Broken off plastic tip on a high-voltage probe - can/should be fixed? - Page 1
Forum section: electronics / testgear

author: K
author_rank: Contributor
timestamp: October 06, 2015, 10:01:40 am
content: Hello,this is my first post!I am not sure in which section this post belongs - this one or "Repair"! Please move if not appropriate!Purchased a Beckman HV-211 high-voltage probe for use with my DMM as I want to fix an old oscilloscope and need to check somewhat high voltages safely. Probe was sold as new and unused, and was not terribly expensive - about a half of price of those TENMA HV probes of comparable spec.Unfortunately, when the package arrived, I found out that probe was not new or even used - it was damaged. I promptly got a refund from seller and basically have spent $15 on a broken HV probe.As can be seen in the pictures, the tip of the probe has been broken off - someone probably dropped it.Also, from what I can tell, an unsuccessfull attempt to repair it had been done. There is a specific smell of what I think is epoxy hardener near the tip of the probe and front part of the probe interior is covered with something clear and oily-sticky. The main resistor itself looks undamaged, apart from being covered in said mystery stuff. The resistor is a bit loose inside the main part of probe - it can be pushed forward and backwards for about a 1cm - but does not rattle when proble is shaken.My question is - can/should the attempt to repair this be done so it can be used safely?As I can not identify the type of plastic the main shaft is made of (to pick a propper, working glue for it), I am thinking about cleaning the surfaces clean and then using several layers of heat-shrinkwrap (the variety with glue inside) to hold the tip in place. Could the sticky stuff be some kind of high voltage dope and therefore be left as is?Or maybe I should just scrap it and have some nice, fat resistors in my parts bin?

author: Andy Watson
author_rank: Super Contributor
timestamp: October 06, 2015, 10:24:23 am
content: When I am fault-finding I like to be able to trust the indications on my test equipment - using potentially broken test equipment adds another lever of uncertainty. Would you trust a device that has clearly suffered significant trauma to its signal path? I would put it in the bin.

author: Performa01
author_rank: Super Contributor
timestamp: October 06, 2015, 10:58:24 am
content: Don't worry too much. A string of HV resistors isn't rocket science. Your plan using heat shrink tubing sounds good, still would fit the broken parts together using some super glue beforehand.Electrical leakage isn't a concern since the damage is only at the front end of the resistor string. That means, accuracy won't change a bit - apart from the fact that HV-probes of this kind aren't terribly accurate anyway.

author: wagon
author_rank: Frequent Contributor
timestamp: October 06, 2015, 11:18:30 am
content: I would have no hesitation in repairing it with a two-part epoxy resin. You must ensure it's cured before you use it, since when it's 'wet' it's a partial conductor and may burn.

author: Gyro
author_rank: Super Contributor
timestamp: October 06, 2015, 11:33:59 am
content: QuoteIf it extends down the resistor then it is almost certainly 'high voltage dope'. It could well be that there has not been any previous attempt to repair it, just broken in transit and dope leaking. You'll probably need the assemble it as tightly as possible and then clean the outside surface of the probe (only) in order to get the heatshrink adhesive to stick Avoid getting any solvent inside.

author: Fraser
author_rank: Super Contributor
timestamp: October 06, 2015, 04:06:34 pm
content: This probe may be safely repaired using plastic weld. Such welding uses the original plastic so there is no change in the insulator characteristics. It is also a much stronger bond than epoxy adhesive. Modelling shops sell plastic weld under various names. I use EMA Plastic weld as it is pure solvent with no filler. Some plastic weld fluids used for plumbing contain a filler and are less fluid. EMA Plastic Weld is available on eBay or at model shops in the UK.In case you are unaware, plastic welding uses a powerful solvent to liquify the faces of two mating surfaces. The plastic of the two surfaces melt into each other and harden as the solvent evaporates. A superb tool for repairing all manner of cracks and breaks in ABS plastics.With regard to high voltage safety. You do need to inspect the red insulated section of the probe fro longitudinal cracks. Such would render the probe compromised for use at EHT voltages as tracking could occur if the crack became conductive due to moisture or other contamination.Fraser

author: HighVoltage
author_rank: Super Contributor
timestamp: October 06, 2015, 10:31:04 pm
content: I have repaired many high voltage probes and have built my own.In this case, I agree with Fraser to use a plastic weld to really bring the plastic parts together to become one.Epoxy will probably not stick well to the surface of the plastic.In addition I would put a shrink tube around it.

author: K
author_rank: Contributor
timestamp: October 07, 2015, 02:54:14 pm
content: Thanks for the answers!As suggested, I have ordered EMA Plastic Weld from ebay and will try to use to glue the tip back.Question about that suspicios smelly stuff around the break still stands. The resistors themselves do not seem to be fully covered with that. I guess I will clean off the metal tip from outside so it is completely clear on the outside surfaces and leave the insides as it is.

author: Fraser
author_rank: Super Contributor
timestamp: October 07, 2015, 06:32:22 pm
content: Clean the gunk away with IPA. You can safely immerse the tip and resistor in IPA without damaging the insulation. Surfaces should be clean before using the plastic weld. None of my EHT probes contain any gunk around the EHT resistors. Unlike some HP EHT probes, the type you have does not rely on gas or fluid as part of the insulation within the casing.