Black Hole in Tek 465 trace

[shakalnokturn]

If black hole position on screen can be changed with a magnet the defect isn't on the screen itself.
So the most likely does seem to be a speck on the mesh as suggested by others.

I certainly would beware of vibrating it too hard in the mesh axis being uncertain of how the dome would react when hitting it's resonance.

[dagema]

After tipping and tapping at various angles with no change, I've paused for a moment to reconsider the physics involved. As the screen is charged at about 15KV and the mesh (and therefore also the speck) is charged at about -115V, the screen would be pulling the speck onto the mesh, holding it in place, allowing the speck to resist any movement due to the friction between speck and mesh. I wonder if a plausible cure might be make the speck repell from the mesh by applying a relatively small positive voltage to the mesh. The problem is, how much positive voltage would the mesh bear without buckling back? A safer way might be to just discharge the screen and then again try the tipp and tap method. Without the 15KV pulling at the speck, it might even just jump away since both mesh and speck should have the same residual negative charge, repelling each other. Any advice?

[med6753]

Obviously applying a positive voltage to that area is completely uncharted territory. No idea how the mesh would react. But it might be worth a try keeping in mind the risks.

Make absolutely certain that the HV is completely discharged first. Disconnect the HV lead. Keep the scope powered off. I would start small, perhaps +30V to the CRT HV lead. Ramp it up slowly and see what happens. The positive charge just might cause the speck to release.

[dagema]

I first discharged the CRT and, with the CRT rear connector removed I felt daring and blasted the mesh contact with a Tesla coil spark.  It dawned apon me too late that that was AC voltage, so I most likely welded the spot onto the mesh.  In any case it did not remove or even move it.  Then I repowered the scope and applied +15VDC to the mesh - again no change.

So, having all but given up, I have to agonize over the choices of leaving it as it is, swapping the CRT with my other unit (live with that and try to get a good price for this one), or taking up on the offer for a replacement CRT.

[tggzzz]

So, having all but given up, I have to agonize over the choices of leaving it as it is, swapping the CRT with my other unit (live with that and try to get a good price for this one), or taking up on the offer for a replacement CRT.

Q1: can you live with the spot?
Q2: what would the overall benefit of swapping CRTs?
Q3: what is the overall penalty of tapping the CRT hard (to try to dislodge crud) and breaking it?
Q4: can you live with the possibility that a replacement CRT might not be perfect?

[dagema]

A1: Absolutely!
A2: I'd have a good 465 to continue to use plus a better 465 I could sell at a presumably better price.
A3: The cost of procuring a replacement CRT - presumably more than the cost of the whole 465.
A4: Well, I would not commit suicide over it, but that is the big agoniizer ("throwing good money after bad?").

[james_s]

I don't know that the spot is going to have a big impact on the value of the scope, certainly not enough to expend the effort needed to obtain and install a replacement CRT if you just want to sell the scope. The 465 is an iconic scope, still arguably one of the best portable analog scopes ever made, but analog scopes in general are worth rather little these days. There is just not much demand for them outside of a few specific niche applications. I keep mine for things like XY mode but I probably use it only a couple times a year.

[andy3055]

I took a look at one of the images of 465 CRT on eBay and it seems like you can see the inside parts in the neck of the tube. I wonder if it is possible to do a visual inspection for anything that might be obvious. If there is something it will be extremely small though. May be use a magnifying glass?

Just a wild idea considering the rarity of those CRTs. The guy in eBay ships worldwide and to Germany it is almost $70.00 (the cost of the tube is only $49.00).

[james_s]

You can see inside much of the neck but there is a significant portion including the mesh that is inside the ceramic portion. I suspect whatever is causing that mark is extremely small and being magnified when it is projected onto the screen.

[dagema]

I don't know that the spot is going to have a big impact on the value of the scope, certainly not enough to expend the effort needed to obtain and install a replacement CRT

I tend to agree with you. It just would be nice to offer a full performer with no kinks.

[tggzzz]

A1: Absolutely!
A2: I'd have a good 465 to continue to use plus a better 465 I could sell at a presumably better price.
A3: The cost of procuring a replacement CRT - presumably more than the cost of the whole 465.
A4: Well, I would not commit suicide over it, but that is the big agoniizer ("throwing good money after bad?").

I'd consider selling the scope with the spot.

If you show pictures of it working apparently perfectly, plus picture where you visibly have to make an effort to see the blemish, plus text stating what you do and don't know, then you will probably get a reasonable price and people can't complain. After all, people buy defective "spares or repair" scopes.

Look at the fleabay price difference between a good 465, a defective 465, and the cost of a CRT. I'll bet there isn't much difference.

If shipping the scope, consider that packing has to be done "properly" (== money), and that breakages still occur.