Old analog oscilloscope questions - Page 1
Forum section: electronics / beginners

author: tedsorvino
author_rank: Regular Contributor
timestamp: August 08, 2018, 05:37:24 am
content: Hello. Another first timer in the forum. I'm also new to the world of oscilloscopes even if I 've studied a bit the concept behind them. But there are many many things to learn. So I decided to buy a cheap ADVANCE INSTRUMENTS - GOULD OS 240 10MHZ Dual Channel Analog Oscilloscope. Mainly for Audio work.Unfortunately I can't find the exact model manual, but I 've found some quite similar models' ones.So there are some questions I 've got.Firstly I would like to know if I have to ground (is it necessary) the device since its mains plug has no mains ground. It's an old two prong plug and there's also a "banana plug" ground socket above it (how can I use that??). I don't know if it's dangerous.Also, in the front panel there is no test- probe calibration input. How can I calibrate the probes?There is a strange binding post called Gate o/p. It must be an output. There is something like 15v dc voltage on it. An old Tektronix manual (different brand but seems like it also had the same style of output) writes that it supplies a positive pulse for the duration of the timebase. What does it mean and how could it be useful? Could it be a test point for the probes?There is also another ground socket next to it. I think this is not useful since there is a ground clip on the probes. Am I right? Should I always use the ground clip for anything I measure (like a multimeter ground)?Under the devices there are two hole for the two channel balance. Are they test points, or just potentiometers? What do they set?How can I understand that the oscilloscopes functions and measures correctly.Here are some photos of the front panel and the particular parts on questionhttps://ibb.co/nHkmVehttps://ibb.co/czcpizhttps://ibb.co/bZ9DAehttps://ibb.co/ds59izThese seem really important questions to start with (even if more seem to arise).Thank you in advance for your time.

author: Jwillis
author_rank: Super Contributor
timestamp: August 08, 2018, 06:29:28 am
content: Similar to the 240http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Gould%20Advance/OS250.pdfcircuit diagramhttps://elektrotanya.com/advance_os240_oscilloscope_sch.pdf/download.htmlfurther information herehttps://www.radiomuseum.org/r/advance_in_os_240_os240.htmlCalibrate probes at the probe it self.

author: tedsorvino
author_rank: Regular Contributor
timestamp: August 08, 2018, 06:59:03 am
content: Thanks JWILLIS.I' ve already had found all the relevant information that can be found on the net. At least the obvious one.Unfortunately the OS 250 manual hasn't got the features I'm asking for. The radio museum site gives no free information and the schematics is not helpful if you 're not a service - repair technician.How can I calibrate properly at the probe itself?

author: tggzzz
author_rank: Super Contributor
timestamp: August 08, 2018, 07:03:34 am
content: Quote from: tedsorvino on August 08, 2018, 05:37:24 amI don't know that specific scope, but unless it is a very unusual scope the protective mains earthmustbe connected. If it is not connected and a scope probe's shield is connected to a dangerous voltage (e.g. mains), then then entire scope's case will be at that dangerous voltage.It is also possible, depending on why the scope was "floated", that there is internal damage to the mains transformer.Have a look at the safety references and scope probe references inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/The remainder of your questions are not specific to this scope, so any introduction to how to use a scope should help.

author: tedsorvino
author_rank: Regular Contributor
timestamp: August 08, 2018, 07:09:52 am
content: Thanks TGGZZZ.But the scope is constructed "floated" and it only has a socket for mains ground. How and where can I connect the ground from there.On the other hand the calibration issue is not generic. I mean most of the scopes have a test input. This one hasn't. And what this binding post is all about.Believe me I've read quite a lot of material since last week and nothing answers these particular questions (yet...).

author: tggzzz
author_rank: Super Contributor
timestamp: August 08, 2018, 08:28:53 am
content: Quote from: tedsorvino on August 08, 2018, 07:09:52 amI'm afraid,literally, that you are mistaken.It appears to have a metal case that is connected to the probe shields, which is normal for a scope like this. Such instrumentsmustbe earthed - for other people's safety and to prevent other equipment being damaged.What do you mean "constructed floated"? Do you mean that:the original documentation states it is designed to be operated when floated. But you don't have that informationyou have examined the interior circuit, and it is clear that it has never had a protective mains earth and never needed one. But you are a beginner, and so could not tellsomebody has modified it to disconnect the protective mains earth, possibly by replacing the mains leadSo I'll bet it is the last alternative.You have wisely acknowledged that you are a beginner. You would be unwise to assume that you know more about safety than the hard won wisdom of the last century.Seehttps://info.tek.com/rs/584-WPH-840/images/3AW_19134_2_MR_Letter.pdfin particular the bottom of pages 4 and 5. what you do with your body is your decision

author: Gyro
author_rank: Super Contributor
timestamp: August 08, 2018, 08:41:02 am
content: Looking at the partial schematic that Jwillis linked, it clearly shows the mains ground input connected to the chassis. It most certainly needs mains grounding.

author: tggzzz
author_rank: Super Contributor
timestamp: August 08, 2018, 08:49:04 am
content: Quote from: Gyro on August 08, 2018, 08:41:02 amYes, without reservation.The OP would be very foolish to ignore that.

author: nfmax
author_rank: Super Contributor
timestamp: August 08, 2018, 12:02:30 pm
content: Quote from: tedsorvino on August 08, 2018, 05:37:24 amDo you mean there is an earth 'socket' (instead of a pin) in the mains plug itself? The sounds like a CEE 7/6 plug, which is intended to be used with a CEE 7/5 socket. This has an earth pin sticking out of it instead of the normal two earth contacts top and bottom. They were common in France and a few other areas. You will need to obtain a suitable adaptor or fit a replacement plug.YouMustearth your oscilloscope for safe operation.

author: drussell
author_rank: Super Contributor
timestamp: August 08, 2018, 12:34:57 pm
content: Lots of older scopes were floating by default.My Stark OSK-2 only has a 2-prong plug with no provision for grounding via the AC input. I ground it using one of the front panel ground terminals to bench ground for regular operation when the chassis and probe "grounds " should really be ground.Obviously if you're floating a measuring instrument to do something, you need toknow exactly what you're doingand not touch the chassis of your instrument or have any stray accidental connections touching the chassis anywhere, etc. Certainly could be a shock hazard and not something that a beginner should be doing foranyreason.

author: tedsorvino
author_rank: Regular Contributor
timestamp: August 08, 2018, 03:11:03 pm
content: Thank you very much for your time.These pictures will help you understand what I mean by saying "constructed not grounded - floated". Drussell is closer to what's going on.https://ibb.co/k8i3bKhttps://ibb.co/hVcwwKAny ideas how to ground this thing in order to be safe?I attach the proper schematic

author: Gyro
author_rank: Super Contributor
timestamp: August 08, 2018, 03:39:04 pm
content: You might be best just replacing the mains cable if you're confident enough - that one looks pretty manky. According to the schematic, there ought to be an easily identifiable grounding point where it terminates inside. Posting a photo would help identify it. If you do open it,don't touchany components associated with the CRT or the EHT lead, they might still have a high voltage charge.Also have a look inside that plug, you wouldn't be the first person to discover a ground wire either cut short or tucked away in a 2-pin plug! [Edit: In fact it's a round section cable on a piece of equipment manufactured in the UK, I'd lay odds that there's a ground wire in there!]P.S. That ground banana socket looks more like a convenient ground reference for the Z-mod input than a mains earthing point.

author: drussell
author_rank: Super Contributor
timestamp: August 08, 2018, 04:08:51 pm
content: Quote from: tggzzz on August 08, 2018, 07:03:34 amTrue, but if it were earthed it would probably just blow up your probe or scope.You shouldn't be attaching your input/probe ground to anything other than a grounded point on your device under test or bad things are bound to happen.QuoteNo, it was designed to be floating, just like most older scopes I've seen, especially the old vacuum tube based ones intended for the TV repair industry.QuoteHyperbole much?You're more likely to damage your scope or your DUT if your scope is grounded. Don't get me wrong, I would normally run with it grounded too, but it actually makes equipment damagemorelikely, not less. It reduces the chance of you shocking yourself if you do something silly but not the chance of equipment damage. If you're probing a device where the spot that you put your measurement ground is at a voltage that will give you a shock, you should be running your DUT on an isolation transformer.QuoteHe has the schematic. It shows that it is floating. He could probably earth it using the banana jack on the back (assuming it is reasonably well connected to the chassis, not just connected to the transformer shield like it shows in the schematic) if he chooses (my Stark has no such provision, BTW...) but itcame from the factory this way, it hasn't been modified.He could change the input cord to permanently ground the chassis but that really doesn't make sticking your finger or probe ground to anything but ground any safer and removes the ability of sticking the scope ground on, say, a 12v power rail, to reference to that for measurements. Having your scope chassis at 12v isn't dangerous, you just need to know what you're doing.If you want to be safest, use a GFCI upstream of the scope and ground the scope frame. Then your GFCI will trip (hopefully) preventing any damage to anything should your probe ground go astray.

author: tedsorvino
author_rank: Regular Contributor
timestamp: August 08, 2018, 05:00:53 pm
content: Thanks everybody. I replaced the mains plug to a modern one. There was a ground connected but it was floated at the end. Here are some some pictures.https://ibb.co/fUYsmKhttps://ibb.co/ijoQ6KWhat about my other questions regarding the binding post and the probe compensation - calibration. Any help.

author: tggzzz
author_rank: Super Contributor
timestamp: August 08, 2018, 05:39:34 pm
content: Quote from: drussell on August 08, 2018, 04:08:51 pmThat would be a better outcome.QuoteThe OP's scope wasn't. This thread isn't about your scope.QuoteNo hyperbole.I don't think you are replying to what I wrote, viz: "preventotherequipment being damaged".QuoteYou are hallucinating. His schematic shows the mains input should be connected to the protective mains earth.QuoteThere are many ways in which a GCFI can fail to protect you. A couple of examples, but it is easy to think of others:https://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=4503https://midas-uk.co.uk/its-alright-guv-its-got-an-rcd-on-it/(Nice URL)You appear to be forgetting that the OP is a beginner and doesn't know what he is doing. In that context your statements are dangerous.

author: tggzzz
author_rank: Super Contributor
timestamp: August 08, 2018, 05:43:58 pm
content: Quote from: drussell on August 08, 2018, 12:34:57 pmLike the person in this incident?... Fromhttps://groups.io/g/TekScopes/topic/7632418#10795with myemphasis:What makes it so dangerous to float the scope is that it is very easy for you to come in accidental contact with the floating scope chassis and receive a very bad shock, possibly lethal. One of my first customer contacts as a Sales Engineer for Tektronix was to call on the Sylvania Lighting Center in Danvers, MA and investigate a rumor about an engineer working there that was killed while using a Tek scope. I found it it was true. During lunch, one of the engineers was working alone in the lab on a lighting experiment that was using some 3 phase, 220 volt power. He needed to make some measurements between points none of which were at earth ground. So, he floated the scope . . . He even has the scope sitting on a scope cart with a sheet of insulation material between the bottom of the scope and the metal tray it normally sits in so the scope cart would not be "hot" with the scope. He also had a "tunnel" of plexiglas on both sides and over the top of the scope in a crude attempt to prevent anyone from accidentally touching the hot scope. The back was not covered with plexiglass in order to allow the fan to do its job and the front was not covered so the engineer could access the scope controls.This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway.Anyone who teaches you that floating a scope by defeating the power cord ground lead is a very poor teacher, indeed. They simply do not know enough about making SAFE measurements to be in a position to teach electronics.

author: Gyro
author_rank: Super Contributor
timestamp: August 08, 2018, 05:48:06 pm
content: Quote from: drussell on August 08, 2018, 04:08:51 pmThe fact that itdidcome from the factory with a 3-core mains lead (and still has it) rather spoils your assertion.As both I and tggzzz pointed out, the schematic quite clearly shows mains ground connected to chassis.

author: tedsorvino
author_rank: Regular Contributor
timestamp: August 08, 2018, 05:53:58 pm
content: I think it's about time to leave behind the disagreement. All of you help in your way. And I'm REALLY THANKFUL .So PLEASE help me with the rest of my INITIAL questions, since the GROUND issue is SORTED.

author: Gyro
author_rank: Super Contributor
timestamp: August 08, 2018, 05:55:43 pm
content: Quote from: tedsorvino on August 08, 2018, 05:00:53 pmThe lack of a probe calibration output is certainly a bit of a problem for compensating your probes. What you need is a nice clean squarewave of around 1kHz. You could maybe make a basic oscillator using a 555 timer or logic gate - as long as there isn't too much overshoot. My feeling is that this would probably be 'good enough'.[Edit: Sorry, yes you're right, I was just getting around to trying to give you some practical advice.]P.S. I've lost track - which binding post? ... If it's the gate output, then yes, it's the same as the Tek manual. It can be used for things like frequency counters (to monitor the exact timebase frequency - not the signal frequency directly), to measure the exact sweep time, possibly detecting that the scope has triggered. With a some delay logic and the Z-Mod input, you could make a 'cursor' that accurately times between two points on the trace, etc. Not a lot of use to you to be honest.

author: tggzzz
author_rank: Super Contributor
timestamp: August 08, 2018, 06:48:41 pm
content: Quote from: tedsorvino on August 08, 2018, 05:53:58 pmWe are being insistent since somebody else might come along in the future, read this thread, and somehow come away with the (dangerous) misapprehension that it is OK to float a scope. We're gladyou'vegot the message!Probe calibration. Some people think a scope's CAL out enables the timebase (ns/div) and vertical deflection V/div) to be calibrated. It doesn't, and usually has aroughly1kHz frequency androughlya volt amplitude. What is does have is reasonably fast edges (<1us) and a flat top with no ringing. That enables a probe's low-frequency compensation to be tweaked so that the displayed signal has a flat top as shown in the usual pictures.So, in order to "calibrate" a probe (i.e. tweak its frequency response), all you need is such a flat-top square wave from some source or other. If you tell us what you have, we could say whether it could be used.

author: james_s
author_rank: Super Contributor
timestamp: August 08, 2018, 11:29:11 pm
content: I wouldn't worry too much about whether the scope is grounded or not. If it originally had a grounded plug which has had the ground cut off or been replaced with a 2-prong plug then it should be replaced with a grounded plug. If it was never grounded to begin with then don't worry about it, it's ok that it's floating. When people speak of "floating a scope" they often mean deliberately disconnecting the ground in order to use the scope to measure a high voltage non-isolated device, this is dangerous, not the fact alone that the scope is floating. So long as you are staying away from the mains side of whatever gear you're working on then there should be no problem. A good rule to follow is if you wouldn't touch something with your bare finger, don't connect a scope to it.Now as for probe calibration, you don't need anything fancy there. A simple square wave oscillator capable of reasonably fast edges is all that is required. Something like this should make a pretty good probe calibration oscillator.http://www.epanorama.net/circuits/tdr.html

author: tedsorvino
author_rank: Regular Contributor
timestamp: August 09, 2018, 01:01:44 am
content: Thanks guys for all the advice and effort.I'm using an android app. Keuwlsoft Dual Channel Function Generator. It's pretty basic and quite simple.I know how to set a particular frequency of any shape and amount but I don't know how to set the voltage of its amplitude, since the generator only measures the amplitude percentage.Could you please help me on how to set it. Is it an appropriate signal generator or do I need something else, even for other proper measurements?

author: james_s
author_rank: Super Contributor
timestamp: August 09, 2018, 04:09:01 am
content: It depends on what you want to do. The volume control will set the amplitude, it should be adequate for starting out at least.

author: tedsorvino
author_rank: Regular Contributor
timestamp: August 09, 2018, 07:51:13 am
content: I understand that amplitude it's volume relative but what exactly do I need to do in order to calibrate the probes as good as possible?After some audio tests I realized that my cheap probes function quite well, in both settings ( x1 - x10) and in all available voltage divisions. But I need a good reference to be as sure as possible, with my measurements.Also some of the audio things I need to measure demand a particular peak to peak amplitude voltage.I think I need a different signal generator as it seems more reliable to work it from a computer soundcard than an android phone (there is a lot of extra noise). One capable to set the voltage too. Any suggestions?And another question is how does a set amplitude work along with volume? I mean if you set the amplitude to 1V p/p with the volume up to 30% and then you lower the volume down to 10% isn't the new amplitude around 330mV? And what is the real 1V p/p (or whatever amount) amplitude that it's demanded ? How does it really work?Sorry if I confuse you, it's all new to me.

author: Gyro
author_rank: Super Contributor
timestamp: August 09, 2018, 08:23:50 am
content: As previously indicated, the actual voltage and frequency don't matter - as long as it's somewhere in the region of 1kHz and the output voltage is high enough to give you a reasonable sized trace... and it's a squarewave.tggzzz used the wrong word - it is not probe "calibration" it is probecompensation. The probe automatically has a DC divide ratio of 10 (and 1 in the other switch position), the compensation trimmer in the probe is there to match the probe to the input capacitance of your particular scope. When compensation is correct, you will see a nice square wave with square leading corners, the tops of the squares should be nice and flat (not tilted). A bit of noise shouldn't stop you being able to do this.The probe compensation trimmer isonlyneeded for the X10 position. In the X1 setting, the probe is basically just a straight-through 'bit of wire', the circuit sees the full scope input capacitance in this case. The X10 position divides the probe input capacitance (not by the full 10 because there are stray capacitances in the probe and clip) and increases the input resistance from the scope's 1Meg Ohms to 10Meg Ohms. It does this at the expense of dividing the voltage by 10 at the same time.The X10 position is normally the best position to use (particularly at higher frequencies) because it reduces loading on your circuit and helps to protect your scope from accidental overloads.The probe compensation adjustment needs a squarewave with reasonably fast edges, your PC soundcard or Android appmightbe ok (try them), if you are not seeing any reasonable visible difference when you adjust the probe compensation trimmer then you will need to make something with faster edges, such as the circuit that james_s linked.As for voltage calibration of the scope itself - you should be able to check this using a variable DC source and your DMM. Remember that a scope is not a precision measurement instrument - it shows you what waveforms look like and gives to a reasonable indication of amplitude - probably around 5% accuracy for your scope. Only adjust its voltage calibration if it is significantly off.For checking the timebase accuracy (us/div - ms/div etc.), your PC soundard and Android app should both be fine - their frequencies are derived from a crystal oscillator, so will be accurate. Again, expect your scope to be within a few %.I hope this clarifies things for you.

author: tggzzz
author_rank: Super Contributor
timestamp: August 09, 2018, 03:27:28 pm
content: Quote from: james_s on August 08, 2018, 11:29:11 pmNo, that is dangerous advice: it presumes an ideal world with complete knowledge and complete understanding.If there is a mistake, or a misunderstanding, or a fault, then a floated scope can be lethal.Example mistake: a probe slips.Example misunderstanding: you misread a diagram, e.g. not recognising whether a protective mains earth connection is specified.Example fault: mains has jumped to an unexpected place in the UUT - and that's why you are probing it in the first place.And with a little imagination or knowledge, many many other examples can be found.

author: tggzzz
author_rank: Super Contributor
timestamp: August 09, 2018, 03:33:12 pm
content: Quote from: Gyro on August 09, 2018, 08:23:50 amSigh. Quite right.

author: james_s
author_rank: Super Contributor
timestamp: August 09, 2018, 03:39:37 pm
content: Quote from: tggzzz on August 09, 2018, 03:27:28 pmNo, it presumes a very basic understanding that poking around in mains powered equipment is potentially dangerous. Lots of old scopes never had grounded power cords even when they were new, no need to modify the equipment to add one. If a person can't figure out not to poke around in dangerous stuff when they don't know what they're doing then they should not be using a scope. There is too much safety hysteria these days, can't protect everyone from themselves.Your example fault of mains jumping to somewhere it shouldn't would be dangerous even with no test equipment involved. Loads of consumer equipment has no earth ground.

author: james_s
author_rank: Super Contributor
timestamp: August 09, 2018, 03:45:22 pm
content: Quote from: Gyro on August 09, 2018, 08:23:50 amThat's true, however on quite a few scopes the terminal used to adjust the probe compensation is called "Calibrator" so I don't think the term is entirely inappropriate. If Tektronix thought it was reasonable to call it a calibrator that's good enough for me.

author: tggzzz
author_rank: Super Contributor
timestamp: August 09, 2018, 04:50:13 pm
content: Quote from: james_s on August 09, 2018, 03:39:37 pmThe OP is a beginner. Responsible responses should be tailored to that.QuoteI've never over-protected my family; I backpacked round India with my 13yo, and before she could start to learn to drive a car, she was doing aerobatics that could have killed her within 10s, making forced landings, travelling at 90kt.However,it is necessary to protect people fromotherpeople, and from other people's bad advice.QuoteThat's false, and a failure of your imagination.

author: tedsorvino
author_rank: Regular Contributor
timestamp: August 09, 2018, 05:04:37 pm
content: Thanks a lot Gyro. Your advice really really helps on what the next steps should be.Thanks everybody for your kind effort and help.

author: Gyro
author_rank: Super Contributor
timestamp: August 09, 2018, 05:49:33 pm
content: Quote from: james_s on August 09, 2018, 03:45:22 pmYou're splitting hairs a bit too much there. Whatever you use as a source, the process is known as probe compensation and probe specs state a 'compensation range'. Many scopes call the terminal something like 'Probe Comp'.The reason Tek (and a few others) call theirs a Calibrator is... (1) They normally implement it as a current loop for active current probe level calibration, and (2) It has specified accuracy for voltage and current. That's not general practice across all manufacturers.Just because Tek implement it in a specific way doesn't justify an 'If it's good enough for Tek then its good enough for me' type response. More fundamental understanding is needed.EDIT: ...and yes I am guilty too:Quote from: Gyro on August 08, 2018, 05:55:43 pm

author: tedsorvino
author_rank: Regular Contributor
timestamp: August 19, 2018, 04:05:21 pm
content: Finally, I found and purchased the manual (if anyone cares or needs it I can send the pdf) and I also bought a decent old Farnell signal generator. So I opened the scope, calibrated it and tested it. Now it's really functional.Some things (pots and trimmers) would be better replaced in the future (for more precise measurements), but after working out some solutions (changing the vertical subdivision to "calibration friendlier" values) and cleaning the whole thing with certain contact cleaners and compressed air sprays, I managed to revive it and make it work properly.And in order to answer the question regarding the binding post ... after finding, purchasing and reading the manual. ...According to the manufacturer: "This output provides a positive going sq. wave of approximately 20V amplitude from a source impedance of 15kOhm. Its freq. is dependent on the setting of time/div and is suitable for probe compensation.Mine is missing the jumper to the source impedance.But in my eyes it's quite risky (things may fall apart or stop working) and troublesome to remove three old boards from an old oscilloscope. One day it may happen if it really needs to.Thanks a lot everybody.