Scope DIY calibration/verification - Page 1
Forum section: electronics / testgear

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 09, 2016, 04:27:25 pm
content: Hello Guys,Scored a bunch of T&E equipment, mostly scopes, quite some time ago. They are taking about two months longer than they were supposed to arrive (which is another, lovely, story) but I should have them in a week or two. I may keep some stuff but most off it will be sold.What I'd like to do is... well, not exactly a calibration but at least some kind of a verification/sanity check/confidence boost so that folks who buy stuff may be fairly certain they are not buying a lemon.Truth is, I don't have a budget to buymorestuff, so preferably I'd be able to do this with what I have and/or will have. So, if possible, I'd like some advise/input on what is feasible with what I have,preferably without 20 hours of work on each one and what would boost your confidence if you would be considering buying a scope...So, what do I have for sure:- Rigol DP832 (still in calibration). I was thinking this would be good to demonstrate DC voltage accuracy?- Rigol DM3058E (still in calibration). Not sure how this would help, though...- B&O TG8 sig gen but I still have to verify how good it still is (still in the box). No calibration, obviously- Other scopes and SI- analysers to double check but none of them are in calibration. I suppose that it would still be nice to compare waveforms on a 500MHz scope against a 1.5GHz Yoko. Or a 6GHz Tek... Then again, how would I be testing those?Unsure:I have also 2 HP 8664A's inbound but both of them are listed as dead. I kinda hope/dream (when I'm not having nightmares of my container being knocked off the ship) I'll be replacing a fuse and get going but... Well, no guarantees. And no calibration. Then again, I'm guessing that even out of call they should be good enough for the accuracy expected from a scope? And I suppose that if I hook up 5 scopes, have frequencies relatively close to each other that would be fairly decent as a vote of confidence...So, thoughts? What would boost your confidence in a scope with the stuff (perhaps) at hand?

author: singapol
author_rank: Frequent Contributor
timestamp: October 09, 2016, 05:28:47 pm
content: I think you may be taking on too much for yourself...20 hours on each scope even if it's a low end scope?It's nice of you to do these basic tests like if scope powers up and the display is working. With what you have on hand you can verify that scope and measure and display a waveform. Almost all ebay sellers just say scope powers up and there's a display but we do not have knowledge to test further or sold as is.Of course if it's a high end scope then to make it more saleable and give customer confidence more tests would be appreciated but if you need to ask then you may not be knowledgable enough to do what needs to be done.Anyway for better scopes customer already know what it's capabilities are. If you are going to spend 20 hrs. each then you are not going to sell cheap?A 20 foot container can pack more than a handful of scopes?

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 09, 2016, 05:41:48 pm
content: Hey,Thanks for taking the timeQuote from: singapol on October 09, 2016, 05:28:47 pmThis may have gotten lost in translation. I havenodesire to spend 20 hours on each scope. But if by spending an hour (example) on it I could raise confidence in potential buyers then that's a great investment.QuoteExactly my point: I have the knowledge and the tools to set my gear aside from those folks and I would like to do so.QuoteThe knowledge is not a problem. But, well, I'd want to get maximum confidence from a minimum of effort (there, I said it). Put another way: I can measure DC voltage accuracy easily. But what voltage. I could go over the range in the datasheet but I really,reallydon't want to. I could measure noisefloor, rise times, square waves and basically try to verify all the specs from a datasheet. But I'd much rather boil it down to a short list of a few representative measurements.QuoteFor your reference:https://www.eevblog.com/forum/buysellwanted/fs-various-high-end-tm-equipment-auction-score/

author: tautech
author_rank: Super Contributor
timestamp: October 09, 2016, 05:43:46 pm
content: IME PSU's are the most common point of failure IF the scope has not been abused, therefore a return of the PSU to correct operation and exact expected rail voltages returns the scope accuracy to previous levels.Then basic checks like you're thinking confirms it's operating correctly more or less.CRT and other display failures provide a similar scenario, restoration of the display should leave calibration unaffected.The higher V/div input attenuations aren't normally subject to any damage but all the low ones should be checked for basic accuracy in case somebody's had amoment.50inputs should all be checked in scopes that have it, again amoment can damage them as they're commonly used with direct coax connection and have only 5V max ratings. 50termination uses the same resistors for all input attenuations so any V/div setting on each channel will indicate if they're OK or not.

author: nctnico
author_rank: Super Contributor
timestamp: October 09, 2016, 05:45:33 pm
content: Do what the over enthousiastic Ebay seller does: turn the scope on and show the probe calibration signal on each trace. Otherwise just power on and show some traces. People will know the equipment is used and will spend their money accordingly. Self calibrating a scope is a waste of time and sending the scopes away for traceable calibration is a waste of money. As Tautech wrote: do a quick check with some DC levels and the probe calibration signal to see if the attenuators work and that's it.

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 09, 2016, 06:18:55 pm
content: Quote from: tautech on October 09, 2016, 05:43:46 pmExcellent point. Hadn't considered the 50 ohm input. Thanks!Quote from: nctnico on October 09, 2016, 05:45:33 pmEh, well, I want to do betterQuoteAnd that's what Idon'twant. I'd prefer to have a number of points adressed (such as the concern for the 50input) so that doesn't have to be taken into account when determining the priceAlso: just checked my inventory-to-be: there's a Lecroy 9210 pulse gen. Up to 300psec edges, 300MHz. If none of the HP sig gens "make it", this might be a very decent option...

author: tautech
author_rank: Super Contributor
timestamp: October 09, 2016, 06:38:38 pm
content: Quote from: Ice-Tea on October 09, 2016, 06:18:55 pmNow you're talking.You can do many basic checks with that but for any adjustments using it you'll need the scope service manuals to use it in the correct part of the Cal procedure.

author: nctnico
author_rank: Super Contributor
timestamp: October 09, 2016, 07:26:41 pm
content: Quote from: Ice-Tea on October 09, 2016, 06:18:55 pmTo put it bluntly: there are so many other things which can go wrong with knobs, displays, power supplies, etc that you can't know for sure an instrument still works 5 minutes after you tested it. If you want to instill confidence then offer a one year warranty. In the long run that will probably be the cheapest option for you because you don't need to spend hours on testing an instrument thouroughly.

author: tautech
author_rank: Super Contributor
timestamp: October 09, 2016, 07:43:37 pm
content: Quote from: Ice-Tea on October 09, 2016, 06:18:55 pmQuality pulse gens are quite precision instruments, quite exacting amplitudes, pulse and off times and can also be used as mark space generators for timebase adjustments. Quite specialised really but perfect for you upcoming needs. Be sure to check it out carefully with a "good" scope first and also become quite familiar with it.If you're gunna be fussy with your checks it might be worth getting it professionally calibrated/checked.Stating that all scopes have been checked against calibrated equipment could well provide the return in investment with increased sale prices. That'll be your call as to whether it will pay off in the end. But if you do at least you'll know all's OK and you'll still have it at the end for otherprojects.The Sig gens, if any good can be used to check higher frequencies as needs be.Pulse gens and voltage references are the basic tools for checking and adjusting scopes.

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 09, 2016, 07:55:53 pm
content: Offer a warranty? Might actually make sense. Had some nasty experiences with that in the past though and logistics tend to be a nightmare. But for folks on EEVBlog and inside the EU I might go for it. On eBay? Not a chance in hell.Might end up having the sig gen calibrated, good advise. Anyone have a balk park for this? As for a voltage reference, how "believable" is the DP832?

author: Vgkid
author_rank: Super Contributor
timestamp: October 09, 2016, 08:01:06 pm
content: You can do some decent verifications with just a power supply, and a stable frequency source.the dp 832 is 0.01% accurate, much better than most scopes. What dmm's do you have?

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 09, 2016, 08:09:46 pm
content: Rigol DM3058E. And a shitty Batronix one

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: October 09, 2016, 08:35:16 pm
content: Quote from: Ice-Tea on October 09, 2016, 04:27:25 pmWhat you have are essentially two choices:A. You do a power on check, run any built-in test routines, and do some very basic testing (like the mentioned probes connected to the cal port). Then sell it as it is for a somewhat lower price.B. You get the instruments properly (i.e. trace-able) calibrated, i.e. by a professional calibration facility.Don't waste your time with spending hours on some DIY pseudo-"calibration" which isn't trace-able and thereby worthless to any commercial buyer, you won't get a higher price that way.B is worth considering for younger high-value test gear for which there is some demand in the market, however when I remember right most of the stuff you listed in the Buy&Sell forum was >10 years old and most not exactly the most sought-after, so I'd avoid any further investments for calibration as you're unlikely to see a return.Option A allows you to check instruments in reasonable time, then sell as "basic functionality tested" for a somewhat lower price and move on. You could offer a warranty but I'd think carefully about that as it could make you potentially liable for a hefty repair bill. Also, since you're inside the EU, if you sell as business to end users (consumers) then you're on the line if the instrument stops working within a certain period after the sale anyways (and this can't be waived through contract).

author: nctnico
author_rank: Super Contributor
timestamp: October 09, 2016, 09:41:11 pm
content: IMHO a healthy 'self-test passed' screen is a big plus when trying to sell equipment.

author: IconicPCB
author_rank: Super Contributor
timestamp: October 09, 2016, 09:58:22 pm
content: In the old days of purely analog scopes a voltmeter, a DC power supply, a signal generator and a pulse generator were sufficient to test and calibrate a scope.Pulse generator to chek various time base settings.Signal generator to chek bandwidthand a voltmeter and Power supply to check the vertical calibration of the display.Consider current crop of digital scopes are predominantly 8 or 10 bit resolution instruments.You dont need much in the way of a multimeter to check the vertical gain.A digital pulse generator will give You a precise frequency signal to check the time base and a decent sig genny will be parts of a dB leveled source to check the bandwidth of the scope.Good luck.

author: David Hess
author_rank: Super Contributor
timestamp: October 10, 2016, 01:40:25 am
content: With one exception, doing a performance check is a lot less demanding than a full calibration. Most failures will be gross rather than subtle.Any frequency source and frequency counter can verify the timebase and delay functions if necessary.Checking the vertical calibration is the most time consuming because suitably accurate low level references sources are uncommon. An AC voltmeter can be used with the square waves from a function generator. Low reference levels can be created with a switched attenuator.Testing the transient response is easy if you have a fast reference level pulse generator and difficult without one but you can make a pretty good one from some 74AHC logic gates good to 200 MHz or so and acceptable for gross tests above that.

author: IconicPCB
author_rank: Super Contributor
timestamp: October 10, 2016, 06:25:14 am
content: Vertical cal is done against a DC source

author: David Hess
author_rank: Super Contributor
timestamp: October 10, 2016, 12:08:34 pm
content: The same restriction applies for DC; even if the user has a voltmeter which can measure 8 to 16 millivolts to within 1%, how many voltage sources can accurately produce it for testing 2mv/div? Some leveled signal generators may be able to do this but they are even less common.