Tektronix 465 horizontal issue - Page 1
Forum section: electronics / testgear

author: bob808
author_rank: Frequent Contributor
timestamp: November 16, 2013, 03:23:23 am
content: Hello.I got a Tek 465 scope from ebay and I was surprised to see that it worked with no problems. Perfect cosmetic condition, came with original service manual, pouch and a blue screen (I don't know what that's used for, there's already a clear one installed, and a metal mesh in front of that).After a few tests it was clear that I need to recalibrate it.Calibration signal was about 270mV with 960hz. I could get it to 300mV but there's nothing on frequency adjustment in the manual. If anyone knows how that's done...Voltages are very close to spec:5v - 35mV ripple55.74V - 47mv ripple15V - 29mv ripple-8V - 10mv ripple112V - 30mv ripple.This is my first scope (somehow, as I just got a digital one as well so it's a first for me anyway).I followed the calibration procedure about 4 times until I understood what was happening, and I got everything right. I don't have a function generator so I used the 1khz signal from the digital scope, I had another 4mhz tcxo and 16mhz from arduino crystalI only used the 16mhz sine wave for trigger calibration as that's not that reliable. 1khz and 4mhz generators are spot on with digital scope and dmm.The problems that I have are with the horizontal part of the oscilloscope.First the horizontal line goes beyond the visible area of the CRT. I can adjust the horizontal knob and get more of signal both ends. The trace also fades away in a fuzzy way instead of a sharp dead stop (only for sine waves, for rectangular it ends nice). If I increase the brightness then I get even more trace at both ends. At maximum crt intensity I get really fuzzy endings on the trace.On faster time sweeps (faster than 5us) I get a dimmed portion of the display that moves with the horizontal knob, it's signal dependent, doesn't seem to be from the crt itself. The "faded" part of the signal moves with changing the knob, with signal and with gnd couple on either channel.Also when I gnd couple both channels (on slower sweep speeds) the line is nice and sharp but it has a "belly" in the center part, oriented upwards, and is about a micro division on horizontal. If I move the line to the upper part the shape inverts, with the "belly" downwards. On the left side the trace tends to distort more on the extremes. I couldn't test the HV as I don't have proper probes but I did adjust the CRT bias as per instructions.Any ideas?

author: bob808
author_rank: Frequent Contributor
timestamp: November 16, 2013, 10:58:46 pm
content: I made some more tests and as I cleaned my face plate I must have offset the horizontal knob a bit, but let's say the trace starts from the first division, my trace extends to the right for about one more division.Also I tested the horizontal amplifier and all transistors seem ok, also I got the same shapes as per manual in the test points indicated.Also I did a measurement for the signals going to the CRT plates and they are identical.I did another test in XY mode with 5Vpp sine wave on CH1 and I get a 45 degrees line with the same vertical divisions as the signal so the horizontal amplifier seems ok.I was thinking maybe the sweep generator but there are two for A and B modes and the dimming problem presents itself in both modes, although in B mode the trace moves a micro division to the right and the dimming moves on the trace.Also I put a 1khz signal on ch1 and 16mhz signal on ch2 and in ALT mode I get the dimming but as I switch to either channel the dimming becomes brighter, although is still there.

author: sync
author_rank: Frequent Contributor
timestamp: November 17, 2013, 01:34:04 am
content: Quote from: bob808 on November 16, 2013, 10:58:46 pmThat's ok. The trace on an analog scope should be longer than the screen. Usually 11 divisions.

author: bob808
author_rank: Frequent Contributor
timestamp: November 17, 2013, 02:11:16 am
content: I so than that's very good but I'd like to solve the dimming issue.I hope I get some time tomorrow to check the sweeping circuits.

author: sync
author_rank: Frequent Contributor
timestamp: November 17, 2013, 02:41:04 am
content: Quote from: bob808 on November 16, 2013, 03:23:23 amIt's ok. The calibrator is only spec as "approximate 1kHz".QuoteFading in and out is normal for faster sweeps. The intensity modulation has only a limited bandwidth. At what sweep speeds do you get this?QuoteThis is not normal. Maybe caused the z-axis (intensity) modulation circuit. Nasty area. Parts of the circuit are at -2500V!btw: Only do adjustments when you know what you are doing and have the necessary equipment. Make the performance tests from the service manual first.

author: bob808
author_rank: Frequent Contributor
timestamp: November 17, 2013, 01:12:20 pm
content: Quote from: sync on November 17, 2013, 02:41:04 amOn a tektronix scope the test signal is "approximately" 1khz? Didn't know that but ok, not that important anyway.I get the dimming on some portions of the signal at faster than 5us sweep speeds. Also as I said if I move the horizontal knob the trace moves with the dimmed part together, so it's like it's "glued" to the signal. It's there even without any signal.I attached pictures with GND coupled CH1 at all sweep speeds where the first problem appears in the order: 5us, 2us, 1us, 0.5us, 0.2us, 0.1us and 0.05us.*later editAlso even if the extension beyond the visible area of the trace is normal, can I adjust it somehow to stop at the last graticule?

author: sync
author_rank: Frequent Contributor
timestamp: November 17, 2013, 03:05:46 pm
content: Quote from: bob808 on November 17, 2013, 01:12:20 pmOn faster sweep speeds raise the intensity until you see the beginning of trace. I think you will see the trace in dimmed portion then. Reduce the intensity on slower sweep speeds again.QuoteWhy? You would loose the availability to do measurements over the full 10 divisions.You should learn how to use an analog scope first and get experience. On all of your pictures the scope is working. There is this small dimming issue. But that's only cosmetic. No need to try a repair and risk damaging the scope or hurting yourself.

author: bob808
author_rank: Frequent Contributor
timestamp: November 17, 2013, 03:35:39 pm
content: Well I'd have a full signal to measure. Maybe I didn't said it right, I mean I'd like the trace to stop where the last vertical line is noted on the CRT. So I have signal over the whole 10 divisions, but not extend to the outside of the display.I do like to work on tube amplifiers so I learnt my lessons a few years ago regarding HV O0Also there would be no problem if I end up breaking the whole thing. It's not like it will ruin me financially and I am also learning in the process of repairing. So if something is not working right I pretty much would like to try and repair it. I have my digital scope for other kinds of work/repairs.

author: tautech
author_rank: Super Contributor
timestamp: November 17, 2013, 06:37:44 pm
content: Good on you! IMO there is nothing worse than 2 pieces of your kit giving you 2 different measurements. There will be a way to adjust the trace in manual, as I have done a few, with similar problems and it may be linked to HV if my memory serves me. Check the HV is correct first then the plate amps. Just remember the voltages you are working with!

author: bob808
author_rank: Frequent Contributor
timestamp: November 17, 2013, 07:12:49 pm
content: I was just looking for enough 1Mohm resistors in the box

author: bob808
author_rank: Frequent Contributor
timestamp: November 18, 2013, 12:08:01 am
content: I adjusted the HV supply to -2450V, but I couldn't find any other problem. All test waveforms are as the manual shows. I think the problem is that the manual suggests you do the tests with 1ms sweep times. The problem presents itself at the fastest speeds so if anyone is bored with their 465 and want to make a measurement I'd appreciate a photo/screenshot of the waveform for test points 64, 87, 88 and 89 at 0.05uS sweep

author: bob808
author_rank: Frequent Contributor
timestamp: November 18, 2013, 11:36:50 pm
content: Well I fixed the horizontal extension problem, now it fits perfect on screenI totally screwed up the previous calibration and it seems that X1 gain adjusts this.Now all that's left is get to the bottom of the faded signal. It's really annoying over 1mHz and I need to crank up the "brightness" of the CRT and it hurts my eyes. Since all the waveforms check out ok at the settings in the manual I'm really in the dark right now. I notice that those waveforms do start to look very ugly once I am past 5us and go to worse when going faster to 0.05us. And I saw that in many test points. All the transistors that I've checked are working, nothing is shorted out. Power supply voltages are spot on. I do have some 20ish mV ripple on some of them but I have lots of noise around me, lights, UPS, computer etc.

author: sync
author_rank: Frequent Contributor
timestamp: November 19, 2013, 12:18:37 am
content: Quote from: bob808 on November 18, 2013, 11:36:50 pm*double facepalm*The X1 gain is for adjusting the time/div value. Read the service manual. Now your timebase is not calibrated anymore. Measuring times and frequencies give wrong results now.

author: bob808
author_rank: Frequent Contributor
timestamp: November 19, 2013, 01:00:31 am
content: I was suspecting that it maybe alters the initial calibration but that was just something I did while searching for the fault. Ofcourse that I will calibrate it again and see if I can somehow manage that issue. Initially I did that with gnd coupled so the line got shorter and I had a brief moment of joy until I remembered that I used that pot in calibration so I wasn't 100% sure.I am not even discussing calibration at the momenti need to find the problem and unless someone who knows this issue is found, it is going to take a whileIt's nice at the moment as it keeps me busy but I hope I won't become frustrated in the near future.Again, I screwed the calibration completely while searching for the issue and I'm not planning on using this scope in the near future unless I fix it.Any ideas at least on what kind of problem I'm facing? Is it an interference or a blanking problem or something recognizable? On B sweep the problem is less obvious and starts at about 2us but still pretty ugly at 0.05us.

author: bob808
author_rank: Frequent Contributor
timestamp: November 19, 2013, 11:15:25 pm
content: This issue has been addressed in the manual, but I couldn't fix it in the first calibration I did so that's why I resorted to looking somewhere else.I added the part of the manual that describes the procedure, I also made some photos from the 465 screen and screenshot from the signal viewed on digital scope.Basically there's this variable capacitor that you adjust for best square wave from a test point, and also has to fill the screen with the signal, especially the first two divisions. This test is made at 0.05us. I even cut one of the resistors leads in series to test individually and they test in spec. I just re-soldered back the leads. My dmm cannot read 0.25-1.5pF so I don't know if that variable cap is in spec but seems well built and there's some change in the shape between fully screwed in and almost all out. It's just not enough, the change is minimal.Is there something obvious that I'm missing?

author: PaulAm
author_rank: Frequent Contributor
timestamp: November 21, 2013, 04:19:22 pm
content: That's a weird enough problem that it might be worthwhile to send it off to the yahoo tekscopes group. It sort of looks like something is wrong with the blanking or Z modulation. You horizontal display switch is in "A Lock Knobs", right?

author: bob808
author_rank: Frequent Contributor
timestamp: November 21, 2013, 05:29:26 pm
content: Yes, it's in that position.I looked in the manual and Q1466 is part number 151-0223-00 and I found it to be MPS2369A so I bought a couple of 2N2369 today. They are a faster type of transistor with turn-on/turn-off times of 12/18ns. I thought maybe the bc549c couldn't make it that fast but...nothing changed.Unblanking comes on the Q1466 emitter. I measured the emitter of the Q1466 and the unblanking signal looks perfect at 0.05us, but on the collector the thing is a mess. The signal on the collector starts to get ugly when I get past the 10us setting and faster, until it's really bad at 0.05us. I added some photos with side by side comparison of emitter/collector for Q1466. Also the description of the circuit.The emitter is the red signal.p.s. I joined that group a few days ago but I still haven't been accepted.

author: bob808
author_rank: Frequent Contributor
timestamp: November 21, 2013, 05:46:15 pm
content: Well, I just remembered that I was looking at waveforms with averaging on. In sample mode I can see something else that's happening there. And I can see it at slower sweeps. It looks like there's another ~50khz riding on the collector signal, and at faster sweeps it just messes up the signal. Maybe this could be the cause. I need to understand what's happening at Q1466My knowledge is minor so I have to do some research.I added some screenshots in sample mode. Duh, I forgot to change Acqu Mode to Sample sometime ago...As before this is emitter vs collector on Q1466. Red line is the emitter.

author: PaulAm
author_rank: Frequent Contributor
timestamp: November 21, 2013, 06:12:12 pm
content: That looks seriously bad.Do you see anything like that at tp 1486? Looks like it might be a little bleed through from the hv supply oscillator.

author: bob808
author_rank: Frequent Contributor
timestamp: November 21, 2013, 06:20:34 pm
content: Yes, seems like it.

author: PaulAm
author_rank: Frequent Contributor
timestamp: November 21, 2013, 07:48:37 pm
content: On the bright side, there aren't too many parts in that section.Just to be on the safe side, check the +55V supply R1480 connects to. I got caught once because there are a couple of +55V sources on some of these scopes.Otherwise, C1488, C1487 and C1420 are possibilities as well as a leaky diode.

author: bob808
author_rank: Frequent Contributor
timestamp: November 21, 2013, 08:06:12 pm
content: Seems like the HV oscillator is at around 50khz and that's consistent with what I get.Another thing to notice is that the previous owner replaced C1418/C1419 with a 47uf capacitor. I don't know if there was supposed to be two 22uF in parallel or a double capacitor but the actual capacitor looks new, I mean new production. And I can see that one pin in the + hole is not properly covered in solder, but it has good contact.On the + pin of the capacitor I get about 23.4V unregulated and on the fuse about 22V. On the fuse I don't get the 50khz extra but on capacitor it's there as you can see in the attached photos. On test point 90 there's 316V at 52.45khz+15V supply is unregulated and at a fast look in the manual I couldn't find it in the power supply schematic. Maybe derived from +55 unregulated.

author: bob808
author_rank: Frequent Contributor
timestamp: November 21, 2013, 08:27:27 pm
content: C1488 shows about 2000pF, C1487 about 5000pF and C1420 I've tried with LC meter, ESR meter and DMM on capacitance but I get high values of 700-1000pF and it's oscillanting. I guess 47pF is way too small and in circuit to be measured. But it's small and sturdyAnd is not shorted.

author: tautech
author_rank: Super Contributor
timestamp: November 21, 2013, 08:38:35 pm
content: "On the fuse I don't get the 50khz extra but on capacitor it's there..."So the inductor is doing its job.Suggest you revisit previous cap replacement and research OM spec for best replacements. I feel 2 caps were put there for good reason. Not sure that low ESR is required.

author: bob808
author_rank: Frequent Contributor
timestamp: November 21, 2013, 08:58:59 pm
content: I looked in the parts list and saw that until an earlier model than mine (28xxxx series) they used only C1419 with a value of 47uF/25V then moved to two 22uF/35V caps. Mine is now 47uF/40V. Maybe they decided to up the voltage rating on the capacitor since I get there about 23V. Don't know about the two, a higher voltage spec cap usually has lower ESR than same capacity and lower voltage spec cap. I will first try to get a low ESR 22uF/50V caps and try it that way. The thing is that I have to remove the trigger cicuit board to reach on the other side. Also as I saw in the manual one of the caps may be on the other side. I may leave it alone for the moment and see if I can find something else.

author: PaulAm
author_rank: Frequent Contributor
timestamp: November 21, 2013, 09:03:48 pm
content: That stuff looks fine. L1419 keeps it from the fuse. The later 465 diagram shows a single 47uF as C1419. That does look replaced, however.The problem is on the other side of the transformer since it doesn't show up on the emitter of Q1466.C1488 is rated at 4KV and C1487 is 6KV. Be careful around that area. I would suspect one of those, but your values are close enough. They may exhibit different characteristics at operating voltage though.Probably a long shot but check C1481.Check the resistors as well and it could be one of the diodes. Something is allowing a little 50KHz ripple through.If you don't find anything, it will be a good Tekscopes question.

author: bob808
author_rank: Frequent Contributor
timestamp: November 21, 2013, 09:10:54 pm
content: Well there are 2500V on the board and I'm only measuring that area with the thing unplugged. Not taking any chances on something like that.Will check the diodes and resistors that are left, and C1481 as you suggested.Thank you very much for your support!

author: bob808
author_rank: Frequent Contributor
timestamp: November 21, 2013, 09:23:24 pm
content: C1481 checks at 0.5uF/7.5ohms ESR so seems ok. I even paralleled it with another 1uF/100V cap and nothing change. There are 10mVpp of 50khz signal on + lead of C1481 thou.And C1419 is 50uF/0.3ohm ESR.UpdateCR1482/CR1483/CR1487/CR1488 diodes check out ok with 0.6V drop and no reverse conductivity.

author: PaulAm
author_rank: Frequent Contributor
timestamp: November 21, 2013, 10:12:24 pm
content: That's a good puzzleWith any luck someone on tekscopes will have had a similar problem. Sometimes parts look good on a static check but do not work as intended under operating conditions. That can be a real bear to track down, particularly in a section where lethal voltages are present.

author: tautech
author_rank: Super Contributor
timestamp: November 21, 2013, 10:43:59 pm
content: In my experience of CRT's many of the old components subjected to 1Kv+ are under stress and fail or "do not work as intended under operating conditions"Caps and high value resistors particularly.I have previously just gone about replacing such BEFORE they fail.Nothing worse than kit that fails halfway through a project.

author: bob808
author_rank: Frequent Contributor
timestamp: November 21, 2013, 11:46:52 pm
content: All resistors also check out perfect. I got two 10k that read 9.98k or something like that. And one 22Mohm resistor that neither one of my DMM don't want to measure accurately. I guess it's at their limits, they sometime show me 10-13Mohms or nothing.I guess I don't have anything more now... I'll have to start again tomorrow and think of something else. Maybe build the voltage divider again.

author: tautech
author_rank: Super Contributor
timestamp: November 22, 2013, 12:01:29 am
content: "And one 22Mohm resistor that neither one of my DMM don't want to measure accurately. I guess it's at their limits, they sometime show me 10-13Mohms or nothing."Replace it!Get a couple of 10M's in seriesandto check your meters

author: bob808
author_rank: Frequent Contributor
timestamp: November 22, 2013, 12:09:10 am
content: Luckily I have some 10Mohm resistors I just got today so I could build the Jim Williams pulse generator and it seems that neither dmm wants to go to 30mohm. But I insisted, I turned off the CFL lampand I managed to get a read on one of the 22Mohm resistor. And it tested out good, but the other I forgot to mention that it isn't your classic resistor, it's a piece of pcb I guess with a trace on it, mounted perpendicular on the main PCB. The trace looks nice but the reading oscillates. I could get a good reading on the other as I saw that there were some diodes in series with the resistor so I just had to switch the test probes and I got the 22Mohms.

author: tautech
author_rank: Super Contributor
timestamp: November 22, 2013, 12:23:45 am
content: Thinking out loud.All the HV stuff is operating at milli-amps and any leakage will load the supplies up and create extra ripple. Dividers, caps, dirty boards etc. etc.I suspect additional loading is what you are looking for.

author: bob808
author_rank: Frequent Contributor
timestamp: November 22, 2013, 12:30:14 am
content: At this point I may wait to be accepted in the TekScopes yahoo group and see if someone else had this problem before. Also I'm considering on building a good HV voltage divider so I can "see" what's happening in that part of the circuit.

author: tautech
author_rank: Super Contributor
timestamp: November 22, 2013, 12:33:05 am
content: "that it isn't your classic resistor, it's a piece of pcb I guess with a trace on it, mounted perpendicular on the main PCB."Could there have originally been 2 of these, one having been replaced with 22M?

author: bob808
author_rank: Frequent Contributor
timestamp: November 22, 2013, 12:48:24 am
content: No, the pcb one is a multi tap resistor and pretty large about 6x2cm. I marked them both in the picture.edit: wrong picture, fixed it now

author: oldway
author_rank: Super Contributor
timestamp: November 22, 2013, 07:40:22 am
content: Check ou replace C1421, C1422, C1423 and C1427.

author: PaulAm
author_rank: Frequent Contributor
timestamp: November 22, 2013, 10:10:22 pm
content: That's a ceramic thick film resistor network. Be careful around that. I banged my head against a wall for a week before finding a cracked lead on one of those. Also, they are practically impossible to find (and not cheap when you do)

author: bob808
author_rank: Frequent Contributor
timestamp: November 22, 2013, 11:46:58 pm
content: Out of curiosity, can't you just replace them with HV resistors? You could make up the required values.I've been admitted to the Tekscopes yahoo group and the guys there gave it a go but no luck so far. I'm back to where I was last night. Z amplifier seems ok. HV caps all check out.I got some today but these are real small, round blue ones. 4k rated same capacitance, ceramic. Why the size difference? Better technology or?

author: PaulAm
author_rank: Frequent Contributor
timestamp: November 23, 2013, 01:21:51 am
content: I've been tracking the discussion; looks like your puzzle has caught the interest of some good guys. Keep at it; you'll learn quite a bit in the process.If you had to, you could make up a new network, but you'd need to pay attention to power dissipation and particularly leakage. You are dealing with a HV regulated supply. Tek did some good engineering since we're talking about 30 year old scopes.If the caps are really small, I'd worry about leakage and arc over in a HV situation. If they're Chinese, I'd worry period.

author: bob808
author_rank: Frequent Contributor
timestamp: November 23, 2013, 02:26:06 pm
content: Here's a size reference of the old vs new capacitor. Same values. To be honest the original ones are 5kV and new ones are 4kV but that's very close. Both 0.0068uF.Is there some other type of HV ceramic capacitor?

author: bob808
author_rank: Frequent Contributor
timestamp: November 23, 2013, 08:01:18 pm
content: I have cut the R1485 resistor so I can separate the Z-axis amp from the DC restorer. The problem remained the same, rounded top on the unblanking pulse on Q1478 collector and complete crap on the collector of Q1466. The pulse on the emitter of Q1466 looks nice. I've tested all the transistors in the Z-amp amplifier and they check out ok, no shorts. I've even replaced all of them with identical ones from all over the oscilloscope. Absolutely no change. I don't know what else to do at the moment.The guys over Tekscopes group have been amazing in the level of commitment to my problem. They gave a lot of insightful suggestions and I managed to exclude the HV area from my problem (and that's good also as I'm not that exposed to HV anymore O0 )

author: markce
author_rank: Regular Contributor
timestamp: November 23, 2013, 11:38:16 pm
content: Hi Bob,Just to encourage you, I've a similar problem with a Tek2236. It's smaller (trace starts a little late at fast sweep settings, the scope had other problems as well. Needed a power supply revision, so did that first. Thats fine now. Very interested in your findings on this one. Lately, I've tried to measure the horizontal trace issue, which is not easy to pinpoint.Replaced a booster ceramic cap in the Z-amp output stage (have't looked at 465 schematics yet). Seems to change things a bit, but cap voltage of the one I used is too low (50V, need 200V), so I'm gooing to order a new one.

author: markce
author_rank: Regular Contributor
timestamp: November 24, 2013, 10:05:55 pm
content: Had a quick look at the 465 schematics. The Z-amp is nearly the same in the 22xx series. Now I'm even moreinterested in your findings from the TekScopes yahoo group.

author: bob808
author_rank: Frequent Contributor
timestamp: November 24, 2013, 10:51:46 pm
content: Hello.At the moment I am in the same spot. I managed to replace some transistors only for a minimal improvement. I still don't have the solution.I will make a full explanation when I fix itIn the meantime maybe you find the problem first

author: bob808
author_rank: Frequent Contributor
timestamp: November 27, 2013, 11:39:51 pm
content: Finally, with the help of the pro guys over at TekScopes group I managed to solve the riddle.It was a faulty transistor, more exactly Q358 on the vertical preamp board. I replaced it and it works good now. I'm still missing about half division on 0.05uS sweep speed but hey, I can live with that. It's pretty good now!But there are a lot of things that could go wrong in that area. Do check the amplifier's transistors and capacitors. Voltages as well.A giveaway for the bad Q358 was a higher voltage on it's base. It didn't fail completely so these things are harder to pinpoint.I will post some photos along the way with some bad signals and test points, but now I will calibrate it one more time so I can put the case on.

author: tautech
author_rank: Super Contributor
timestamp: November 28, 2013, 06:42:42 am
content: I have always been careful to have a vertical input setting that will not overdrive the vertical amps and display a vertical waveform that not will be larger than the screen. In my mind this only applies to analogue CRT scopes. Anybody in EEVBlog land like to add to this or shoot me down?Likely this was the cause of all your grief bob808

author: PaulAm
author_rank: Frequent Contributor
timestamp: November 28, 2013, 05:35:19 pm
content: Marginal parts can be a real bear to track down. Congratulations on tracking down a tough problem.There was some excellent analysis of the circuit; I've save the whole discussion for further study.