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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 22nd Sep 2018, 6:17 am   #1
FrankB
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Default Low PF Capacitor Home Brew

If you need a low PF capacitor, there are 3 ways I have made them.
1. Twist 2 wires together and cut down until you get the correct PF rating.

2. Use a length of coax cable. The type of coax and pf/ft varies with the type (and sometimes the mfg.) It also varies with the percentage of shield. Mfg. specs. will give you pf/ft rating.

3. Use zip cord, speaker wire or similar twin wires and cut to length for the correct pf you need. You can cut a ft., meter, or whatever unit you want and measure the PF for the length, then cut it to size for correct pf.
Added bonus: I have seen twin wire cord used as spark gaps in TV sets too.

This type of capacitor is great for tweaking that last little bit to get an oscillator on freq. down to a gnats hair.
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Old 22nd Sep 2018, 8:11 am   #2
Cobaltblue
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Default Re: Low PF Capacitor Home Brew

Manufacturers sometimes use homebrew capacitors as well. ISTR that Philco used an insulated wire wrapped around the tag of the tuning capacitor on models such as the 444.

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Old 22nd Sep 2018, 9:09 am   #3
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Default Re: Low PF Capacitor Home Brew

During my early career in video Broadcast, the modulator (using MC 1495 Ic's) card used this method. The technique was to twist some Kynar wire together and then cut it down to reduce the capacitance until the vectors were in the boxes.

Problem was when you tried to neaten up the arrangement and secure it the capacitance often changed. Depending on which way it went, you might have to de solder the Kynar and start all over again....... Then you used to go off for a tea break and come back to find everything out of spec - happy days - not
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Old 22nd Sep 2018, 9:34 am   #4
Craig Sawyers
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Default Re: Low PF Capacitor Home Brew

I did exactly that in my first job in the early 80's. There was a video amp, and to tweak up the pulse response I used the twisted wire method.

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Old 22nd Sep 2018, 10:38 am   #5
Trigon.
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Default Re: Low PF Capacitor Home Brew

When you need a 1kV 27pF for a late night emergency fix for an LCD monitor backlight, and you've only got some PTFE covered wire:-

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A 'temporary' fix, still in place three years on...

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Old 22nd Sep 2018, 10:39 am   #6
Silicon
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Default Re: Low PF Capacitor Home Brew

The twisted wire capacitors are also called 'gimmick' capacitors.
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Old 22nd Sep 2018, 5:34 pm   #7
ionburn
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Default Re: Low PF Capacitor Home Brew

One presumes that if you use enamelled wire twisted together the capacitance can be fairly high (relatively)
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Old 23rd Sep 2018, 1:31 pm   #8
Herald1360
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Default Re: Low PF Capacitor Home Brew

And the breakdown voltage disappointingly low?
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Old 24th Sep 2018, 9:48 am   #9
ionburn
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Default Re: Low PF Capacitor Home Brew

Quote:
Originally Posted by Herald1360 View Post
And the breakdown voltage disappointingly low?
Yes there is that but on a quick check online it seems that it might not be as bad as it sounds, at least for receivers:


http://www.magnetwire.biz/soderon155_data_sheet.pdf

I do notice though that with the standard stuff from RS (for example), the voltage rating is not stated.
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Old 24th Sep 2018, 10:32 am   #10
Herald1360
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Default Re: Low PF Capacitor Home Brew

Interesting that its breakdown voltage is higher at higher temperature.

Certainly looks good for 300v or so operating voltage. I wonder what the dielectric properties of the insulation are like?
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Old 25th Sep 2018, 6:21 pm   #11
G6Tanuki
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Default Re: Low PF Capacitor Home Brew

The coax "gimmick" capacitor is quite well-known in ham radio circles: one popular use is as the resonating capacitor in antenna-traps, where the coax 'capacitor' can be conveniently stuffed into the inside of the coil-former once you've GDO'd the trap to the required frequency.
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Old 26th Sep 2018, 6:09 pm   #12
ex seismic
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Default Re: Low PF Capacitor Home Brew

Spot the gimmick cap in this WW2 dinghy homing beacon transmitter.

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Old 26th Sep 2018, 6:42 pm   #13
VT FUSE
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Default Re: Low PF Capacitor Home Brew

I presume that TX is "Walter" somewhere I have two brand new triodes as shown in your example and often wondered what the unit looked like out of its casing.

Are you able to post another photo,nice implementation of a gimmic capacitor in the tank circuit (I presume).
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Old 27th Sep 2018, 8:38 am   #14
ex seismic
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Default Re: Low PF Capacitor Home Brew

Yes, a Walter. Another photo and full info here: https://www.royalsignals.org.uk/photos/AN_CPT-2.htm

gmb
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Old 28th Sep 2018, 7:25 pm   #15
VT FUSE
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Default Re: Low PF Capacitor Home Brew

Most kind of you,no idea how that reference had slipped me. I bet it would have been quite effective, but do wonder how well a dunked Airman,possibly with bad injuries would have coped deploying it.

Thanks for posting the link.
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