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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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31st Oct 2019, 12:34 pm | #1 |
Octode
Join Date: Jun 2013
Location: Huddersfield, West Yorkshire, UK.
Posts: 1,782
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LOG/ANTI LOG pots
Hi peeps. Just a massive thanks to the forum....my latest project is re purposing and Eddy EC10 receiver.......as most of you know it was not the best product Eddystone produced, but at the time was.. the "cats whiskers".. other terms are available.....
Quite a while ago I rebuilt it using silicon devices and Plessey SL series IC's and it was re jigged as an Amateur bands only radio. The problem was the Local Oscillator was still very microphonic and drifty... as the long wires to each coil set didnt help I decided to strip out the whole shebang and fit 5 x Oscillators each relay switched with varicap tuning. The problem was using a linear tuning pot obviously the tuning was cramped at LF and stretched at HF ends. I tried a log pot, and then reversed it so that the tuning was HF at the left end... but that was a bit "odd". Fortunately the forum had the solution... i remember seeing something about making Lin pots into log pots and anti log pots. The situation now... while not ideal, is much better, so a Dual 10 K lin pot strapped with a 2K2 at the high end to slider makes a psudo antilog pot. Why a dual pot ?? If two tracks are being used, it should lower the chances of dead spots...well thats my theory anyway.
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Should get out more. Regards Wendy G8BZY |
31st Oct 2019, 3:59 pm | #2 | |
Rest in Peace
Join Date: Sep 2010
Location: Ripley, Derbyshire, UK.
Posts: 785
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Re: LOG/ANTI LOG pots
Quote:
However, your comment about using two pot tracks might be Ok in audio applications, but with the pots controlling D.C., the "dead spots" simply become half-dead spots, and the output from the pot slider will still fall or rise to some indeterminate level at the dead spot, therefore, I believe, dead spots will still upset the tuning. Tony. |
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1st Nov 2019, 12:02 am | #3 |
Octode
Join Date: Jun 2013
Location: Huddersfield, West Yorkshire, UK.
Posts: 1,782
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Re: LOG/ANTI LOG pots
Well Boxdoctor... As the pot is new.. and on its way from China.. hopefully its overkill anyway.
__________________
Should get out more. Regards Wendy G8BZY |
1st Nov 2019, 9:58 am | #4 |
Rest in Peace
Join Date: Sep 2010
Location: Ripley, Derbyshire, UK.
Posts: 785
|
Re: LOG/ANTI LOG pots
Ah ! I did not know you were using a brand new pot for the job. I guessed from your comments that you might be using a somewhat less than pristine one. A brand new one obviates the necessity for such precautions anyway.
Good luck with the project. I am a great believer in in adapting equipment to a specific purpose instead of letting it sit idle for evermore. Cheers, Tony. |
1st Nov 2019, 4:33 pm | #5 |
Octode
Join Date: Feb 2009
Location: Bletchley, Buckinghamshire, UK.
Posts: 1,205
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Re: LOG/ANTI LOG pots
I have used a few 10k 10-turn pots for tuning varicaps. Normally the pot sits in a voltage divider chain between a pair of fixed resistors to set the upper and lower limits. I've always been able to get good linearity by slugging the tuning pot with a fixed resistor to earth, i.e. turning it into a log or semi-log type.
A typical value for a 10k tuning pot is 18k but you have to experiment, as with the limit resistors. I start out with trimmers and substitute with fixed resistors when the range and linearity have been optimised. |
1st Nov 2019, 11:44 pm | #6 |
Octode
Join Date: Jun 2013
Location: Huddersfield, West Yorkshire, UK.
Posts: 1,782
|
Re: LOG/ANTI LOG pots
Hi Dave.... In the EC10.. I replaced the Variable ganged cap with a Dual gang pot, obviously you only get a half rotation from end to end. So I gave myself about 5% at the top end to stop crashing the endstops of the pot. With 5K in total I will end up with a 1K in parallel .. or somewhere abouts.
Just making a new main pcb, as the MC1496 is good for SSB, but not for AM, even unbalancing the balance pot. So using a SL1623 AM/SSB detector. So the line up will be SL1610..MC1496 first mixer, 4.43 Mhz filter.. SL1612 IF amp, then SL1623, and SL621 AGC control/S Meter. As its amateur bands only, I can use fixed tuning RF Filters, and just a varicap VFO. I will post some details when its more complete.
__________________
Should get out more. Regards Wendy G8BZY |