Black Hole in Tek 465 trace - Page 1
Forum section: electronics / repair

author: dagema
author_rank: Contributor
timestamp: December 14, 2019, 06:56:03 pm
content: I have been using my Tek 465 for decades, and am still quite satisfied with it. But nevertheless, I bought a cheap broken unit on Ebay. It had the description "verticle switches defect", so I was expecting to fine the plastic joints between the vernier rods and the potentiometers to be broken due to old age wear-and-tear. Instead, I found that both potentiomenter/switch units were torn back off of the verticle preamp PCB. The whole scope inside was immaculate - no dust or residue on either the chassis parts or any of the PCBs, so I surmised that the unit had been disassembled, sent through an ultrasound bath, reassembled, and then per mishap bumped severely against something hard which rammed the vernier rods into the unit, tearing out the potentiomenter/switch units.After repairing the damage, I began to go through the funtional verification process when I noticed a "black hole" in the trace (see pictures). The "hole" only appears a trace crosses it, and the position of the hole is not affected by any of the control settings, so I suspect that the severe bump may have jolted some phyiscal part the CRT assembly out of alignment. But I have not been able to find any documentation on removal/replacement of the CRT assembly, and I do not want to jump too hastily into unfamiliar territory.So here my question / call for help: has anyone seen this kind of anamoly before and/or does anyone have an idea of what may be causing it? The problem is not serious, but I would like to solve it if it can be done with reasonable effort, since the unit appears otherwise to be in a very good working condition.Thanks for any helpful advice!

author: ferdieCX
author_rank: Regular Contributor
timestamp: December 14, 2019, 07:04:28 pm
content: I have seen this kind of anomaly more than 35 years ago in a Tek 475A, bought from a dealer in the USA.We ended assuming that it was a defect in the phosphor coating.

author: dagema
author_rank: Contributor
timestamp: December 14, 2019, 07:16:16 pm
content: Thanks. I guess I will have to live with it.

author: tggzzz
author_rank: Super Contributor
timestamp: December 14, 2019, 08:02:34 pm
content: Sometimes some flakes can become loose inside the CRT, and end up on the expansion mesh, blocking the beam at that point.What falls onto the mesh can fall off, so try gently by firmly holding the CRT in "strange" orientations and tapping it.

author: dagema
author_rank: Contributor
timestamp: December 14, 2019, 08:11:28 pm
content: Thanks for the tip - it seems to fit in well with the jolting scenario. I`ll give it a try!

author: MadTux
author_rank: Frequent Contributor
timestamp: December 14, 2019, 08:21:59 pm
content: Looks like a burned spot on the phosphor.Happens, if some idiot turns intensity full up, while scope is in X/Y mode or very slow sweep.

author: dagema
author_rank: Contributor
timestamp: December 14, 2019, 08:25:41 pm
content: aarrgh. That would explain why it spot is so perfectly round.

author: TurboTom
author_rank: Super Contributor
timestamp: December 14, 2019, 09:39:06 pm
content: Is there any difference to the surrounding screen visible in that place when the scope is powered down and the screen illuminated with a strong light (maybe U/V) externally? If there is, it's very likely screen burn-in, though I don't remember to have seen anything like this on a TEK CRT for a long time.Cheers,Thomas

author: james_s
author_rank: Super Contributor
timestamp: December 14, 2019, 10:00:54 pm
content: It certainly looks like screen burn, and it's definitely possible to burn it by turning up the brightness in XY mode. Before completely condemning it I would remove the bezel and filter and make sure there isn't a blob of something on the face of the tube.The burn is annoying although it shouldn't really be a big impact to using the scope.

author: dagema
author_rank: Contributor
timestamp: December 14, 2019, 10:10:03 pm
content: I inspected the position with a bright LED penlight and could not see anything telltale. I`m pondering now where I can come up with a source of ultraviolet light to check with that. Thanks for the tip!

author: shakalnokturn
author_rank: Super Contributor
timestamp: December 14, 2019, 11:31:17 pm
content: Because of the small brighter spot above and the aura around the black hole, I'd be more inclined to think it is a manufacturing defect in the conductive coating before the phosphor layer rather than screen burn.

author: Electro Detective
author_rank: Super Contributor
timestamp: December 14, 2019, 11:35:55 pm
content: 99% looks like it was accidentallyor stupidlyleft on unattended in XY mode for a weekendIt looks too perfectly round to be a defect =If the rest of the scope works great, it's still a winnerand a handy buddy sitting next to a DSOjust don't expect good money if you sell

author: james_s
author_rank: Super Contributor
timestamp: December 15, 2019, 01:26:50 am
content: I seriously doubt Tektronix would ship a tube with a defect like that, they were pretty much the gold standard for quality.An easy way to tell if it's the phosphor or the mesh is to take a magnet and hold it near the screen, that will bend the beam and make it hit that spot even if there's a flake of something further back blocking it. I'd be real careful banging on the CRT unless you have to, it's not unheard of for parts of the electron gun to break loose or become misaligned.

author: Miti
author_rank: Super Contributor
timestamp: December 15, 2019, 03:00:39 am
content: A burn spot would still be there when the scope is turned off. If you don’t see it it is not burned, it is the shadow of a debris that fell inside. If it is magnetic, you can try a coil with a short current pulse to shake it without tapping the glass.

author: med6753
author_rank: Super Contributor
timestamp: December 15, 2019, 05:22:30 am
content: Agree with the other 99%. It's a phosphor burn. I've seen similar on B/W TV's that have lost deflection voltage. It doesn't take much for that highly concentrated electron beam to damage the phosphor.Given that as mentioned you can live with it or replace the CRT. Removing a CRT on a 465 is not difficult. It comes out the front and the service manual has instructions. I've done it. The only hassle is the horizontal deflection plate wires. It's tight in there especially when installing the replacement.465B's and later S/N 465's share the same CRT. Early 465's have a different P/N CRT but I think the newer P/N can be installed without circuit modifications. So if you can find a junker 465 or 465B it can be swapped out.I do have a parts mule 465B which I believe has a good CRT and I would be willing to give up for only the cost of shipping. But that's the rub. Shipping to Germany would probably be cost prohibitive and I would have to pack it to withstand below. I'd be willing to check on a price but after Christmas if you're interested. And I would also be willing to test the CRT first to make sure it's good.

author: dagema
author_rank: Contributor
timestamp: December 15, 2019, 07:17:47 am
content: That`s a great offer! I`ll review the SM regarding CRT removal and get back to you after christmas with a decision.Many thanks!

author: tggzzz
author_rank: Super Contributor
timestamp: December 15, 2019, 07:37:04 am
content: I would beverycareful of any magnet around the side of the tube.The expansion mesh, which the electron beam goesthroughis extraordinarily fragile - breathing on it will irreparably damage it. If the mesh is magnetic, a strong magnetic field could be "suboptimal".For pictures, seehttps://entertaininghacks.wordpress.com/2016/03/09/rescuing-a-broken-tektronix-465-crt/

author: james_s
author_rank: Super Contributor
timestamp: December 15, 2019, 07:51:24 am
content: Don't put the magnet around the side of the tube, and don't use a monster rare earth magnet, just a small refrigerator magnet type thing held up to the face of the tube is plenty to distort the image, you shouldn't even have to touch the tube with it, you only need a very slight distortion.

author: dagema
author_rank: Contributor
timestamp: December 15, 2019, 11:40:39 am
content: OK, the magnet test (I was very carefull) proves that it is NOT a phosphor burn (the spot moves around on the screen when the magnet approches the screen).I haven't had any luck yet with the tilting-and-tapping routine yet, but may be just a matter of applied patience. I'm back to being hopefull. Thanks to all for all the tips!

author: med6753
author_rank: Super Contributor
timestamp: December 15, 2019, 12:13:18 pm
content: Interesting, I had that pegged for a phosphor burn.Since you CAN move the spot with a magnet is it possible to move it (carefully) out of the field of view? Remove the front bezel and try to move it to the bottom of the screen. That will save doing any shaking or tilting and risking damage.

author: dagema
author_rank: Contributor
timestamp: December 15, 2019, 12:28:27 pm
content: Hmm, looks like a misunderstanding. When I approach the screen with a magnet, the black hole moves around due to "bending" the electron stream - the spot returns to the original position when I move the magnet away. So, I have not yet been able to move the cause of the spot. It appears to be more of a shadow of something in the electron path than an actual fleck on (or near?) the screen. I now suspect that it might be a defect in the mesh. If the unit received it's "bump" while the front side was pointing down (e.g. it was dropped) then I can imagine that a small bit of debris could have been jolted loose an landed on the mesh. I'll try standing the unit on it's back feet and tap gently. I don't think trying to move it with a magnet would be a good idea

author: dagema
author_rank: Contributor
timestamp: December 15, 2019, 12:56:16 pm
content: I just had an idea that I would like to hear opinions on, before I actually try it. The idea is to use a variable low frequency sound wave to jostle the spot away - like maybe mounting a bass speaker to the chassis and then go through various frequency and volume combinations. Does anyone think I might harm the CRT assembly doing so?

author: MadTux
author_rank: Frequent Contributor
timestamp: December 15, 2019, 01:44:25 pm
content: Weird problem, but quite interesting.In the pictures above, the beam is moved outside the phosphor and set to high intensity.So the beam hits some other parts inside the CRT and the phosphor is illuminated by reflected/secondary electrons.In my opinion, this would exclude any problems with very fine -150V focusing mesh in the middle, since the beam isn't focused in any way.So it's likely some problem close to the phosphor, but not the phosphor itself, if you can move it around with a magnet.Is there another grid/mesh behind the phosphor, ie for the 10-15kV acceleration voltage, that might be damaged?Perhaps burned/destroyed aluminum coating of the phosphor? Phosphor itself is fine and the spot with bad coating gets charged negatively, so that the beam gets deflected away from it.

author: dagema
author_rank: Contributor
timestamp: December 15, 2019, 03:15:34 pm
content: I played around with the focus and intensity, and found a cloverleaf-like plumage around the black hole (see photo) which moves with unchanged geometry with the spot when deflected with a magnet, but grows and schrinks asymmetrically as I move the trace into, through and out of the spot. It looks to me as if the plumage is formed by electrons bouncing off the outer edge of the hole, back onto the phosphor at a slight angle, just skimming it - as if the hole was made of a microscopic thin dot with a relativly sharp edge which refelcts back any eletrons hitting it. Still hopefull on getting it to fall off.

author: med6753
author_rank: Super Contributor
timestamp: December 15, 2019, 03:30:04 pm
content: Based upon your latest picture I am now pretty much convinced that it is some sort of particle either on the mesh or the screen.

author: shakalnokturn
author_rank: Super Contributor
timestamp: December 16, 2019, 12:43:13 pm
content: 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.

author: dagema
author_rank: Contributor
timestamp: December 16, 2019, 01:00:49 pm
content: 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?

author: med6753
author_rank: Super Contributor
timestamp: December 16, 2019, 01:47:47 pm
content: 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.

author: dagema
author_rank: Contributor
timestamp: December 16, 2019, 05:13:30 pm
content: 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.

author: tggzzz
author_rank: Super Contributor
timestamp: December 16, 2019, 10:51:23 pm
content: Quote from: dagema on December 16, 2019, 05:13:30 pmQ1: 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?

author: dagema
author_rank: Contributor
timestamp: December 16, 2019, 11:06:00 pm
content: 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?").

author: james_s
author_rank: Super Contributor
timestamp: December 17, 2019, 03:10:55 am
content: 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.

author: andy3055
author_rank: Super Contributor
timestamp: December 17, 2019, 03:57:22 am
content: 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).

author: james_s
author_rank: Super Contributor
timestamp: December 17, 2019, 05:17:36 am
content: 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.

author: dagema
author_rank: Contributor
timestamp: December 17, 2019, 08:51:41 am
content: Quote from: james_s on December 17, 2019, 03:10:55 amI tend to agree with you. It just would be nice to offer a full performer with no kinks.

author: tggzzz
author_rank: Super Contributor
timestamp: December 17, 2019, 09:47:02 am
content: Quote from: dagema on December 16, 2019, 11:06:00 pmI'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.