Pot characteristics - UK Vintage Radio Repair and Restoration Discussion Forum

squinancy · 23rd Oct 2007, 8:51 am

Could someone help me please?
In my quest for a pot to replace a 500 k log switched volume pot that was completely beyond hope (track simply crumbled away), I managed to come into possession of quite a haul of used and unused 470 k log's. I measured the resistances of the tracks for all of them which produced an unexpected result. None measured exactly what is stated on it, more importantly, few go down to 0 ohm at the end of the track.

To put it simply, my question is: Should I choose one that measures for instance, around 560 k at one end but would not reduce below 30 - 10 ohms at the other end (wouldn't this result in sound still being heard when the set should be completely quiet?) or one that measures max 440 k at one end but duly diminishes to practically zero at the other end? My instinct calls for the latter, especially as it has a metal shaft as opposed to the 560/10-30 ones which are newly manufactured and come with a plastic shaft all the more difficult to attach couplings to to fit to original conditions. I'll be most interested to see expert opinions. Many thanks! - Squinancy

ppppenguin · 23rd Oct 2007, 9:21 am

The tolerances really don't matter. For a 500K nominal pot anywhere between 400K and 600K is fine. Even 200K or 800K would likely cause little or no change in circuit performance.

As for end resistance, 30 ohms as a fraction of 500K represents a huge attenuation, more than 80dB. Clearly not a problem.

GMB · 23rd Oct 2007, 9:21 am

I think you expect too much from the accuracy of pot values! Unless part of some measuring equipment I think being within 20% is fine and is as good as it got.

squinancy · 23rd Oct 2007, 10:02 am

Sounds reassuring. Thanks.

Leon Crampin · 23rd Oct 2007, 4:11 pm

Some time ago I was restoring a DAC90A for a friend to a high standard. My favourite pots are the Plessey ones with the solid carbon brush for a wiper and a smaller carbon brush for the centre ring. These are almost everlasting.

Not having a 470k + switch to hand, I made a hybrid from a 100k switched pot and a 470k log unswitched pot. By swapping the tracks, I ended up with the required 470k log + switch.

On trial in the radio it was useless. I hadn't realised that the tracks are different in switched pots and have a dead zone around the point at which the switch operates, to give minimum volume. "My" pot had a fully active track throughout its length, meaning that the volume could not be reduced below a highish minimum until the switch operated - useless.

So, make hybrids with care. Mine had no noise though.

Leon.

squinancy · 23rd Oct 2007, 10:56 pm

This is why I insisted with Bowood Electronics that I needed 470 k switched pots and could wait for them as long as necessary. While I did not know about the "dead zone", I have given up on transplants some years ago due to a series of total failures. Bowood are exceedingly nice people trading in increasingly rare items. They are among the last sources of new 2M log switched pots which they stress they obtain from a non-Chinese manufacturer.

23rd Oct 2007, 8:51 am	#1
squinancy Retired Dormant Member Join Date: Aug 2007 Location: New Malden, London, UK. Posts: 70	Pot characteristics Could someone help me please? In my quest for a pot to replace a 500 k log switched volume pot that was completely beyond hope (track simply crumbled away), I managed to come into possession of quite a haul of used and unused 470 k log's. I measured the resistances of the tracks for all of them which produced an unexpected result. None measured exactly what is stated on it, more importantly, few go down to 0 ohm at the end of the track. To put it simply, my question is: Should I choose one that measures for instance, around 560 k at one end but would not reduce below 30 - 10 ohms at the other end (wouldn't this result in sound still being heard when the set should be completely quiet?) or one that measures max 440 k at one end but duly diminishes to practically zero at the other end? My instinct calls for the latter, especially as it has a metal shaft as opposed to the 560/10-30 ones which are newly manufactured and come with a plastic shaft all the more difficult to attach couplings to to fit to original conditions. I'll be most interested to see expert opinions. Many thanks! - Squinancy

23rd Oct 2007, 9:21 am	#2
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Pot characteristics The tolerances really don't matter. For a 500K nominal pot anywhere between 400K and 600K is fine. Even 200K or 800K would likely cause little or no change in circuit performance. As for end resistance, 30 ohms as a fraction of 500K represents a huge attenuation, more than 80dB. Clearly not a problem.

23rd Oct 2007, 9:21 am	#3
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,100	Re: Pot characteristics I think you expect too much from the accuracy of pot values! Unless part of some measuring equipment I think being within 20% is fine and is as good as it got.

23rd Oct 2007, 10:02 am	#4
squinancy Retired Dormant Member Join Date: Aug 2007 Location: New Malden, London, UK. Posts: 70	Re: Pot characteristics Sounds reassuring. Thanks.

23rd Oct 2007, 4:11 pm	#5
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,874	Re: Pot characteristics Some time ago I was restoring a DAC90A for a friend to a high standard. My favourite pots are the Plessey ones with the solid carbon brush for a wiper and a smaller carbon brush for the centre ring. These are almost everlasting. Not having a 470k + switch to hand, I made a hybrid from a 100k switched pot and a 470k log unswitched pot. By swapping the tracks, I ended up with the required 470k log + switch. On trial in the radio it was useless. I hadn't realised that the tracks are different in switched pots and have a dead zone around the point at which the switch operates, to give minimum volume. "My" pot had a fully active track throughout its length, meaning that the volume could not be reduced below a highish minimum until the switch operated - useless. So, make hybrids with care. Mine had no noise though. Leon.

23rd Oct 2007, 10:56 pm	#6
squinancy Retired Dormant Member Join Date: Aug 2007 Location: New Malden, London, UK. Posts: 70	Re: Pot characteristics This is why I insisted with Bowood Electronics that I needed 470 k switched pots and could wait for them as long as necessary. While I did not know about the "dead zone", I have given up on transplants some years ago due to a series of total failures. Bowood are exceedingly nice people trading in increasingly rare items. They are among the last sources of new 2M log switched pots which they stress they obtain from a non-Chinese manufacturer.