scope: faulty, not faulty? - Page 1
Forum section: electronics / beginners

author: 3n2323
author_rank: Contributor
timestamp: April 07, 2012, 03:14:00 pm
content: man do i have questions!a friend showed me his Tektronix 465, not A, not B, just 465, where he hooked the probe to the Calibrator bar.the top of the wave was so fat, i wonder whether this is normal or not, how could a reasonably accurate amplitude reading be obtained from a wave displayed this way?my question is, is this normal, or is it that only a faulty scope will do that?oh, the volts/div was set at 10m (10mV) on the dial, if that matters.

author: firewalker
author_rank: Super Contributor
timestamp: April 07, 2012, 03:25:33 pm
content: Was the focus properly adjusted? Did you played around with time/div to see if there is a signal on top of the square wave? If you ground the channel is the line slim?Alexander.

author: Unknown
author_rank: Unknown
timestamp: April 07, 2012, 03:47:02 pm
content: Looks like a focus issue to me.

author: 3n2323
author_rank: Contributor
timestamp: April 07, 2012, 04:13:16 pm
content: Quote from: firewalker on April 07, 2012, 03:25:33 pmfocus was adjusted to the best we could. would the thin and clean rising/falling edges rule out the focus issue?there was no signals on the top, they were just fat, further focus adjustments would make them thinner, but fuzzier, and longer.without a signal, the line was a whole lot thinner, although not as sharp and clean as i would like it to be. i don't know enough to tell whether the line is normal or not, i'll snap a picure of it next time.

author: 3n2323
author_rank: Contributor
timestamp: April 07, 2012, 04:15:07 pm
content: Quote from: alm on April 07, 2012, 03:47:02 pmthank you alm!is focusing a weak spot of Tektronix 465's?is it easy to fix?

author: 3n2323
author_rank: Contributor
timestamp: April 07, 2012, 04:19:42 pm
content: oh, forgot to ask, could this be a probe issue?

author: ModemHead
author_rank: Frequent Contributor
timestamp: April 07, 2012, 04:26:54 pm
content: Pull out the "Trig View" button. This engages the bandwidth limiter. The scope may be picking up high-frequency noise.

author: 3n2323
author_rank: Contributor
timestamp: April 07, 2012, 06:19:38 pm
content: Quote from: ModemHead on April 07, 2012, 04:26:54 pmBingo! thank you ModemHead! love you guys! love this forum!Alexander asked me whether the probe was grounded, and i was investigating, it's really weird, grounding made the top even fatter, and if i hold the probe cable the fatness changed, i was going to report to Alexander on this later. i know there is a 20 Mhz bw button, i read the manual, but it never occurred to me, newbee!thank you for the help! you the guy!funny that the scope was next to a live wireless modem

author: ModemHead
author_rank: Frequent Contributor
timestamp: April 07, 2012, 06:47:34 pm
content: Electro-magnetism. It really works. Sometimes even when you don't want it to.Glad you got it sorted.

author: 3n2323
author_rank: Contributor
timestamp: April 08, 2012, 02:44:36 am
content: thank you all guys for your great help! here are the results of my investigation.Trigger View button pulled out, i.e. 20 mhz bw limitation on.probe not grounded, since that doesn't seem to have decernible effects on display quality.[noprobe.jpg]the two traces of channel 1 and 2, without probes plugged in. i suppose they are ok, normal, or acceptable as normal.[wave.jpg]now the wave display looks like this, great!but the verical rising/falling edges are barely visible, and they don't seem to be "in focus", when focus is adjusted so that they are "in focus", the tops get fat.[intensity.jpg]intensity turned up quite a bit.the vertical rising/falling edges are visible now, but still rather faint, at the same time the tops have become abnormally fat.[vfocus.jpg]with intensity up, and focus adjusted so that both the tops and the vertical edges are "in focus".the tops are abnormally fat, and cannot be focus adjusted to thin and crisp.in this state, if the probe is unplugged, the flat line trace of the channel will be fat to the same degree, not "in focus" that is.turning the intensity back down, and re-adjusting the focus will get the display back to [wave.jpg].so what's the deal here?

author: ModemHead
author_rank: Frequent Contributor
timestamp: April 08, 2012, 03:39:43 am
content: The trace appears on a CRT because an electron beam is striking a phosphor coating on the inside of the tube. The amount of light emitted by the phosphor depends on the speed at which the beam is moving. The slower the beams moves, the more light that is emitted. Notice that as you turn the sweep speed up, the trace gets dimmer. So for a square wave, during the rising and falling edge, the beam is moving very very fast, therefore very little light will be emitted. It is normal and expected that a square wave will look like two separate broken/dashed lines on an analog scope.With a grounded, flat trace, adjust the intensity so the beam is just easily visible in your ambient light conditions. Adjust the focus for sharpest trace. You will need to re-adjust the intensity if you change sweep speeds, but the focus should need little or no further adjustment.Edit: @robrenz, yea same thing but you have numbers!

author: robrenz
author_rank: Super Contributor
timestamp: April 08, 2012, 03:48:06 am
content: Edit @ModemHead, your post came in while I was posting this. Sorry but we are saying the same thing.I am no expert but this may help you.All you experts: I am intentionally simplifying some aspects of this description, don't crucify me.This is the way an analog scope is supposed to behave. For a given intensity setting, trace brightness is roughly proportional to beam speed. On the horizontal portion of a trace the beam speed is equal to the horizontal sweep speed. But the approximately vertical portions of the trace the beam speed is a function of the volts per division setting and the rise time of the scope and signal.That roughly vertical beam speed can easily be hundreds of thousands times faster than the horizontal speed. That is why the rising and falling edges are very faint or not visible at all.Example: 1ms/div sweep speed equals 1cm/ms beam speed on the horizontal parts of the trace. apply a square wave with a 10ns rise and fall time (includes rise time of scope) and adjust v/div to have signal 1 division high. The beam speed on the rising and falling edges are 1cm/10ns or 100000cm/ms. 100000 times faster beam speed, no wonder it disappearsIn general:Adjust focus for the finest trace and leave it alone.Adjust intensity for a comfortably visible trace on the sweep speed you are using and leave it alone.Realize you are never going to get the rising and falling edges to be as bright as the horizontal part of the trace.

author: rf-loop
author_rank: Super Contributor
timestamp: April 08, 2012, 06:28:44 am
content: Brightnes related to time what beam activates phosphor is feature what they very hard try copycat also to digital oscolloscopes. In expensive high-end digital oscilloscopes it works well but not still ads good as in analog. But also this is disadvantage in analog when need capture fast things. If people have lot of experience with analog osciloscope he can use this brightness differencies also as information. This feature have pros and cons. With very fast single shot things more just cons. But with repetitive signals it may also give big advantage over entry-level or middle level digital oscilloscope.----Focus.Use sinewave. Around 6 div high and speed so that cycle is 2 or 3 divs.Then adjust brightness so that it is not dim and not too bright!Then adjust together focus and astic (in front panel) for best overall sharpness. (focus and astic affect different direction of focus)This need be well focused in x and y axis. Also you can use square but there is problem due to risetime and top and bottom is easy too bright and edges barely visible or you need adjust brightnes too bright. For Focus/astic together adjustmet it is better use sine so that signal brightness is nearly same every place.I do not remember if in this model is also separate "focus" for highest brightness or other compensation.Also you can easy find full service manual and there is full detailed paragraph about adjustments. Follow it in order.There is also exact procedure for display (service) adjustments. All display related adjustments need be adjusted and values measured as service manual tell. Tektronix service manuals are very good and every word need take serious, also there are things what need do in order stated in manual.Basically it looks in pictures like there is not bad problem.Here you find service manual:http://bama.edebris.com/download/tek/465(2)/465.pdfParagraph 6 start with power supply check but adjust ONLY in extremely need becouse it affects to many things.Look table 6-1 about adjustment interactions.Then display and z-axis. Do not go inside adjustments if you have not experience and knowledge about High Voltage working. There you need example measure and adjust over 2kV .For best focus - astic adjustment you can also use X-Y mode and dim dot (not too bright!) near center screen. Adjust both together for best dot.

author: onlooker
author_rank: Frequent Contributor
timestamp: April 08, 2012, 10:45:01 pm
content: Has anyone mentioned CRT aging? That is always a fear when buying any old scopes.

author: 3n2323
author_rank: Contributor
timestamp: April 09, 2012, 12:48:29 am
content: Quote from: ModemHead on April 08, 2012, 03:39:43 amthank you modemhead!good information, and it all makes sense; good practical advices as well; both are just want i needed, thanks a lot!so judging from the pictures[noprobe.jpg][wave.jpg]the scope looks alright, not an issue, correct?

author: 3n2323
author_rank: Contributor
timestamp: April 09, 2012, 12:53:23 am
content: Quote from: robrenz on April 08, 2012, 03:48:06 amthank you very much robrenz for taking the time answering my newbee question!you are the expert, the numbers made the point really clear, totally convinced! same as to modemhead, very good, useful, and practical advices too on how to treat focusing when using an old school scope, thank you!

author: 3n2323
author_rank: Contributor
timestamp: April 09, 2012, 01:23:39 am
content: Quote from: rf-loop on April 08, 2012, 06:28:44 amthank you rf-loop for such a thorough and rigorous treatment to my stupid naive newbee questions, really appreciate it!i've already downloaded the service manual, very useful and necessary document to a piece of equipment, thank you!what i learned from you is that everything comes with its own characteristics, which are not good, neither bad, they just are. the key lies in smart people take advantages of such characteristics."use this brightness differencies also as information", just brilliant! wow!such seriousness, such rigor, such understanding of things!fantastic!(C)love it!(C)i'll trying following such examples in learning electronics, thank you so much rf-loop!

author: 3n2323
author_rank: Contributor
timestamp: April 09, 2012, 01:36:21 am
content: Quote from: onlooker on April 08, 2012, 10:45:01 pmthe CRT looks good to me, no burn-in's, and the intensity can be adjusted from very dim to very bright. now, do the traces get too fuzzy when intensity is turned up, i don't know where to draw the line between normal and not normal there.what sort of things/signs should i look for in this regard?do you think pictures [noprobe.jpg] and [wave.jpg] look normal and acceptable?thank you for mentioning this!

author: onlooker
author_rank: Frequent Contributor
timestamp: April 09, 2012, 02:48:12 am
content: Burn-in is only one part of CRT aging. The other aging processes are the electron gun aging and vaccum level degradation. If what were shown in the two low brightness pictures were the best you can get by all adjustments, I would say CRT aging is the likely problem (less likely are the circuitry problems), since the trace width is a lot thicker than the grid line on the tube. When a CRT aged, to reach the same brightness, a lot more current is needed; this will make the circuitry working outside the specs. For example, the driving high voltage will drop, and the beam can't be focused (or can only be focused in one direction, not both).Ppl has been suggesting some kind "shock" treatment to cure CRT aging. I think it may risk a half working CRT to a sudden death. Anyway you can google it to find the details.

author: rf-loop
author_rank: Super Contributor
timestamp: April 09, 2012, 08:20:30 am
content: Specially ageing maybe problem if tube is used very special way (and tube is also littlebit "special"). Some kind of example is HP8568 spectrum and other similar or nearly similar. There is one beam and it is used for drawing all. Signal trace of course and also then data txt etc but then also scale lines. It need large dynamic becouse different things draw with different speed and different current. (driving circuits are extremely complex) After tube have extremely high power on hours there can see problem so that thin scale lines, txt and sweeped trace can not focus together. There is special way to stretch lifetime but it need special knowledge and skills to do it. I do not know power on hours where this problem come clearly visible so that inside service adjustments can not anymore help. Tube dynamic change. Overrall these very expensive special tubes are very good.TEK 465 do not use multi speed multi current drawing in same CRT picture. So problem can only see when you adjust brightness (what need do as speed change. In this case also some small focus adjust need) rising edges (dim) and other part of signals still draw with same current. Old tube this focusing maybe can not do perfect but very rare it go so bad it can not use. But many times if really do all service adjustments inside scope optimum it may help some amount.Some Tek scopes have also more adjustments inside for example adjustments for high brightness focus and other compensation adjustments. Also geometry adjustments.Tektronix oscilloscope (vector type) tubes (most of) are maybe best what ever have done.More new and complex scopes have cursors and lot of numbers and text in same picture, and all drawed with one beam all together. (in TV type raster monitor situation is totally different and driving it is kids play)I have repaired and service adjusted (how many - simply: lot of) many different Tektronix oscilloscopes. I have not seen any really end of life tektronix vector tube so that it have happend in normal use without extra things for damage. Yes if people forget brightest line over monitor weeks or months, there may be burn in. In some use scpe may be power on in full use 24/7. Then then year give 87000 hours! But in some lab, it is maybe average used 2 hours per day or 2 hours per week.In old scope there may be 200 or 5000 hours and in this case tube do not normally come any kind of problem. Of course it is rarely possible tube fails but more typical is just some other parts in 20 to 40 years old scope (if it is Tektronix. If it is other than Tek or HP it may be really end of life state becouse most of them have made as entertainment consumer electronics)I have seen Tek 2465 with >60000 (!) power on hours. Allwhere in front panel can see it was really used. Not any kind of problem with tube itself.I have seen example ~2000 hour used 2246. can not focus and other small problem seen. One failed resistor. Not burned. Just as nice as new, but resistance have changed really lot of and tube work wrong.

author: 3n2323
author_rank: Contributor
timestamp: April 21, 2012, 12:02:17 am
content: Quote from: rf-loop on April 09, 2012, 08:20:30 amthank you very much rf-loop for such detailed and expert account of old scopes!i'm leaning, all is much appreciated, i'll keep it in mind when looking at an old scope to buy.

author: jgbena
author_rank: Contributor
timestamp: April 25, 2012, 05:34:01 pm
content: 3n2323, I have the same scope... and this is just the way things are.as someone alluded to earlier, its the amount of time that the electron beam dwells on that particular spot on the phosphorescent screen. when viewing a sine wave it looks nice and continuous because its a steady rate at which the beam moves. on a square wave the beam has to jump VERY rapidly in the Y direction, so the time it is able to dwell on a particular point on the screen is minimal. Hence the dimmer appearance of the waveform on the vertical axis. Generally I adjust it such that the horizontal portions are at a "comfortable" setting for me, and ignore the vertical lines because I know they are there anyway!There are two things that can affect the scope trace if it is out of focus.. first is "FOCUS" and the other is a small pot called "ASTIG" (adjusted with a non conducting screwdriver) short for astigmatism. ASTIG helps adjust the beam to compensate for the shape of the screen, once set, you generally dont touch it again until you notice a beam that is sharper in the center than it is on the sides. After this adjustment "FOCUS" sharpens or blurs the entire image and is the only one you touch on a daily basis.

author: 3n2323
author_rank: Contributor
timestamp: April 25, 2012, 05:43:34 pm
content: Quote from: jgbena on April 25, 2012, 05:34:01 pmthank you very much jgbena for your input! it is nice and a relief to hear from owners of the same scope.now, i may end up buying this scope from my friend.meanwhile, all the info from every one is really useful and helpful, i learned a lot about old scopes, great!

author: pullin-gs
author_rank: Regular Contributor
timestamp: April 26, 2012, 04:30:28 pm
content: It's spurious RF noise.Take your probe, coil up all the loose slack, and lay it on top of your scope, and see it clear up some.Focus is not selective (the vertical legs would be fuzzy also.Scope is fine.Another way to get rid is to build a simple low-pass filter which will not pass those higher frequencies relative to your target rate (some scopes have a switchable one built-in already, usually around 20Mhz or so).