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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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Old 28th Apr 2020, 11:46 am   #1
RF Burn
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Default Variable Capacitor Types

Can anybody tell me which law these two capacitors in the pictures follow?

I gather that other types with both sets of plates semi-circular and mounted at the centre are Straight-Line-Capacitance (SLC) law.

There are other types which I can't find in my hoard that have both sets of plates semicircular but mounted offset from the centre (A) and one set of plates semi-circular but the other set reducing in diameter like part of a spiral also mounted offset from the centre (B). I would guess these are Straight-Line-Wavelength (SLW) types.

Looking up some American info there seems to be several variations on the simple laws anyway, It is confusing!

Adrian
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Old 28th Apr 2020, 12:04 pm   #2
M0FYA Andy
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Default Re: Variable Capacitor Types

Connect a capacitance meter and a pointer against a scale and measure them!

Andy
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Old 28th Apr 2020, 12:34 pm   #3
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Default Re: Variable Capacitor Types

Thanks Andy, not having a capacitance meter, I had thought about rigging up an oscillator to plot the change of frequency with rotation on my frequency meter.

However, I am in the middle of sorting out keepers, sellers and binners (from an enormous hoard of parts and kit collected over a lifetime) ready for our retirement move to a smaller home, the missus would kill me if I stopped sorting and started building something! I was hoping that one of you guys with experience of these parts might avoid that ;-)
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Old 28th Apr 2020, 12:38 pm   #4
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Default Re: Variable Capacitor Types

Just photograph the capacitors, showing the shape of the plates like you've done for that one, and let the buyers decide.

David
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Old 28th Apr 2020, 2:02 pm   #5
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Default Re: Variable Capacitor Types

The more you look at something the more likely you are to keep it. The more quickly you get rid of it the more likely you are to need it very soon.
An aspect of Murphy's Law.
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Old 28th Apr 2020, 6:07 pm   #6
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Default Re: Variable Capacitor Types

Quote:
I would guess these are Straight-Line-Wavelength (SLW) types.
My guess to at first glance.
 
Old 29th Apr 2020, 6:08 am   #7
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Default Re: Variable Capacitor Types

Variable caps have laws? Who knew? It's only 6.00AM and I've learnt something new already! Will look into this futher and have a little drawer of these, will check em out.

Andy.
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Old 29th Apr 2020, 7:17 am   #8
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Default Re: Variable Capacitor Types

Not only that, Andy, but a lot of them have little cuts in the outermost plates of each section on the rotor so you can bend the plates outwards a little to do vernier adjustment of the law to twiddle it so each marked point on the tuning dial is spot on. Everyone's seen the slots but very few know what they were for.

The skilled capacitor tweaker must be one of the most obscure lost professions.

David
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Old 29th Apr 2020, 10:59 am   #9
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Default Re: Variable Capacitor Types

Would you have to start the tweaking by bending all the tabs out a bit so that increases as well as decreases in local capacitance could be made?

I guess only professional equipment would get this attention to detail, not your common or garden wireless!
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Old 29th Apr 2020, 12:54 pm   #10
RF Burn
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Default Re: Variable Capacitor Types

Quote:
Originally Posted by greenstar View Post
The more you look at something the more likely you are to keep it. The more quickly you get rid of it the more likely you are to need it very soon.
An aspect of Murphy's Law.
I agree with you on that, got to be ruthless otherwise the keepers pile just gets too big! Most of my stash are the ubiquitous JB C804 SLC types, so just keeping the good clean examples of those, however I will definately keep any that I can determine are SLF.

I have found a couple of similar two gang jobs, the two sections contain different numbers of plates (as expected for RF and LO tuning) but strangely they have different plate profiles, the front one (usually RF) is SLC but the rear one (usually LO) has a different law - maybe designed for some specific kit?
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Old 29th Apr 2020, 1:40 pm   #11
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they have different plate profiles,
That keeps the LO and RX frequency in step without the need for a padder capacitor. If you make 1000's of sets the saving can be significant. Best for one band only (MW is the usual setup) here we have LW and it can be a bit of a compromise for that band, if centred on 198kHz (200kHz when made I suppose) they work well.
 
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