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 Post subject: Rebuilding the 1" diameter FP twist lock capcitor.
PostPosted: Apr Fri 03, 2020 11:57 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 569
In the past I have not re-built the twist lock style capacitors because of the availability of the new new stock (NNS) twist lock types available from AES. However, the 1" diameter type are not as common in the re-manufactured version or a large range of values. Recently I needed a replacement for a 1" diameter type. So I decided to buy some vintage NOS capacitors to act as a donor canister.

I started with a vintage Sprague type, by drilling a closely spaced array of 2mm holes in the phenolic base, and cutting between them with sharp nose side cutters to allow removal of the connector pins and make a start to get inside the capacitor. Then I wrapped tape around the body of the capacitor and held it in the chuck of my mini lathe and machined it out to about 18.7mm diameter. This is about as large as it can go without damage to the twist lock pins.

Then it was a matter of slowly picking out the old paper & foil. It would not slide out as a whole because the 18.7mm was smaller than the outer diameter of the foils, but I broke down the central part with a 10mm drill to help fragment it.

Inside the capacitor I found "The nemesis" Latex rubber. This is very destructive to the internal metalwork and workings of not only vintage electrolytic capacitors, but also vintage radio vibrators as well. With age the latex melts into a corrosive brown liquid. This eats into aluminium and other metals. I published an article once showing how it caused some capacitors to go open circuit by corroding through the internal interconnects. The most likely reason it does this is Halide contamination of the Latex.

http://worldphaco.com/uploads/ELECTROLY ... FRAMES.pdf

(one interesting thing is that the latex rubber has three phases, the soft sponge when it is new, the tacky brown liquid when it melts and the vapour phase where in a sealed container it re-deposits out on surfaces. This is why the contacts in radio vibrators get a tacky coating become insulated from each other and stick together too)

In any case when I opened the donor capacitor up there was significant Latex induced corrosion on the top of the inside surface of the aluminium canister where the latex had melted onto it. I cleaned this off as best possible and varnished the surface. If left alone I think one day it would have corroded through the capacitor's top to the outside world. I'm not sure if it was only Sprague that put latex sponge rubber inside electrolytic caps, but it turned out to be a very bad idea. It has appeared to have been used as a space filler and affects capacitors of the 1965 to 1975 era, check out the article cited above.

I made a plug/disc for the base of the capacitor out of 6mm thick (close to 1/4") insulating material, the white material I used (called Bramtie) is a lot like fibreglass, which would also have worked, or another type of thick insulation material like phenolic or Paxolin plate, Garolite or Tufnol. Acrylic or polycarbonate would also work. This disc of material holds the new terminals and the replacement capacitor.

I attached some terminals. It is important to have permanent marks for the polarity of the terminals that won't wipe off, so I drilled a small countersink and filled it with red & black paint. Also a label was applied (from a Brother label machine) I find its better to wrap the label over itself about half a turn on top of its circumference and its less likely to peel off later. The plug was made about 18.65 mm diameter and was placed into position and now held there with 24hr epoxy resin.

The attached photos tell the story.

So this is a first attempt. I'm going to re-build a few more with alternate methods & materials.


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capx.jpg
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capy.jpg
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 Post subject: Re: Rebuilding the 1" diameter FP twist lock capcitor.
PostPosted: Apr Fri 03, 2020 5:43 pm 
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Joined: Jan Mon 29, 2018 5:58 pm
Posts: 241
Location: Nipomo, CA 93444
That looks great! What holds the plastic disk in place? Friction??

_________________
-Jim
W6JCR


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 Post subject: Re: Rebuilding the 1" diameter FP twist lock capcitor.
PostPosted: Apr Fri 03, 2020 10:41 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 569
Jimmler wrote:
That looks great! What holds the plastic disk in place? Friction??


As noted in the text above it is held there by 24hr epoxy resin (Araldite here, JB Weld in the USA)

Since the original insulating base is made of two phenolic plates and a middle rubber layer, when its machined out, the rubber machines well (I didn't expect that), but because it is softer it comes out about 0.1mm undersize compared to the phenolic part. So the white machined disc that just slips through the 18.7mm hole in the phenolic plates interferes with the rubber, so when it is pushed into position, it is firm. So prior to doing that I coated both mating surfaces with the 24 hr resin and pushed it into position, where it stayed until the glue dried. It is incredibly strong now, and would be a job to remove it.


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 Post subject: Re: Rebuilding the 1" diameter FP twist lock capcitor.
PostPosted: Apr Sat 04, 2020 5:30 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 569
I've done a second one.

This time I made the plug for the base out of brown phenolic material that better matches the original and gone to a 10mm thick plug. The material supports a thread well, so I got rid of the nuts & washers of the previous design and cut a 2mm metric thread with a taper tap. This results in the 2mm x 10mm round head screws just starting to bind in the thread when they are tightened up making them shake proof. However, once the lugs are soldered to the capacit wires, they cannot rotate anyway, being an extra margin of safety.


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 Post subject: Re: Rebuilding the 1" diameter FP twist lock capcitor.
PostPosted: Apr Wed 08, 2020 2:28 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 569
I have settled on the method now (see photo) it appears the better material is the Brown 10mm thick phenolic, it is a better color match for the base and it also takes a thread well. Two 2mm diameter 10mm long screws and two lugs, no washers or nuts required.

If you are going to bother doing this sort of capacitor re-build, it pays to use high quality caps as replacements. The ones in the photo are Panasonic brand 105 deg C types (other good ones are Nichicon or Rubycon- my advice is to "Go Japanese" for electrolytics) In addition don't forget to label the polarity and and the electro's value & voltage, I have also added a date code so that I know when the part was rebuilt.

The replacement caps in this case are 47uF 450V and said to be 18mm diameter, they measure about 18.1mm.


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 Post subject: Re: Rebuilding the 1" diameter FP twist lock capcitor.
PostPosted: May Tue 05, 2020 5:39 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 10710
Location: Omak,wa,usa
Hello ACORNVALVE,
nice job


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 Post subject: Re: Rebuilding the 1" diameter FP twist lock capcitor.
PostPosted: May Tue 05, 2020 6:12 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3526
Location: Seattle WA US
Very nice work. Can you suggest a source for the brown phenolic, in small quantities ?
Are you starting from phenolic rod, or using a plug cutter on sheet stock ?
Thanks,
Chuck K7MCG


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 Post subject: Re: Rebuilding the 1" diameter FP twist lock capcitor.
PostPosted: May Tue 05, 2020 6:28 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 569
K7MCG wrote:
Very nice work. Can you suggest a source for the brown phenolic, in small quantities ?
Are you starting from phenolic rod, or using a plug cutter on sheet stock ?
Thanks,
Chuck K7MCG


I just start with flat plate and a hole saw in the drill press (without he central drill bit).

The insulating plate and variants of it is common, its on ebay, but you could probably find it locally as used a lot in the electrical industry.

https://www.ebay.com/itm/300x300x10mm-B ... SwGJlZJ-7B

Fiberglass plate would also work.


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