22-12-2016, 11:53 PM
Yes, they are quite a bit smaller:
Time to open up one of them. This one had a resistance between plug and tip of anything between <1 ohm and infinity, depending on how you wiggled and squeezed it. Clearly something wrong...
Under the plastic, a brass rod has the wire crimped at the end. It's quite a decent wire, BTW, and as such I'll make use of it by attaching a spring hook or similar to the end.
At the other end, something else is crimped. But what?
The probe tip is removable, and you can just about see the end of it poking through the end of the metal sleeve.
It's hard to be sure, but it looks like the metal sleeve has broken away at the point it exits the brass rod. Next time I take one apart, I'll do it more carefully.
This isn't a brilliant photo, but best I could do in a hurry:
You can see the tube which I think was once the same as the tube you can see in the other part of the probe, crimped in place. And note the red plastic between it and the outer brass rod...
Note that the end of it wasn't quite as rough at first - I had a gentle "prod" before I took the picture.
I'm not sure how the plastic got there - perhaps it's just filling a gap that occurred? I assume these probes are injection moulded (polypropylene) before the grey soft-touch overmoulds are added.
Anyway, the interface between the two metals appears to be the problem. There is a good sub-1 ohm resistance between 4mm plug and the brass rod, but careful probing reveals a higher resistance between 4mm plug and the inner (stainless steel?) tube. To get to the bottom of that, I guess I'd need to file away a section of the outer brass rod and somehow open it up. Hmm. Maybe tomorrow...
Incidentally, I don't know which Pomona probe this is. From a quick look on the Farnell website, there are at least 3 possibilities. However, I'd assume they are all much the same, and possibly prone to this high-resistance failure mode. Of all the probes we bought, this is the only one that could be persuaded to go O/C - it's possible that someone bent it too hard and severed that internal tube. I will take apart another at a later date. Any suggestions about the best method? I'm considering a blow-torch!
Time to open up one of them. This one had a resistance between plug and tip of anything between <1 ohm and infinity, depending on how you wiggled and squeezed it. Clearly something wrong...
Under the plastic, a brass rod has the wire crimped at the end. It's quite a decent wire, BTW, and as such I'll make use of it by attaching a spring hook or similar to the end.
At the other end, something else is crimped. But what?
The probe tip is removable, and you can just about see the end of it poking through the end of the metal sleeve.
It's hard to be sure, but it looks like the metal sleeve has broken away at the point it exits the brass rod. Next time I take one apart, I'll do it more carefully.
This isn't a brilliant photo, but best I could do in a hurry:
You can see the tube which I think was once the same as the tube you can see in the other part of the probe, crimped in place. And note the red plastic between it and the outer brass rod...
Note that the end of it wasn't quite as rough at first - I had a gentle "prod" before I took the picture.
I'm not sure how the plastic got there - perhaps it's just filling a gap that occurred? I assume these probes are injection moulded (polypropylene) before the grey soft-touch overmoulds are added.
Anyway, the interface between the two metals appears to be the problem. There is a good sub-1 ohm resistance between 4mm plug and the brass rod, but careful probing reveals a higher resistance between 4mm plug and the inner (stainless steel?) tube. To get to the bottom of that, I guess I'd need to file away a section of the outer brass rod and somehow open it up. Hmm. Maybe tomorrow...
Incidentally, I don't know which Pomona probe this is. From a quick look on the Farnell website, there are at least 3 possibilities. However, I'd assume they are all much the same, and possibly prone to this high-resistance failure mode. Of all the probes we bought, this is the only one that could be persuaded to go O/C - it's possible that someone bent it too hard and severed that internal tube. I will take apart another at a later date. Any suggestions about the best method? I'm considering a blow-torch!
