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Old 16th May 2019, 7:28 pm   #5
David G4EBT
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Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,737
Default Re: Help with pot codes

Unfortunately, when it comes to trying to identify particular pot tracks there isn’t one standard worldwide practice as to defining the type of track – it depends in which country the pots are made and who makes them. IE:

Linear:

Asia ‘B’
America ‘B’
Europe ‘A’
Vishay ‘A’

Log:

Asia ‘A’
Europe ‘C’
America ‘A’
Vishay ‘L’

Anti-log (also known as 'inverse log' or 'reverse log'):

Europe ‘F’
America ‘C’
Vishay ‘F’

Thus, depending on which country their pots are sourced from, the same supplier may even use both conventions. 20mm Omeg pots sold by several suppliers, linear are marked ‘A’, and log are marked ‘B’. However, smaller 16mm diam pots of different manufacture have the opposite markings – linear are ‘A’ and log are ‘B’. (That was the case with Maplins).

As an aside, almost all of the so called ‘log’ pots that we encounter aren’t actually true ‘log’ pots – they’re what's sometimes known a ‘commercial log’ because they're made down to a price. Basically they use two sections of linear track, one section of a much lower value than the other, which are joined. Hence, instead of getting a smooth log law track, we get two straight lines with a kink to roughly mimic a true log taper, quite adequate for most purposes. You see this if you attach a paper dial marked in degrees of rotation. Most pots have 270 degrees of rotation, so each tenth of the rotation equate to 27 degrees. When I was doing some experimentation in connection with reverse log pots, I made a paper dial to attach to the bush on the pot shaft, then made a pointer from a paper clip to push onto the pot shaft. I then plotted the resistance versus degrees of rotation on a graph. I've attached the dial below in case it might be of use to someone.

For most of the pots in radios that we encounter, generally, it’s a safe bet that volume controls are log, and tone controls are linear, but that can only be verified by resistance measurements at various points on the track. Unless a pot is stamped 'Lin' or 'Log', that's only way of being sure. The reason I went into this was that I was restoring a Portadyne 'Princess' and a similar 'Noble' set. Both have 'minimalist' circuitry, are almost identical and are TRF sets, but unusually the volume control also acts as the reaction control, making for easy tuning. In each case, the on-off switches, ganged to the pot had failed open circuit.

There's only an R&TV circuit on the Princess (nothing on the 'Noble'), and the pot track isn't mentioned. I saw what I thought was a 'C' on one of the pots, so plotted the resistance v rotation which confirmed that it was anti-log. The originals had been Omeg and I got in touch with them to see if they could supply replacements. They could, but with a minimum order quantity of ten. (Blore Edwards couldn't help). As it was, I managed to dismantle the pots and get the switches to work.

There's an excellent paper entitled 'The Secret Life of Pots' which some members of the forum will be familiar with.

It has lots of interesting facts and graphs to show the different tracks:

http://www.geofex.com/article_folder...s/potscret.htm

Hope the ramblings might be of interest.
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