing as before and mounting two 50W metal clad resistors on a heat sink that I've set about making, and can get on with other work on the little Noble.

Hello David, hows about a plot of a linear pot with taper resistor fitted
(top to slider) to simulate a reverse log curve would that be possible?

Lawrence.

[Yorkie]

Hello David, hows about a plot of a linear pot with taper resistor fitted
(top to slider) to simulate a reverse log curve would that be possible?

Lawrence.

When it comes to plots, I'm afraid I've 'lost the plot' with regards to using a taper resistor. It looks so simple and an excellent solution on the 'secret life of pots' diagrams - just add a taper resistor - job's a good 'un! However, try as I might, I either end up measuring just the taper resistor, or the taper resistor + linear pot, and get a linear curve. I understand the concept - at each point on the curve, it is bent upward by the value of the pot at that point + the value of the taper resistor added to it to simulate an anti-log pot. Simples one would think, but not 'numpty proof'!.

I'll have to revisit it hoping for a 'eureka moment' Lawrence

The older I become, the less I seem to know.