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Capacitor Trimmers..

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Capacitor Trimmers..

Post by Wolfie » Thu Aug 11, 2016 10:20 am

You guys will know...

How do we tell what value these are?? I can't find any numbers or even marks, this yellow one is supposed to be 2-25pf, but then I have these green ones also in a drawer marked 2-25pf.

I've signed up for a Uni course to learn to be psychic or psychiatric, can't remember which.


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Re: Capacitor Trimmers..

Post by sideband » Thu Aug 11, 2016 10:46 am

I don't think colour has anything to do with value. Normally there is a value code on them.

The only other thing you can do if your test meter has a capacitance range is to measure them

 
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Re: Capacitor Trimmers..

Post by Refugee » Thu Aug 11, 2016 4:13 pm

With those I have within reason been able to "guestimate" the value from the number of vanes in them.

 
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Re: Capacitor Trimmers..

Post by Marconi_MPT4 » Thu Aug 11, 2016 6:42 pm

Hi,
Well, according to my crystal ball, these are Mullard (Philips?) 808 series Polypropylene Film Trimmer Capacitors.

Rated at 250V DC, Insulation resistance >10000MΩ

They are (were) available in four flavours:

808 00004, 1.4pF to 5.5pF Colour GREY, diameter 7.7mm, Power dissipation 0.35W*, Self resonance > 800MHz
808 00005, 2.0pF to 10.0pF Colour YELLOW, diameter 7.7mm, Power dissipation 0.35W*, Self resonance > 600MHz
808 00006, 2.0pF to 22.0pF Colour GREEN, diameter 7.7mm, Power dissipation 0.35W*, Self resonance > 400MHz
808 01001, 5.5pF to 65.0pF Colour YELLOW, diameter 10.5mm, Power dissipation 0.9W*, Self resonance > 200MHz

* @175MHz

Rich

 
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Re: Capacitor Trimmers..

Post by sideband » Thu Aug 11, 2016 6:59 pm

Well,well,well....you live and learn! :aad

 
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Re: Capacitor Trimmers..

Post by Wolfie » Thu Aug 11, 2016 11:05 pm

Cheers Rich, that's the sort of guide I was hoping for..

But now I think my Circuit diagram is wrong.. drat.. in the position where it shows a 22pf, there was actually a Yellow (larger diameter) trimmer, it was crackly on adjustment, as if shorting or going open in some positions, but I did have up to a watt or so of OP Power (it should be 10W).
Replacing it with a 2-22pf has killed it totally, but assuming the diagram was correct I then went looking elsewhere and now my hair is much thinner.... So I should be fitting a 65pf there instead. Doh! :zx:

AND... I need lessons in how to peak up a Transmitter (step by step) since I've forgotten nearly everything I ever knew on the subject, stand by for another Post.

 
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Re: Capacitor Trimmers..

Post by Marconi_MPT4 » Fri Aug 12, 2016 3:21 pm

Wolfie wrote:Cheers Rich, that's the sort of guide I was hoping for..
..there was actually a Yellow (larger diameter) trimmer, it was crackly on adjustment, as if shorting or going open in some positions...


Over time these trimmers tend to develop a slightly higher contact resistance that varies with rotation. At VHF it causes unacceptable frequency drift and inability to set up properly. I have found that to improve matters just use one or two drops of Philips Contact Cleaner and Lubricant (CCL) as shown in diagram. Be careful not to use too much! Give it a few turns and usually good to go.

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Rich


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