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Old 13th Sep 2021, 6:05 pm   #1
Superscope
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Default Strange Pre-set Capacitor

Another odd componant, at least for me!

Can anybody tell me anything about this strange pre-set Capacitior?
Not seen one like this before.

I had assumed it was an odd connector for the wiring loom, but finaly
I realized it was the Pre-set Variable Capacitor I was looking for on the
Schematic.

Are these Factory pre-set and fixed, or can they be re-adjusted?

There are no markings on it, but the Schematic suggests it's 3-30mmfd.
I have mearsured it at around 10.77 nF
Is mmfd nF or pF ?


Thanks in advanced for any enlightenment!


Ian
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Old 13th Sep 2021, 6:12 pm   #2
G6Tanuki
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Default Re: Strange Pre-set Capacitor

Is it in a Philips radio??

The basic idea is a tube with a wire up the centre and fine wire wound over the outside. You adjust it by unwinding turns of the fine wire.

A commercially rendered version of the twisted wire ' gimmick ' capacitor.

Last edited by G6Tanuki; 13th Sep 2021 at 6:19 pm.
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Old 13th Sep 2021, 6:18 pm   #3
ms660
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Default Re: Strange Pre-set Capacitor

Quote:
Originally Posted by Superscope View Post
Is mmfd nF or pF ?
pF.

Lawrence.
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Old 13th Sep 2021, 6:29 pm   #4
turretslug
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Default Re: Strange Pre-set Capacitor

Trombone trimmer. Similar principle to the piston trimmer, but more often seen in low-cost applications, I've seen a few in radio front ends and test-gear compensation, sometimes there's a simple friction lock with a couple of brass ears to squeeze to release it.
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Old 13th Sep 2021, 7:12 pm   #5
PJL
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Default Re: Strange Pre-set Capacitor

Piston trimmer also seen in Philips sets from early 30's but that doesn't narrow it down much as Philips changed their trimmer type every couple of years.
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Old 13th Sep 2021, 7:19 pm   #6
TowerRadio
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Default Re: Strange Pre-set Capacitor

This is in the Avo test bridge,probably used to pad a cap used in the lower capacity range.Les
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Old 13th Sep 2021, 8:37 pm   #7
Superscope
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Default Re: Strange Pre-set Capacitor

Thanks for the replies.

Well, I have to say, on closer inspection, it does indeed appear to have
a very Fine winding round the outside.

Who would have known? Apart from everybody except me by the looks of it!

Almost invisible if I hadn't been drawn to looking closely (under a
Magnifiying Glass) by Post #2.
It looked like just a Brass Tube!

Yes, Les, it is indeed an Avo Test Bridge, the subject of my other Thread,
and Yes it appears to Pad C1 (85pF) in the Lower Capacitance Range.
I assume it is there to get the 85 pF 5% Capacitor to within 0.5% as specified
on the Schematic.
I thought the 0.5% Tolerance was a bit optimistic! Actually, I thought it was a Typo!

If mmfd is pF, then I guess it is way out of Spec at 10.77 nF!
So it will need changing out like the rest of the Caps.

Not surprisingly, pretty much all the Caps are well out of Spec, but most the
Resisters are still in.

If it's a Cheap and cheerful componant as suggested, it's a bit of a surprise
Avo would have used it in such an Instrument.




Ian
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Old 13th Sep 2021, 8:56 pm   #8
ms660
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Default Re: Strange Pre-set Capacitor

Quote:
Originally Posted by Superscope View Post
Yes it appears to Pad C1 (85pF) in the Lower Capacitance Range.
I assume it is there to get the 85 pF 5% Capacitor to within 0.5% as specified
on the Schematic.
So far as I can make out from a schematic it's to get the total capacitance to 100 times less than the capacitor on range C2, the capacitor on range C2 being 100 times less than that on range C3.

Lawrence.
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Old 13th Sep 2021, 9:03 pm   #9
Superscope
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Default Re: Strange Pre-set Capacitor

Now that definately makes more sense now you come to mention it!




Ian
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