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General electronics => Electronic Circuits => Topic started by: Hydron on July 18, 2018, 03:55:09 PM

Title: BNC connector repair on (epoxy potted) Pearson Wideband Current Monitor
Post by: Hydron on July 18, 2018, 03:55:09 PM
Just a quick post to share a trick I found useful for replacing a BNC connector without rear access on a Model 4100 Pearson "Wideband Current Monitor" (internally terminated wideband CT).

These items are made with a brass case surrounding the CT core, and are 100% epoxy potted _after_ the BNC connector is screwed to the case and it's centre pin soldered to the CT output. They are then dipped in (green) paint. See below for an undamaged example:
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Example of the type of BNC used:
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The actual part number seems to be a UG-290A/U Amphenol RF 50R BNC, but there are equivalent (cheaper, non MIL-Spec) parts widely available with the same dimensions, including the B6551A1-NT3G-50 I used in my repair.

I obtained one of these CTs on ebay for a steal, but it came with a badly damaged (dented, deformed) BNC. While I was able to beat it back into enough shape to work with some BNC connectors, it was still a problem with most of my cables/connectors, so a solution was desirable.

Although the paint covering the base of the BNC can be removed to access the screws and free the base from the unit, the centre pin is still soldered to a wire trapped by epoxy, so the connector cannot be removed. Brute force could be tried to pull the connector off (leaving the pin behind) but risks breaking internal connections. Thankfully I found another way!

It turns out that it's possible to "remotely" de-solder the wire at the back of a BNC socket by heating up a conductive pin inserted into the front. I found a diode with thick copper legs that _just_ fit into the front socket, and cut one off to use. Heated with a soldering iron set to ~325C and placed just above where the copper pin entered the insulator inside the BNC, it managed to desolder the pin at the back and allow the BNC to be removed. This was probably helped by teflon's high temperature resistance (~350C melting) and the leaded solder used in the old CT, but can probably still be done for lead-free by using a higher temperature and accepting that the insulator may melt and make a mess (not a big issue as that part is being replaced anyway).

Behind the BNC was bare brass with tapped holes at the corners and a hole milled through for the central protrusion and connection (sorry forgot to take a pic of this). Epoxy potting compound had surrounded the space where the rear protrusion of the BNC had been, with the termination wire down a little hole formed by the removal of the central pin. After checking that the new BNC would fit (the rear insulator was thankfully the exact same diameter, and slightly shorter) I sanded down the pin at the back of the new BNC to match the (slightly shorter) pin of the old one, and also opened up the solder cup a bit.

At this point I had two options - do the de-solder trick in reverse to solder the connection again from the front with the new BNC (I tested this on a scrap BNC, and it worked without melting the insulator with a 325C iron and leaded solder), or remove the new BNC pin from the connector and fit it separately. With a teflon insulator the pin could be pressed out relatively easily (it's not actually meant to be removed!), and as the bottom of the hole with the CT termination wire was full of epoxy (i.e. strong enough to withstand the BNC connector being pushed back onto the central pin), I decided to solder the pin on first, then push the BNC back onto it. This step was fairly easy - I was just careful that the pin was straight and at the right height, and got a fine tipped iron into the base to hit both the end of the termination wire and the solder cup for a nice solid joint. The BNC shell was aligned to match the screw holes and pressed onto the pin (alignment is critical, as twisting or pulling after the fact to line things up is risky!). Finished result along with the old bent BNC is shown below - good as new:
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...uh, I take back the description of "quick post", but hopefully this trick can help others with a similar problem!
Title: Re: BNC connector repair on (epoxy potted) Pearson Wideband Current Monitor
Post by: Mads Barnkob on July 22, 2018, 08:23:26 PM
I will this a try on a rainy day :) so do not hang around and F5 every day

As you can see in these pictures I got a similar issue, actually I would say my BNC connector here is damaged beyond proper use, there is only a few "loose" cables that can actually fit on it and its not easy to get the BNC turned all around to the snap lock.

Thank you very much for finding a creative way around the beat-up-bnc-connector-in-epoxy-hell problem.
Title: Re: BNC connector repair on (epoxy potted) Pearson Wideband Current Monitor
Post by: Hydron on July 23, 2018, 11:01:43 AM
Ouch, that's serious damage. Where did you come by that one - was it sold in that condition? Nice unit if you can fix it up.
Title: Re: BNC connector repair on (epoxy potted) Pearson Wideband Current Monitor
Post by: Mads Barnkob on July 24, 2018, 09:45:44 AM
Ouch, that's serious damage. Where did you come by that one - was it sold in that condition? Nice unit if you can fix it up.

Bought it off ebay, in working and fine condition, but seller only gave it a couple of rounds of bubblewrap and in a cardboard box it went half around the globe :(

I got 75% of buying price in return for the smashed up goods as we made a deal on me trying to fix it or return the item. So I got a nice and cheap, but busted CT.

I found the pictures from the shipping that I could not find the last time i posted.
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