How do you evaluate used oscilloscopes before purchasing? - Page 1
Forum section: electronics / beginners

author: ramenbytes
author_rank: Contributor
timestamp: June 27, 2021, 06:35:52 am
content: Hello,I'm slowing trying to learn electronics, with the intent of working on valve guitar amps and eventually RPi/Arduino/Robotics stuff. For budgetary reasons I'm looking to buy a used analog oscilloscope instead of a DSO. I've found some attractively priced local options, such as a Tektronix 2246 Mod A that the seller claims is fully functional. How should I go about testing it before purchasing? Is the Operator Checks and Adjustments section of the manual sufficient to catch most issues, or at least the most problematic ones? Is there anything that you yourself make sure to check when you buy used?Thanks for your time,ramenbytes page 45 of the file available herehttps://www.tek.com/oscilloscope/2246-manual/2246Edit June 28, 2021, 12:18:18 amSo the 2246 was sold to someone else a little earlier while I was replying to this thread. I'm still considering other CRO options and appreciate further advice on evaluating them, however I'm also going to be seriously considering the option of just getting an entry DSO and putting off any other non-essential expenses for a while.Edit July 7, 2021, 08:18:29 amI pulled the trigger on another 2246 in good condition, and got some free gear along with it. I'll post a reply to this thread later with more details and which addresses the numerous bits of input you all gave. Right now I'm trying to make it a cohesive post that shows how all of your advice was applied to my scope search and buying process, in hopes that it will be of use to other people who come across this thread in the same shoes I was in.

author: rstofer
author_rank: Super Contributor
timestamp: June 27, 2021, 03:22:58 pm
content: You can use the Calibrator output to test the scope channels like they show in "Probe Compensation", the next topic in the manual. In x1 mode, compensation adjustments are usually not necessary but you should still see a square wave. You should be able to trigger on the waveform to hold the trace steady and you should be able to adjust the trigger point below the trace and it should roll to the left.Try all the controls (V/div, time/div, trigger, etc) and see if they seem to do something.

author: ramenbytes
author_rank: Contributor
timestamp: June 27, 2021, 08:09:21 pm
content: Quote from: Max2018 on June 27, 2021, 07:48:45 amFor the AC coupling, do I need a signal generator? If I understand correctly, AC coupling dumps the DC component and only shows the AC component. In which case, if I probe a DC source like a AAA battery with the scope AC coupled, then I shouldn't see anything other then a zeroed out signal right? I'm going to go RTFM...Quote from: Max2018 on June 27, 2021, 07:48:45 amGood point, a dim trace would mean a bad/worn down CRT right?ramenbytes

author: WattsThat
author_rank: Frequent Contributor
timestamp: June 27, 2021, 09:12:26 pm
content: QuoteCorrect. Doing that same thing with a power supply can show the ripple and noise of the supply.Spend some time understanding the on screen annunciators, they’re extensive on that scope. Check that they track the different selections such as vertical voltage per div, time, 1/time, delta time, etc.Don’t get too enamored with old crt based scopes, they may be fun to learn some basics with but the functionality compared to a modern digital scope is night and day. Add in the portability and noise levels and a modern digital scope wins in every category every time.Don’t spend more than $50-$75 on such a scope. That money would be best added to upgrades and accessories for a modern scope. Remember that an analog scope cannot display a one shot event, the signal you’re examining must be continuous/repetitive. For what is dead easy to do today with a modern digital scope, required a rather expensive and not so common “storage” scope back in the day.

author: tggzzz
author_rank: Super Contributor
timestamp: June 27, 2021, 09:25:16 pm
content: Quote from: WattsThat on June 27, 2021, 09:12:26 pmModern digitising scopes have more subtle ways of lying to you than old analogue scopes.All tools have their limitations. The keya are to understand the limitations, how they do/don't affect your current measurement, and,where necessary, use them in conjunction with other tools with complementary advantages.And understand hhe relevance of the aphorism in my .sig to this discussion

author: ramenbytes
author_rank: Contributor
timestamp: June 27, 2021, 10:29:30 pm
content: Quote from: rstofer on June 27, 2021, 03:22:58 pmThat is more or less what I figured on doing. Will there be a visible difference in the 1kHz square wave between AC coupled and DC coupled? I just read a keysight article where they used a 100kHz square wave and that showed significant differences. If 1kHz will too, then I don't think I'll need to bring a battery for testing that as mentioned in a previous post, though I suppose it couldn't hurt. The manual says < 50Hz signals are attenuated with AC coupling, so I'm guessing 1kHz signals won't show anything.Quote from: rstofer on June 27, 2021, 03:22:58 pmTriggering, something else I need to RTFM about. I'm starting to think that I should spend some time going through the manual, in addition to this thread, and make a detailed plan for what I will test on the scope.Quote from: rstofer on June 27, 2021, 03:22:58 pmOk, thanks.

author: BrokenYugo
author_rank: Super Contributor
timestamp: June 27, 2021, 10:56:18 pm
content: AC coupled the the test signal will always be centered, DC coupled any DC bias will push the trace up/down, with the built in calibration signal this will make the square wave either centered or all above the center (assuming you started with a centered trace).For a sub hundred buck used scope* just make sure you get a trace and can view a calibration waveform on all channels, a reasonably sharp and bright trace can be had without maxing out any controls, and fiddle with it a bit while subtly watching for nervous behavior from the seller. Unless you know the guy it's kinda rude to spend a hour or more checking everything for such a low buck transaction.*Any more and you're likely better off saving your pennies for a new DSO, between the feature set and questionable dependability of aging test gear. At some point you end up more of an oscilloscope repairer than user, not fun if it isn't your intent.

author: bdunham7
author_rank: Super Contributor
timestamp: June 27, 2021, 11:00:27 pm
content: You can do all the testing you want--and you should--but in the end your best bet is to get as good a deal as you can on the least beat-looking scope possible and hope for the best.I have an old 485 here that looked OK on almost every basic test and only when I went to do an actual calibration/performance test did I discover that there's some sort of severe distortion only on the fastest timebase. I've seen other strange things crop up, like cursor or measurement readings being wrong, that weren't readily apparent and were not minor adjustments. I rarely find an analog scope these days that doesn't have some issue or another.

author: ramenbytes
author_rank: Contributor
timestamp: June 27, 2021, 11:29:55 pm
content: Quote from: WattsThat on June 27, 2021, 09:12:26 pmGood to know, thanks.Quote from: WattsThat on June 27, 2021, 09:12:26 pmI hadn't thought of that, I'll add that to the list.The 2246 just got sold to someone else. More motivation to consider a DSO I suppose.Quote from: WattsThat on June 27, 2021, 09:12:26 pmNo kidding, the Rigol and Siglent that this form always suggests look like incredible tools.Quote from: WattsThat on June 27, 2021, 09:12:26 pmI'll for sure keep that in mind. I've been through several threads discussing this already, including this recent one by a woman looking for an oscilloscope:https://www.eevblog.com/forum/beginners/what-an-oscilloscope-recommended-for-a-woman-passionate-about-electronics/.I think I could conceivably pony up the cash for one of those recommended entry-level DSOs, however I'm hesitant to since that would mean essentially no budget for electronics or guitar related expenses for a while, and I already had some gear upgrades planned. I think I need to re-examine some priorities, and see if I'm willing to put non-electronics stuff on hold for a bit and whether my current electronics setup is sufficient to make good progress until I'm able to invest financially again. I guess tggzzz's aphorism comes to mind here.Thanks for the input, I've got some more thinking to do.

author: ramenbytes
author_rank: Contributor
timestamp: June 27, 2021, 11:46:30 pm
content: Quote from: tggzzz on June 27, 2021, 09:25:16 pmHow so? Aliasing? I don't know what that means, it's just a buzzword I've picked up.Quote from: tggzzz on June 27, 2021, 09:25:16 pmGood point, and something I don't often remember. It seems easy to get tunnel vision when looking for the next lab upgrade.Quote from: tggzzz on June 27, 2021, 09:25:16 pmIf I understand correctly, it highlights the trade off between clever solutions, and buying solutions? For example, the stunts pulled in the demo scene vs just buying a modern workstation?

author: ramenbytes
author_rank: Contributor
timestamp: June 28, 2021, 12:09:59 am
content: Quote from: BrokenYugo on June 27, 2021, 10:56:18 pmOk. Not sure how I'd bias the calibration signal though, maybe a battery in series? I don't know if that'd harm the scope.Quote from: BrokenYugo on June 27, 2021, 10:56:18 pmThat was also something I was considering, how much time a seller is willing to let me play with it. I don't know that many people would be willing to sit there while I run through the entire catalog of features, even if it isn't for something fancy like a 2246.Quote from: BrokenYugo on June 27, 2021, 10:56:18 pmI'm fine with repairs if I can learn something and they don't require unobtainium or another oscilloscope, though I would also like to be able to get work done on my target projects. Like I mentioned above, I'm going to spend some time re-evaluating some goals and priorities. One of the biggest selling points of DSOs for me as a programmer is that I can connect them to my computer and use stuff like SCPI to manipulate them and extradite data.

author: ramenbytes
author_rank: Contributor
timestamp: June 28, 2021, 12:16:02 am
content: Quote from: bdunham7 on June 27, 2021, 11:00:27 pmThis reinforces the growing thought I have that CROs are a gamble. I'd already figured as much, but I thought most of the issues could be caught by thoughtful screening upon purchase. On the other hand, a mostly-functioning scope would still be wildly more useful than no scope. Thanks for the data point.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 28, 2021, 12:24:03 am
content: Quote from: WattsThat on June 27, 2021, 09:12:26 pmThat said, many analog 'scopes with a storage function spent almost their entire, very productive, lives with the storage function used maybe two or three times.In much work, displaying a "one shot" event is a rare requirement, whereas in other activities it is an everyday thing.The important question is, what will the OP use the Oscilloscope for?Another point is, if you are going to get a DSO, don't get averyearly (probably 1980s) model.The memory on these is abysmal, to the point where such signals as analog video are impossible to display.The sampling rate at longish time/div settings reduces so far that on such settings, they are only useful to display sine waves.Fortunately fairly capable DSOs are now available relatively cheaply.

author: tggzzz
author_rank: Super Contributor
timestamp: June 28, 2021, 01:06:05 am
content: Quote from: vk6zgo on June 28, 2021, 12:24:03 amPrecisely.I remember using analogue storage scopes in anger, and they were always a pain. I'd argue they were only used occasionally since the number ofnecessarilyone-shot events is small (PSU turn-on is one, bomb detonation is another). With care and imagination, most events can be turned into repetitive events.QuoteI would limit it to digitising scopes introduced in the last decade. Earlier than that and analogue scopes had some signifiicant advantges. I still have a working analogue storage scope, with dual beams and three input channels. The dual beam is necessary for capturing fast single-shot events on >1 channel.

author: tggzzz
author_rank: Super Contributor
timestamp: June 28, 2021, 01:13:17 am
content: Quote from: ramenbytes on June 27, 2021, 11:46:30 pmDon't panic about aliasing, provided the scope has a peak capture facility - recent digitising scopes do.Instead look at all the options hidden deep inside menu systems: they are there for a reason, need to be understood, and can still surprise the unwary.QuoteSkilled workmen can do wonderful things with very simple tools. The less skilled have to use money as am imperfect substitute for skill - and thinking. When somebody says "you must buy X", which type of person are they?

author: tggzzz
author_rank: Super Contributor
timestamp: June 28, 2021, 01:17:09 am
content: Quote from: bdunham7 on June 27, 2021, 11:00:27 pmI rarely see a low-end digitising scope that doesn't have some issue or other (I don't have experience with recent medium-level scopes). A couple of years ago there were long threads on this forum about that.

author: james_s
author_rank: Super Contributor
timestamp: June 28, 2021, 01:55:31 am
content: Quote from: ramenbytes on June 28, 2021, 12:16:02 amAny old scope is a gamble, that's just the way it goes when buying cheap used gear. If you don't have the budget for something like a Rigol or Siglent then just give the scope you're considering a quick go-over to make sure that you can get a trace and display the calibrator waveform and check the basic controls. Don't waste the sellers time asking a lot of questions or doing an extremely detailed inspection, they're selling a cheap piece of used gear and are going to expect you to take it off their hands with a minimum of hassle. I absolutely agree that any scope is better than no scope, if the price is right and it seems to more or less work then either buy it or don't, but if you sit on the fence for very long and it's a reasonable deal, somebody else will come along and snap it up. When I've bought scopes, most were either known defective or a complete gamble that I didn't even test out, I just bought them as projects and sometimes I've been pleasantly surprised.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 28, 2021, 03:32:04 am
content: Even a dual channel Oscilloscope with one faulty channel is still a useful "single channel" 'scope.Hell, I've used one like that at work, when 'scopes were "at a premium", & you grabbed what you could find!

author: rstofer
author_rank: Super Contributor
timestamp: June 28, 2021, 05:18:40 pm
content: Quote from: ramenbytes on June 27, 2021, 10:29:30 pmIn effect, AC coupling just adds a series capacitor that blocks the DC component of a waveform, if any.On the Rigol DS1054Z, the Calibrator output is a square wave referenced to 0V so it contains a DC component of Vpeak/2. The signal ranges between 0V and Vpeak.Switching between DC and AC coupling results in the waveform shifting down until it is symmetric with respect to 0V when AC coupled. You can just swap between AC & DC and watch the waveform shift.I don't know about any other scope but I would expect them to be similar.

author: mrkev
author_rank: Regular Contributor
timestamp: June 28, 2021, 09:38:58 pm
content: I don't see the problem with starting with an analog scope. It's IMHO a good way how to start, especially if you can buy some advanced one for a good money. But ultimately, it all deppends on the application.For amps and audio stuff, analog is still great and might be even better than some cheap DSOs. For debugging digital circuits (Arduino, robotics) it's good to have a DSO, because you want to be able to check non repeating signals (like communication on any bus). There are ways around this (like changing the program to generate trigger signals), but they are almost always pain in the...

author: tggzzz
author_rank: Super Contributor
timestamp: June 28, 2021, 10:09:14 pm
content: Quote from: mrkev on June 28, 2021, 09:38:58 pmIf you are debugging digital waveforms, then use a digital domain tool such as a logic analyser. Basic ones are remarkably cheap, and more expensive ones can be much more effective than a scope.

author: Old Printer
author_rank: Frequent Contributor
timestamp: June 29, 2021, 02:49:18 am
content: Quote from: ramenbytes on June 27, 2021, 06:35:52 amI think one of the newer digital scopes like the 1054z would serve you better as a first scope. I have seen Amazon sell that scope as a open box return for $350 on occasion. I prefer the Siglent 1104x-e, but that is a pretty solid $500. The older analog scopes like Tek 465 will likely need repairs from time to time, and the later model 4000 series analog-digital are very complicated to work on and often tough to get parts for. They make a better second scope if you can find a decent one cheap enough. Just my 2 cents.

author: tggzzz
author_rank: Super Contributor
timestamp: June 29, 2021, 08:24:47 am
content: Quote from: ramenbytes on June 27, 2021, 06:35:52 amDon't worry: there will be another one along soonDo worry: get the right class of probe for you applications, and understand how to use them without blowing up your scope/equipment or yourself.Do worry: the cost of probes can be non-trivial!

author: Berni
author_rank: Super Contributor
timestamp: June 29, 2021, 10:18:05 am
content: More of a question is how long before an old clunker like that needs some servicing.Old test gear from an unknown origin can sometimes hide marginal faults inside that manifest themselves later on. Perhaps the thermal cycles or a rough bumpy ride home can be the final nail in the coffin of a weak stressed cold solder joint. Sometimes switch contacts get wonky with age as they oxidise etc. But don't let this discourage you. If your scope stops working you can have a go at repairing it yourself. Old test gear is very repairable and tends to come with excellent service manuals to make it easier. In fact once you get a bit better at fixing stuff then it becomes pretty fun to intentionally buy broken test gear dirt cheep, then fixing it into being fully operational. This can be so fun that it turns some people in to sort of test equipment collectors, and its not a bad thing. There is plenty to learn from repairing test gear, since you have to dig in and understand how it works in order to fix some of the more complicated faults.Tho you will likely quickly see the benefits of a proper digital oscilloscope and end up buying one of those too. Nothing wrong with having more than one scope. If anything it can help having a scope to debug a broken scope. Some scopes are also better at certain tasks, so a more modern scope is not always better at everything.EDIT: And yes i had cases where i used one of my working scopes to help fix my other scope that broke.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 29, 2021, 12:57:33 pm
content: Measure some different signals with both probes at the same time. You should only see one trace on the display. If the channels diverge, then one (of the frontends) of the channels has a fault.Do this over the full frequency range and voltage range to check all paths of the front ends.

author: ramenbytes
author_rank: Contributor
timestamp: July 12, 2021, 12:26:55 am
content: Update:Sorry for the delay, I waited on posting this until I could proof-read and edit it.I ended up getting a Tektronix 2246 Mod A for $100 locally, and from what I can tell it appears to be fully functioning or very nearly so. According to the seller, it had sat in storage since 1994. As a bonus, the seller threw in a HP 200CD Oscillator for free, which he would have otherwise thrown out. The oscillator doesn't appear to be functioning, and he said it'd had a tumble, so I guess that can be my first valve device repair victim. For starters, the power cord needs replacing. Someone floated it...I appreciate everyone who gave feedback, I tried to take it all into consideration during the buying process. Below I'm including a list of various quotes from this thread and how they factored in or related to the process, mostly in hopes that the consolidation will be useful for anyone who happens upon this thread later.Evaluating the condition:Quote from: Max2018 on June 28, 2021, 12:59:46 amThis was one of the first things I checked when I saw the scope listing come up, since from this and other discussions on the forum and the TekScopes group it seemed that old test equipment needs some caring for from time to time. I passed on some better priced scope listings specifically because I had issues finding operator or service manuals.Quote from: Max2018 on June 28, 2021, 12:59:46 amYeah, the one I got needs a little cleaning of the potentiometers. I did managed to break a plastic knob during removal for cleaning, but some epoxy fixed that well enough. I've had a quick peek inside, though not a very thorough one, and I didn't see any obviously faulty caps nor any RIFA suprises. I did catch a glimpse of some motorola "ZM" diodes however, which from what I've read so far are candidate #1 for preemptive repair. I'll be looking into a possible pre-emptive recap as well.Quote from: Max2018 on June 28, 2021, 12:59:46 amEase of repair was a definite factor in deciding on a scope, and one of the reasons I didn't follow up on a 2445A listing. It could be working, but for a first scope I don't want to risk U800 issues. If I join TEA then maybe... Before buying, I also checked both this forum and TekScopes for reports on issues that others had with the 2246, to make sure I knew what I'd likely need to repair and that there wasn't any failure prone unobtainium inside.Quote from: Max2018 on June 27, 2021, 07:48:45 amI played with the knobs to see that they had an effect, though I didn't know what all of them did at the time. I used the beam finder to show a basic grounded trace on all the channels, and pushed relevant looking buttons until the B sweep was configured the same as the A sweep and showing a trace. I also messed with the knobs until I was convinced that the B sweep could display a signal across the entire width of the screen. Since it was originally set with non-zero delay and auto-level triggering it was only showing a trace on part of the screen, which confused me at first.Quote from: Max2018 on June 27, 2021, 07:48:45 amThe probes I was going to use from the seller to test things ended up being lost somewhere among three storage containers, so the only signal tests I did were with a multimeter probe acting like an antenna. I did try some other stuff with a 9 volt and also my DMM's resistance setting (as a current source), however at that time I still hadn't realized that I had the coupling set to "ground". When I was trying the antenna test lead I realized this, and was relieved to suddenly see EM noise from the antenna being displayed. This also indicated to me that the DC coupling worked on at least Ch 1. I may have also messed with the AC coupling too just to make sure it would switch over and show a trace.Quote from: Max2018 on June 27, 2021, 07:48:45 amThe brightness looked good too. A quarter turn on the intensity was bright enough for use, so it seems the CRT isn't that worn.Quote from: WattsThat on June 27, 2021, 09:12:26 pmOut of respect for the seller's time, I didn't thoroughly test all of the readout related features. I mostly just checked that when I turned a knob the readout matched this with the appropriate changes to the Volts/Div and Sec/Div, that the focus control worked. When I was messing with the B sweep I was also able to see the readout showing other relevant info. I think I checked to see if the menu showed up too but no in-depth exploration of it.Quote from: BrokenYugo on June 27, 2021, 10:56:18 pmThat ended up being the gist of what I did. Your post and the others that advocated for managing my expectations of used equipment helped me adopt an appropriate mindset. Prior to that I think I may have been expecting too much. The meetup with the seller actually ended up being around an hour, though most of that was just chatting with the seller and him showing off his collection of electronics stuff (I think I saw an old scope labeled "Pacific Bell"?). Actual testing time was a fraction of that.CRO & DSO trade offs:Quote from: WattsThat on June 27, 2021, 09:12:26 pmI've definitely had fun learning the scope so far, and using it to look at PWM signals for Arduino and Pi has been enlightening. I'm also getting a better idea of the limitations of CROs (mainly the lack of sophisticated recording and analysis functions), however I don't think I'll hit any walls because of that for a good while, so I should have plenty of time to save for a decent DSO. I ended up going with a CRO instead of a DSO because the price was good enough (counting the oscillator as part of the purchase) and while I may have wanted some of the DSO features like a computer interface, I don't really need them and the lack of them wasn't a deal breaker. That, and forgoing the DSO for now means I have some budget room for non-scope stuff like the components for a small project.Quote from: tggzzz on June 28, 2021, 10:09:14 pmI also pulled the trigger on a $9 saleae clone, which in conjunction with the CRO should cover just about everything I'm doing with Arduino's and RPi's, at least for now.Quote from: vk6zgo on June 28, 2021, 12:24:03 amFor right now, I'm looking at just audio work (valve guitar amps and effects pedals) and digital stuff with Arduino and RPi's. Unless I missed something, I won't be hampered by the lack of storage for quite a while. Even when I do need storage capabilities, projects like this one should cover a lot of it:https://hackaday.io/project/171961-a-digital-camera-for-analog-oscilloscopesPricing:Quote from: WattsThat on June 27, 2021, 09:12:26 pmQuote from: BrokenYugo on June 27, 2021, 10:56:18 pmQuote from: bdunham7 on June 27, 2021, 11:00:27 pmI think I compromised a little on pricing, paying $100 instead of the recommended $75 max. I did pay $1/MHz though, so bang-on what the recommended CRO pricing seems to be. Given that I didn't have to pay for shipping, I was able to evaluate it before buying, I got a free oscillator that I can at least sell for parts if not repair, the scope appeared to be in good condition, the last 2246 sold in three days flat, and $100 seemed to be about the ebay price floor, I was okay with a bit more than the recommended price ceiling. Now that I've had it for a couple days and can see that it definitely does appear to be functioning well (I haven't done a performance test yet though, and still need to recap and re-diode), I don't think I'll have much of an issue getting my money back later if, worst case scenario, I need to sell it to finance a DSO in the future. Hopefully I won't need to though.Final thoughts:Thanks to everyone who contributed, both through posts and DMs. I now have a functioning 4-channel 100MHz scope + a valve-based oscillator to practice working on before moving to guitar amps.Quote from: tggzzz on June 29, 2021, 08:24:47 amIt looks like I'll need to get a higher-voltage (around 1kV?) probe at some point for checking waveforms on valve pins. A decent one from probemaster looks to be around $90, so yeah non-trivial indeed. Because I didn't go with a DSO though, I should have room in the budge later if I end up needing such a probe. I'm going to wait until I have a measurement I can't make without it before getting one though. Depending on the state of the oscillator, that may end up being soon.Before working on the oscillator though, I'm going to build a TS808 clone as a budget compromise between guitar and electronics and to get more experience with low-energy circuits before learning high-voltage valve stuff. I plan to use the scope to better understand the circuit, and to mod it. Thanks again for all your help and input!Edit at July 12, 2021, 1:30:38 amFixed the formatting on one of the paragraphs, lines are not hardwrapped at 80 columns now.

author: tggzzz
author_rank: Super Contributor
timestamp: July 12, 2021, 12:43:05 am
content: It is always a pleasure to see someone that asks, listens, understandsandthinksNow you can, as I taught my daughter, "have fun - safely". There are a few ways you can hurt yourself with a scope, especially where high voltages and consumer equipment is involved. I suggest you have a look at some of the references inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

author: jmelson
author_rank: Super Contributor
timestamp: July 12, 2021, 01:20:36 am
content: Quote from: Old Printer on June 29, 2021, 02:49:18 amWell, I got a 2465 some years ago that I have been really happy with. a much higher-grade instrument than the 22xx series.I have no problem with analog scopes, but the digitizing ones can do more.Jon

author: ramenbytes
author_rank: Contributor
timestamp: July 12, 2021, 01:22:40 am
content: Quote from: tggzzz on July 12, 2021, 12:43:05 amWill do! I've already started going through some of the stuff (I found the link in a previous post of yours), and it's been helpful so far. The high voltage references I've seen in there appear to mostly deal with floating measurements, however I haven't had a chance to read through them all yet. Do they also cover general operator safety with high voltage equipment?

author: james_s
author_rank: Super Contributor
timestamp: July 12, 2021, 05:27:47 am
content: As far as analog scopes go, the 2465 is a heck of a nice instrument. You just have to hope it doesn't break down because I think those are the ones that use a bunch of irreplaceable hybrids.

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 12, 2021, 05:33:15 am
content: I don't really have a reference to point you at but I must say one must be VERY careful working on tube gear, both for personal safety and the safety of your test equipment. Oscilloscopes and probes are not idiot proofed nearly as heavily as say, a multimeter. The biggest gotcha being that the scope measures everything relative to earth and the ground clip is earthed. Ask if you don't understand how to safely probe something.

author: tggzzz
author_rank: Super Contributor
timestamp: July 12, 2021, 08:24:49 am
content: Quote from: jmelson on July 12, 2021, 01:20:36 amAny working scope is better than no scope; the search for perfection is a great way of delaying action and exhausting the available budget.Bearing in mind the aphorism in my .sig, I'd say the OP has plenty of skill and imagination. He will be able to find ways to use it for everything except one: capturing signals that cannot be made to repeat.

author: Berni
author_rank: Super Contributor
timestamp: July 12, 2021, 09:42:50 am
content: Sometimes its actually a better learning experience to use old gear.Having more direct control of things such as analog scopes forces you to think in therms of how the scope works and how to get it to do what you need. Working your way around the gears limitations makes you think about things that you otherwise would not. It can teach you tricks that are sometimes even useful on modern digital scopes later on.Also with old test gear you can actually get full schematics in a service manual along with an explanation how it works. This lets you see how it all works without even taking the cover off (Tho i do recommend taking the cover off anyway, its cool to see what is in there and how it is made)Besides there there will always be a better scope out there somewhere than what you already have. No point in worrying about it. You can always buy another scope later on.

author: Old Printer
author_rank: Frequent Contributor
timestamp: July 12, 2021, 12:37:26 pm
content: Artek manuals is a good source for Tektronix user and service manuals. Tek made some of the best service manuals of anybody in the business and Arttek Manuals has done an excellent job of scanning and saving them in pdf format. Most have been converted to searchable text format, which can save a lot of time. Prices are very reasonable IMO. I have a Tek 475 and a 2225 and I bought a full set of manuals for each. If/when things get squirrely they will save you a lot of time. Good luck with your new scope.

author: wizard69
author_rank: Super Contributor
timestamp: July 12, 2021, 11:45:21 pm
content: Looks like you got your scope, so no sense in going into depth here. However with used electronics realize that they could run another 10 years or blow a cap tomorrow. The difference between new and used is often nothing more than a warranty. For your indicated audio work any scope or scope like object would likely be OK. I can remember when people in the hobby would have loved a 5MHz scope.I’m not trying to dismiss the value in modern DSO’s here. They can be wonderful in many cases but also complete overkill.

author: james_s
author_rank: Super Contributor
timestamp: July 12, 2021, 11:50:06 pm
content: Normally I'd advocate buying a new DSO, but if there isn't the budget there isn't the budget, and as has been said before, the best scope for you is the one you can get your hands on. When it was new the 2465 cost a fortune and today it is every bit as capable as it ever was. People designed and debugged all manner of stuff using instruments that are primitive by todays standards.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 13, 2021, 04:08:46 am
content: Quote from: ramenbytes on July 12, 2021, 12:26:55 amYou aren't going to need a 1kV rated probe for anything in that oscillator---DC HT is unlikely to be much over 350v, if that.The tube Finals on some big guitar amplifiers just possibly may be up towards 1kV, but nothing much else in the "valve" realm will be anything like that high.If you start playing around in tube radio transmitters of reasonable power levels, all bets are off, however!

author: ramenbytes
author_rank: Contributor
timestamp: July 13, 2021, 04:50:53 am
content: Quote from: vk6zgo on July 13, 2021, 04:08:46 amRight, for the oscillator I think the highest voltage I saw was -160 VDC, 315 VAC on pins 4 and 6 of V5 so -475 Vpeakassuming the VAC doesn't mean VRMSand that I did the math right. If it actually is RMS though, then I think that comes out just over 600 Vpeak. The other reason for possibily going with a 1kV probe is not for the higher voltage rating, but because probemaster's x100 probes just happen to be 1.2kV rated, and going with x100 means better scaling on the scope display. That is, my scope at x1 goes to 5 V/div max and has 8 divisions, so a x10 probe on a ~300 Vpeaksignal would take up more divisions (12 total) than are on the scope screen. Perhaps I've missed something though. I'll have to ask some questions on the repair board if/when I need to actually probe signals that big.

author: Berni
author_rank: Super Contributor
timestamp: July 13, 2021, 05:14:19 am
content: Yeah more of a thing is the x10 of the probe. Not all scopes have the same input voltage range at the BNC. But the 5V/div is a pretty common one that most scopes go up to, so yes that gets you 30V of range on 1x. You can run the channel offset off the screen and get something to show up, but how far that goes depends on the scope.I would recommend getting a high voltage 100x probes for working on tube gear. I like being on the safe side and not running probes right up to there limits. You don't really need one for each channel, having just one is plenty.

author: james_s
author_rank: Super Contributor
timestamp: July 13, 2021, 05:19:27 am
content: Keep in mind the voltage rating of a probe gets derated with rising frequency. If I were poking around in vacuum tube gear I think I'd either use a HV differential probe or be extremely careful and check with a DMM first.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 13, 2021, 08:19:37 am
content: Quote from: james_s on July 13, 2021, 05:19:27 amThe thing is, when everything was "vacuum tube gear", ordinary Tektronix X10 probes were used every day for probing around in equipment.Most of the high voltage stuff in everyday equipment, as distinct from Radio & TV transmitters & the like was DC or 50/60Hz ac, so well within their ratings, even allowing for derating.Maybe not within the ratings of "El Cheapo stuff", although those are often surprisingly good.

author: tggzzz
author_rank: Super Contributor
timestamp: July 13, 2021, 09:03:20 am
content: Quote from: vk6zgo on July 13, 2021, 08:19:37 amBeware of AC coupled scope inputs, where the full voltage is across the scope's AC coupling capacitor.David Hess mentions this frequently, e.g.https://www.eevblog.com/forum/testgear/probing-smps-with-oscilloscope/msg3126680/#msg3126680

author: jmelson
author_rank: Super Contributor
timestamp: July 13, 2021, 02:36:37 pm
content: Quote from: james_s on July 12, 2021, 05:27:47 amYes, they sure do! And, also some very intricate switch components, but my guess on the hybrids is they come with X thousands of hours of life, and I will be hard pressed to use them up.Jon

author: vk6zgo
author_rank: Super Contributor
timestamp: July 13, 2021, 11:02:40 pm
content: Quote from: ramenbytes on July 13, 2021, 04:50:53 amLooking at those quoted voltages in the HP200CD manual, you naturally thought they existed at the same time, & were additive.This is not the case, however.Pins 4 & 6 of V5 are the twin anodes of a 5Y3GT full wave rectifier tube.This link shows the pinout:https://www.radiomuseum.org/tubes/tube_5y3.htmlThe type of rectifier circuit used with these devices is seen in this link:https://www.angelfire.com/electronic/funwithtubes/3_Simple_Power.htmlYou will notice that the power transformer used has a centre tapped secondary.These transformers were commonly referred to as being "so many volts a side".In the second link, the transformer is quoted as "300V CT" which I take to mean "150Vrms a side".In practice, the secondary centre tap is usually connected as the "common" of the HT supply, & in most cases to the metal chassis.In operation, as the ac cycle progresses, each winding outer connection becomes positive w.r.t. the centre tap.As this happens, the rectifier section which has its anode become positive, conducts, completing the circuit from the centre tap, through the load then the rectifier, & back to the outer connection of that half of the winding.On the other half cycle, the other rectifier section, load & winding repeat the process.It is tempting to just imagine the transformer secondary voltage & current staying as a nice clean sine wave at the rated rms voltage, with all the non-linear rectifier stuff down stream in the rectifier & load.This is not the case, & secondary voltage & current contain a considerable DC component, as well as waveform distortion of the ac voltage.The result is that the same voltage waveform is interpreted differently by the DC & ac ranges on a meter.To add insult to injury, many DMMs, & certainly, HP's 1950s VOM are quite happy to lie to you regarding the rms voltage of waveforms other than perfectly sinusoidal ones.

author: Berni
author_rank: Super Contributor
timestamp: July 14, 2021, 05:20:48 am
content: Yeah sort of case of mixed AC and DC can be quite tricky for most DMMs.There are some modern DMMs out there that can be set to measure RMS voltage of only the AC part or to including the DC part too. But this is a pretty rare feature to find, i think the only ones i have that can do it are the two Agilent handheld DMMs (Forgot model numbers, one is OLED and the other is the next generation up LCD one)Still if i was poking around such high voltage gear i would get a probe rated for high voltage. The regular probes probably have a good deal of safety margin that they would survive pretty much anything up to about twice mains voltage (apart from perhaps poking the output of a high power RF amplifier). But for the sake of safety i rather would not push the specs on something like that. Tho working with high voltage its more about having respect for it and being careful, no fancy probe can save you if you end up sticking your fingers into the 350V supply rail.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 14, 2021, 06:52:12 am
content: Quote from: Berni on July 14, 2021, 05:20:48 amOf course, there is 350v &350v!The big fat high value electros in a SMPS just after the input rectifier can give you a much nastier "belt" than the fairly low current capacity power supplies in such devices as the HP 200CD.If you touch the HT line on something similar to the latter, it will be quite disturbing, but seldom dangerous.The real danger with old tube equipment is more centred on the "transformerless" design of domestic radios so beloved of 1950s/'60s North American, & to a slightly lesser extent UK & European manufacturers.With those. you can get a nice "handful of Mains" if the Active & Neutral power connections are transposed.

author: tggzzz
author_rank: Super Contributor
timestamp: July 14, 2021, 08:37:13 am
content: Quote from: vk6zgo on July 14, 2021, 06:52:12 amOr mains current, if there is a significant potential difference between neutral and the scope's earth. An HV isolating/differential probe might be a wise investment.

author: james_s
author_rank: Super Contributor
timestamp: July 14, 2021, 09:00:57 pm
content: Quote from: vk6zgo on July 14, 2021, 06:52:12 amThey're not really that dangerous, the problem is the back cover is typically easy to remove and often missing, and many of them if a knob is missing the shaft can be energized. The transformerless design continued into the relatively modern age, most CRT TV sets have a "hot chassis" where "ground" is connected to one side of the line. The difference is that they have sturdy enclosures and any external metal parts are isolated. I have a couple of old transformerless tube radios and I use them. Just plug them into a GFCI and make sure they're not missing any parts.

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 15, 2021, 07:13:27 am
content: Some are wired with one side of the cord straight to the chassis (and thus knob shafts) but as I understand it most transformerless series string radios and TVs float the chassis with a capacitor and/or resistor.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 15, 2021, 08:30:56 am
content: Quote from: james_s on July 14, 2021, 09:00:57 pmThe last couple of generations of CRT TV sets all used SMPS.There is absolutely no necessity to have a "hot chassis" as the output of the power supply comes from the secondary of a transformer, (albeit a HF one), so is just as much isolated as the secondary of a 50/60Hz mains transformer.All the SMPS powered TV sets I have seen in Australia are of this type, some had the SMPS as a separate module, whereas others had the power supply on the main board, but electrically isolated from the rest of the board.The overwhelming majority of BW "tube type" TVs sold in Oz had mains power transformers (an "Admiral" "transportable" TV in the late '50s was the only locally made transformeress TV I ever saw)The only "transformerless" tube radio sets around were a few "private imports" & very small quantities built for "niche" markets which still had DC Mains in the '50s/ early '60s.The motive to build transformerless radios & TVs was not strong in this country.Radio & TV sets commanded higher prices here, compared to some other countries, transformers were relatively cheap, & "series string" tube were not made here.Such tubes would have had to come from overseas, at high cost, mainly from the UK or Europe, as series string tubes for 110v Mains would require an additional series dropping resistor.

author: james_s
author_rank: Super Contributor
timestamp: July 16, 2021, 06:28:48 pm
content: The stuff I work on is mostly from the 80s and earlier. The vast majority of CRT monitors used in arcade games are hot chassis and require a separate isolation transformer in the cabinet. Some CRT TVs did use a SMPS but most of the older ones I've worked on did not. Modern day stuff yes, it's isolated, although the primary side of the SMPS obviously is not. The point is there's nothing wrong with a non-isolated design *provided* adequate isolation is provided by the enclosure.

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 16, 2021, 09:45:19 pm
content: AFAIK even with the end stage circa 2000 black box CRT TVs many of those super cheap 19 inchers with just a RF jack used a non isolated supply, presumably because a buck regulator is cheaper than an isolated SMPS design. An isolated supply was only used on the models with composite video jacks and whatnot.

author: ramenbytes
author_rank: Contributor
timestamp: July 31, 2021, 11:13:20 pm
content: Quote from: Old Printer on July 12, 2021, 12:37:26 pmThanks for the recommendation! I went ahead and got a full set for the 2246 Mod A, and they are indeed nicely done. Great customer service too: one of the manuals in the set wasn't OCR'd, and Artek resolved the issue the day after I notified them. The schematics are also scanned in such a way as to make it easy to keep your place in them if a big one needed to be split into multiple pages, so that's nice too. Scan quality was better than TekWiki's too.

author: ramenbytes
author_rank: Contributor
timestamp: July 31, 2021, 11:19:41 pm
content: Quote from: tggzzz on July 13, 2021, 09:03:20 amIn which case I probablywillneed to go with some sort of differential or HV probe eventually, assuming I want to use AC coupling in whatever I'm doing. I'll wait 'till I know for sure before spending money though. Thanks for bringing this up.

author: ramenbytes
author_rank: Contributor
timestamp: July 31, 2021, 11:26:23 pm
content: Quote from: vk6zgo on July 13, 2021, 11:02:40 pmIt would seem I have some homework to do. Thanks for pointing this out, I'll have to take some time to work through what you said and make sure I understand it. I also wasn't aware that the situation was that bad with meters, though I suppose that explains why joeqsmith feeds all the funny waveforms into his DMMs when testing them.

author: 0culus
author_rank: Super Contributor
timestamp: July 31, 2021, 11:46:23 pm
content: Quote from: james_s on July 12, 2021, 05:27:47 amIIRC the key "gotcha" with the hybrids is ventilation. Do NOT block the ventilation holes, be careful about ventilation when working inside with the case off (probably blow on them with a small desk fan would be a good idea), and definitely don't run it if the fan isn't working.