BNC Input Connector Replacement for Oscilloscope - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: March 03, 2024, 03:11:42 pm
content: I bought a LeCroy Waverunner LT344 scope a few weeks ago with a mangled BNC connector.Yesterday I disassembled the unit, removed the connector, but can’t locate a similar one on DigiKey or Mouser.Unless I’m wrong, it’s an edge mount, surface mount, straight.Does anyone know where to find these?

author: jmelson
author_rank: Super Contributor
timestamp: March 03, 2024, 03:27:56 pm
content: You may have to find the closest thing you can get and then adapt it. I have seen similar connectors that had the round flange and center pin, and threaded at the BNC end. Then, you might be able to solder some metal strip with a 90 degree bend to the flange. I think LeCroy has been bought out by Teledyne, so you might be able to still buy the repair part, but I'd guess it could be expensive.Jon

author: bostonman
author_rank: Super Contributor
timestamp: March 03, 2024, 03:40:43 pm
content: Makes sense why my search led me to Teledyne at some point.Well, this scope has three issues: the BNC connector, the scope constantly rebooting, and a dim screen.The damaged BNC connector was stated in the CL article prior to buying the scope (I had a feeling it had more issues with only a $100 asking price); the other two weren't. I discovered the power switch is mounted to a PCB with a single wire that I assume is Vcc. When the switch is pressed, it pulls Vcc to ground via the single PCB mounting screw to chassis. After pressing and tapping various areas, I came to this area and discovered the mounting screw was loose causing an intermittent connection.The dim display is the more serious problem. It's a LQ084V1DG21 and replacements are around $150. I'm also finding replacement LED light strips, however, they are similar in price.Since the scope wouldn't stay on long enough to test it, I'm stuck with reassembling the whole thing to test before I make a decision on investing another $150 (plus the BNC replacement cost).

author: nctnico
author_rank: Super Contributor
timestamp: March 03, 2024, 03:50:41 pm
content: Edge mount BNC does exist:Random Google find:https://www.cambridgeelectronics.com/product/C-SX-160About the replacement LED strips for the display: these shouldn't be expensive.https://www.eevblog.com/forum/projects/tft-backlight-led-conversion-ccfl-inverter-alternative/This is around $5:https://nl.aliexpress.com/item/1005002967642135.html

author: BrokenYugo
author_rank: Super Contributor
timestamp: March 03, 2024, 04:44:28 pm
content: It would appear you can still get a new exact fit backlight tube for around $30.E.G. (first one google gave me)https://www.plazmo.com/products/lq084v1dg21

author: bdunham7
author_rank: Super Contributor
timestamp: March 03, 2024, 04:45:18 pm
content: Quote from: nctnico on March 03, 2024, 03:50:41 pmA lot of them, including the ones from Cambridge, are 75 ohms.Maybe this?https://www.digikey.com/en/products/detail/amphenol-rf/031-6233/4746563

author: bostonman
author_rank: Super Contributor
timestamp: March 03, 2024, 04:53:52 pm
content: I typed this message and saw replies were already posted before posting, but for the most part, my statements/questions are still relevant.The BNC link helped because it gave a better idea to the specific name of this connector.This is for a oscilloscope probe but states it's 75ohm impedance. Should it be 50, 75, or open Z?Updated from previously typed copy/paste: the DigiKey one also looks physically good, however, I'm finding from the BNC tip (the part the probe would connect to) the the top of the base is 18.23mm whereas the one in the link is 19.66mm.Also, I didn't realize the LCD display literally have just a row of LEDs around two edges. I thought the entire screen needs to be replaced (until I saw articles prior to starting this thread). Does a possibility exist the LED strip can be installed wrong or whatever the case may be and the screen has shadows?

author: bdunham7
author_rank: Super Contributor
timestamp: March 03, 2024, 04:59:34 pm
content: Quote from: bostonman on March 03, 2024, 04:53:52 pmIt should be 50 ohms. You'll have to measure your old one as precisely as you can and search through some datasheets. You may never find an absolutely perfect match so if you are concerned with appearance, replacing all of them (perhaps with gold-plated ones) is an option.

author: bostonman
author_rank: Super Contributor
timestamp: March 03, 2024, 05:05:26 pm
content: QuoteI never gave thought to this. Obviously physically it will not measure 50ohms (although at my old job I needed to use 50ohm terminations to take RF measurements on outputs of units), but it actually has an impedance of 50ohms? This doesn't get reflections due to the high Z of the scope probe?QuoteSometimes I don't think outside the box. Great idea since I already have the whole thing apart. The only cause for worry is whether the front panel goes together again should the connector deviate slightly in size.

author: bdunham7
author_rank: Super Contributor
timestamp: March 03, 2024, 05:24:08 pm
content: Quote from: bostonman on March 03, 2024, 05:05:26 pmIt won't matter if you are using the 1M input selection on the scope, even the 75 ohm version would be fine for that. Your scope has 50 ohm inputs though, right? So if you select the 50-ohm mode and use a direct connection or a probe meant for 50-ohm inputs, then the impedance characteristic of the connector would matter a little bit. In any case, theory aside, realistically your only two choices for BNC connectors like this is going to be 50 or 75 ohms, unless perhaps you find some weird 93 ohm version.

author: ejeffrey
author_rank: Super Contributor
timestamp: March 03, 2024, 06:29:27 pm
content: 50 ohm and 75 ohm bnc have different pin and insulation dimensions. They mechanically fit together but not correctly. Mismatching can damage or wear out the connectors faster. Assuming you are going to use this with standard 50 ohm cables and probes with 50 ohm connectors you want a 50 ohm connector even if you use it in high Z mode.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 03, 2024, 06:55:19 pm
content: If you look inside a BNC socket, a 50 ohm connector has a white or translucent insulator surrounding the center contact. A 75 ohm connector has no insulator around the center contact.50 and 75 ohm connectors are mechanically compatible. You won't cause any damage by mixing them. I've heard rumors and legends that at one time in the past there was a series of 75 ohm BNC connectors that didn't follow this rule, but that's no longer the case.Note that N connectors also have 50 and 75 ohm versions and in that case, you will cause damage by mixing the two types. AFAIK, the 75 ohm version has a center pin that's as sharp as a needle while the 50 ohm version has a rounded tip on the center pin.Ed

author: bostonman
author_rank: Super Contributor
timestamp: March 04, 2024, 04:56:47 am
content: My BNC has a white insulator. Makes sense it would be 50ohms since a coax cable would most likely be connected to the front panel (as I did at my old job when measuring 5 and 10MHz outputs) and coax is 50ohm impedance.Pasternack has some good ones, but are very expensive. Amphenol has some that are close to the size I need, but are around $8. Still expensive, but less than Pasternak. Later I'll try 3D printing the provided step file they provided. From what I can tell, they are longer, but I don't care about that.I've been researching replacing the CCFL (?) back light with an LED strip (and also disassembled it for fun just to see what it looks like). After reading the datasheet for the front panel, there is a spec called "kick-off voltage" and specs it's up at 1300v. Maybe this sounds silly, I thought this is the voltage needed to turn on the lamp and wondered how a "drop in" LED replacement strip would handle this.The LED replacement strip I'm finding online is selling for $150 and includes a replacement driver board which somewhat answers my question, but not sure if the existing design uses 1300v, or if I'm interpreting the spec wrong.

author: BrokenYugo
author_rank: Super Contributor
timestamp: March 04, 2024, 01:48:37 pm
content: It's a cold cathode light with some distance between the electrodes so it needs HV to start the discharge, 1300V sounds plausible. Driver is usually a simple high frequency inverter with an AC output, not really suited for driving LEDs, they do draw a pretty good arc for their size though. The power will burn a (small, mostly harmless) hole in finger skin faster than you can get away so some caution should be used around a live one if you feel like playing with it.Keep in mind high quality (what you want on a tool) BNC connectors are made out of plated solid brass and PTFE to reasonably high precision, $5-10 for an oddball one that doesn't have a zinc body sounds about right. AFAIK virtually all scopes and general purpose test gear use 50 ohm connectors, 75 is a video/AV thing.

author: bdunham7
author_rank: Super Contributor
timestamp: March 04, 2024, 02:39:33 pm
content: Quote from: bostonman on March 04, 2024, 04:56:47 amThe last time I fixed a backlight on an expensive-ish instrument I was able to get replacement CCFL tubes by size. If it is just dim, the driver may be working fine, otherwise the HV driver boards can be repaired or replaced as well. I don't think it is worth the effort to "upgrade" to LED and you would definitely need a different driver board and appropriate power for it. The latter may be an issue.https://www.plazmo.com/collections/raw-ccfl-lamps

author: bostonman
author_rank: Super Contributor
timestamp: March 04, 2024, 03:00:41 pm
content: I'm uncertain whether to start a new thread for this since it's somewhat of a combination of BNC connectors and the screen.Unfortunately I don't know what that driver board provides for voltage/current. The input connector has four or five wires I think. Most likely Vcc, GND, brightness control, and a few others.I didn't realize the lamps just shine across the screen. The YouTube video for the $150 replacement kit only installs one row of LEDs, blocks the other side, and installs the replacement driver board. My guess is the majority of the cost is in the replacement driver board.

author: bostonman
author_rank: Super Contributor
timestamp: March 05, 2024, 06:07:28 pm
content: I 3D printed the Amphenol RF BNC and it looks nearly identical in size.The threads appear a bit longer by maybe a mm and the solder pins are longer, but can be cut. The cost for a single one isn't much (Update original message: $9), but, if I want to replace all so they match, now I'm up to $40 (four channels plus the ref).May be worth the gamble so they all match and assume the scope is worth the investment.Now I need to figure out the screen issue and how to get some DC convertor for the LED strip. Possibly I need to measure the input connector voltages and take things from there.

author: bostonman
author_rank: Super Contributor
timestamp: March 11, 2024, 04:36:52 am
content: Just an update. Today I received and installed the Amphenol RF BNC.The length and thread length was perfect and the top center pin and two ground pins were a perfect fit. The bottom ground pins gave me some problems. They were a bit too long and had to be cut (not an issue and was expected), and were spaced slightly too close. Thankfully they were curved and I managed to solder them to the pads while the curved part was raised above the coupling capacitor the center pin connects to by way of a via.I'm uncertain why, but trying to get solder to stick to the ground pins was nearly impossible. It seemed the heat was pulling away quickly (I had to screw the front panel on so I could get the BNC to stay in place and to line up correctly) and the solder was just rolling off the pins, but eventually it took.My initial plan was to replace all five so they match, and purchased five, however, after the trouble getting solder to apply, I figured leave what works alone.Afterwards I reassembled the scope to a degree and things seem to work, so now it's a matter of figuring out what to do with the dim display.I plan to measure the input connector voltages to figure out what I'm dealing with, and then move forward.

author: bostonman
author_rank: Super Contributor
timestamp: March 12, 2024, 03:26:38 am
content: I started a thread regarding the CCFL instead of deviating this thread.https://www.eevblog.com/forum/projects/ccfl-tube-on-oscilloscope/new/#new

author: bostonman
author_rank: Super Contributor
timestamp: June 28, 2024, 03:24:48 am
content: Recently I purchased another scope (Agilent 54831M) and one of the problems is a damaged BNC connector on the front panel.Attached is a marked up version of what I'm looking for. It's a typical center pin with four corner ground pins, 90 degree, but, due to going through the front panel, the dimensions are tight. The design doesn't leave any room for error.It has a 'ring' just after the base which I assume is to support the nut (I didn't include this in my mark up).I did a search on DigiKey, but none seem to fit these dimensions. Thankfully my damaged BNC only had damage on the face. It was slightly bent and cracked, however, with the help of a dremel drill, I was able to grind off the cracked piece which resulted in grinding off the bent section. BNC cables and scope probes fit on without an issue and I didn't feel any sharp corners.If possible, although this is "fixed", I'd still like to replace it if possible. My question is: would this most likely be a custom BNC connector and unable to be located, is it just an old design and locating one with the same dimensions be impossible, or does anyone know if a site exists that has more BNC connector options?Unfortunately it seems I can't sacrifice any of the dimensions with the exception of a longer "neck" (the part after the threads) due to how the PCB sits along with the plastic cover that doesn't fit over the threads.Just thought to ask if locating a replacement is easy. If not, no big deal, I'll just leave it as is.

author: ajb
author_rank: Super Contributor
timestamp: June 28, 2024, 05:16:10 am
content: Quote from: bostonman on March 11, 2024, 04:36:52 amYou might want to preheat the board/connector with hot air. You have to be careful to not overheat the plastic in the connector, but that amphenol part has a rating of 165C. (*operating* temp range?) Getting it up there reduces the dT to your soldering temp by 2/3, which reducesheat loss into the connector by the same amount. Similar for the PCB, especially relevant with heavy planes involved.You may also need to use additional flux. I like paste flux in a syringe for stuff like this, although it can be annoying to clean up afterwards.

author: tooki
author_rank: Super Contributor
timestamp: June 30, 2024, 02:27:03 pm
content: 100% agree with ajb’s advice to preheat and add flux.And use a bigger tip. Soldering the body of a BNC takes a lot of heat, and a big tip at a moderate temperature is way better than an undersized tip at a high temperature.

author: bostonman
author_rank: Super Contributor
timestamp: June 30, 2024, 03:31:05 pm
content: Next time I'll try that advice. I have a hot plate, so maybe it would have been perfect.Per my other question about another BNC, I gave up trying to locate a replacement as the dimensions seem like they don't exist.Thankfully grinding the face slightly not only removed the bent area, but removed the crack. Looking at the BNC straight on, it looks perfectly round. From the side you can see a slight dip, but it's a tiny dip.Now the BNC and probes go on with little effort - same effort as the others. Not sure what I would have done if it was damaged beyond this basic repair.