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Repairing the ceramic end bar on a large variable capacitor




 
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Author Topic: Repairing the ceramic end bar on a large variable capacitor  (Read 1022 times)
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VA3AEX
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« on: September 09, 2018, 05:08:52 PM »

Got a large 7500v variable cap at a Hamfest with a broken ceramic end bar. Looks like someone over torqued the nuts on the cap. Tried using Crazy glue to repair it but it didnít hold (at all). Whatís the best way to repair ceramic?
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KD6VXI
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« Reply #1 on: September 09, 2018, 05:24:51 PM »

Epoxy.

Get a non electrical one.  JB Weld makes several types.

--Shane
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Patrick J. / KD5OEI
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« Reply #2 on: September 09, 2018, 11:23:36 PM »

JB Weld replied to an inquiry from someone here  (can't find this now) that the classical stuff with 'metal' in it for fixing engine blocks is not conductive.  

glass & ceramic epoxy from Home Depot was suggested in google never tried it myself.
https://www.homedepot.com/p/PC-Products-16-oz-PC-Clear-Liquid-Epoxy-070161/203565720
says 2450 psi tensile strength. Maybe one of the mechanical engineers here can comment?

an industrial possibility?
https://www.atomadhesives.com/Porcelain-adhesive-24-hr-cure

The two above may be overkill but why not throw a couple options in.. ..


Just my opinion based on 2 such repairs:
Epoxy "jb weld engine repair with metal" becomes at best plastic-like so it will stick to the rough sides of the crack but not always as well to the shiny ceramic finishes. Even so, overlapping onto the shiny part may be helpful. It was perfectly fine for plate tuning /antenna tuner caps where no movement force was put on it.

Please post your repair images and your opinion of the results for the sake of everyone's learning how to do it better!
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VA3AEX
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« Reply #3 on: September 10, 2018, 08:19:06 PM »

Thanks for the pointers. Went to a big box store tonight and got some J-B Weld ĎClearí (apparently its good for ceramics).  I plan on getting to it in the next couple of days. Will post pics and let you know how it worked. 73
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MikeKE0ZUinkcmo
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« Reply #4 on: September 13, 2018, 08:11:23 AM »

Be sure to remove all vestiges of the Super glue before applying the JB weld.   Acetone is a good solvent for the SG, I understand.
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WD8BIL
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« Reply #5 on: September 13, 2018, 12:15:04 PM »

Get your hands on some 1/4" Lexan and make a new one.
Done it many times.
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PA0NVD
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« Reply #6 on: September 13, 2018, 03:02:03 PM »

Lexan?  That is no very good RF insulator, has quite some losses at RF. I did fry some lexan parts with high RF voltages and this cap may be used with very high RF voltages. I normally use glass filled PTFE of HDPE  or PP  Many kitchen cutting boards are PP and are very low loss for RF, a good source.
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WD8BIL
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« Reply #7 on: September 13, 2018, 03:46:39 PM »

Never had a problem with lexan all the way up to legal limits.
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N1BCG
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« Reply #8 on: September 14, 2018, 10:34:56 AM »

I picked up a roller inductor at a hamfest at a great price because, as the seller explained "the broken end made it hard to sell".  Indeed. While all the parts were present, a terminal and some stainless steel hardware had separated and was hanging loose due to broken off and chipped ceramic.

Rob, N1ALF, offered to take a shot at repairing it and the end result was remarkable. The method included mixing a compound of tile grout and HST-7 Super T glue and applying it into a mold made from construction paper. A dremel was used to shape the section to match the original ceramic.

A "before" image would have illustrated the extent of damage that makes the repair that much more amazing. Thanks Rob!



* RollerRepair2.jpg (983.35 KB, 4032x3024 - viewed 92 times.)

* RollerRepair1.jpg (991.81 KB, 4032x3024 - viewed 94 times.)
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VA3AEX
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« Reply #9 on: September 30, 2018, 09:55:16 AM »

Used Acetone to clean up the ceramic bar end (it removed the Suoerglue handily) and JB Weld Clear (for ceramics amongst other materials) to glue it back together. Worked great and appears as strong as original. Iíve attached a few before and after pics of the bar (you can see the break point when glued back together) and it installed back on the cap. Thanks to all for the great suggestions.

73 Alex


* AAF78E80-7CD8-43A9-B6F4-6F0BCC3B916D.jpeg (2525.41 KB, 4032x3024 - viewed 34 times.)

* 9B50E7C1-D32C-4869-840F-A9C3C45969C3.jpeg (2244.67 KB, 4032x3024 - viewed 49 times.)

* 81352D66-FCCA-4AAE-96F8-1900D2D0DE9D.jpeg (1889.69 KB, 4032x3024 - viewed 46 times.)
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PA0NVD
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« Reply #10 on: September 30, 2018, 11:57:01 AM »

Never had a problem with lexan all the way up to legal limits.


Nothing to do with legal limits but with a high voltage, high impedances. I did fry Lexan boxes that did hold small capacitive antennas to light TL tubes for testing using only 30 watts at 27 MHz. The lexan did show bubbles inside close to the HV part, cracked and melted
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KB2WIG
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« Reply #11 on: September 30, 2018, 06:49:07 PM »



A,

It looks mahvelous. I've a few jackbars that could use the makeover.

KLC
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