cardboard covered caps dead space ? - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: January 16, 2023, 05:35:07 am
content: I am restoring a old HV keithley power supply. It had a bunch of HV caps in it (400V 20uF).The caps are the kind that twist in a bakelite disk, or a PCB, and then are soldered, and the paper cover is kind of done like a shot gun shell or something, with a paper crimp on top.But the odd thing is, these caps have a card board tube much taller then the capacitor, so there is basically a crimp on top, and if you look inside the top of the cap is an 1/2 inch down.https://www.westfloridacomponents.com/mm5/graphics/F09/CC0740A_411x500.jpgLike these.Is there a reason they don't trim the tube down? So it would be like if you looked on the top of the cap in the picture, the lid would be where the CDE is.It just looks bizzare with the big void on top.

author: jonpaul
author_rank: Super Contributor
timestamp: January 17, 2023, 12:08:09 pm
content: These are "fabricated plate, twist lock" style capacitors, often seen in aluminum can housings.qthe Fiber plate gave insulation from châssis.The empty space might be in case of leak of electrolyte or failures or an industry standard size.Some vintage restorations gut the old capacitor and place a modern axiale or radial inside.The empty space can be filled with foam or wax filler.Preserves the appearance of original cap.Jon

author: coppercone2
author_rank: Super Contributor
timestamp: January 18, 2023, 01:01:50 am
content: Well the weird thing is the top cover is loose. But there is 10 caps I replaced, and all of them are built the same way. So the top cover is free to rattle about essentially. and I found them all sitting on top of the capacitor top, so its a bunch of little wells/pools.They are mallory brand, and there is no scoring on top, I don't think they have a over pressure seal there. I reuse the bases of twist lock capacitors if they are in bakelite, but since they are soldered to a PCB, there is no point and its alot of extra work and possibly a reliability issue IMO.FP144A under the card board.If it was scored it would kind of make sense to use the card board as containment, but its notReusing the terminals not tied to shield is dodgy too, because they look to be made of a non solderable material inside of the capacitor, so you would need to drill a hole in it anyway to wire it to the center terminal.And also that bakelite smells like electrolyte, I tried to ultra sonic clean it, wash it in baking soda, bake it at 80C for a few hours, etc.. I think it builds up in the old bakelite. Can't shake the smell. I don't like using the chemical smelling materials if possible., so I only bother if their chassis mount isolated ones (where it requires a mechanical support), so I put new capacitors + silicone on PCB mounted twist locks with a direct connection.But I am surprised Malory or keithley did not bother cutting the tube to length so they are built the same as the other capacitors. It makes it look dilapidated/dodgy

author: coppercone2
author_rank: Super Contributor
timestamp: January 18, 2023, 01:25:33 am
content: And yeah I thought that I would find some DIY capacitor replacement inside when I saw those tubes like that, but it looks stock...

author: coppercone2
author_rank: Super Contributor
timestamp: January 18, 2023, 01:27:19 am
content: and is the covering meant to be thermal insulation? The new caps are like 1/4 the size though, so they should be much further away from the tube, hope it will be alright, I should measure with TI after I repair it. Run it on a load for a while and then pop the cover and check temp.