Calibrating an oscilloscope - any tips?  - Page 1
Forum section: electronics / beginners

author: py-bb
author_rank: Regular Contributor
timestamp: November 05, 2022, 12:52:20 pm
content: Hi there,Theoretically there is no difference between theory and practice however I have come to note that in practice there is, as such I'm asking for advice from those with practice first!Both general (that applies to calibrating pretty much anything) and specific are welcome.To be explicit:I'm asking for stuff you wish someone had told you.or ways you have accidentally broken stuff that's quite easy to do.Thanks.THE SPECIFICS: (which may not matter to answers).I have a lovely old analogue 'scope for personal use, a Gould OS300 and the voltage calibrations are way off (on the order of ~17 to 23%)Manuals are readily available:https://www.steampoweredradio.com/pdf/gould/Gould%20OS300%20Dual%20Trace%20Oscilloscope%20Manual%20and%20Schematic.pdfandhttp://www.radiomanual.info/schemi/ACC_instrument/Gould_OS300_user.pdfI prefer the 2nd one, it's sharper and has R1 and R2 in the parts listing (otherwise I've found few differences)I have no problem finding the adjustment points, anyone want pictures? It has a fairly well described procedure.

author: Ian.M
author_rank: Super Contributor
timestamp: November 05, 2022, 01:20:04 pm
content: This is general advice for any CRO from the era before digital controls, not Gould specific.Don't twiddle *ANYTHING* before you've located and fixed the fault and confirmed all power rails are in spec!An ~20% error is most likely due to one or more components changing value due to aging, rather than lack of calibration. Attempting to cover up the fault by twiddling adjustments will compromise accuracy (you've got no idea of how the fault + twiddling will affect linearity, or the bandwidth and flatness of the frequency response) and may decrease reliability.Once you've found and fixed the fault you can calibrate, but still don't twiddle any adjustment you don't have the specified test equipment (or comparable specification equivalents) + the specified test jig (if any) to do properly.

author: py-bb
author_rank: Regular Contributor
timestamp: November 05, 2022, 01:32:48 pm
content: I will check the power rails, the manual lists a bunch of test points - that's a great place to startbut it does work well and I don't consider it faulty.Quote from: Ian.M on November 05, 2022, 01:20:04 pmEverything inside looks good (no bulging caps and stuff like that) and it does work well, just the y scales are not accurate. So for example on 0.2 div in the calibrated position a 1v signal should be 5 divisions tall - it's a little over 4.There are loads of variable components inside (detailed in the manual) designed to be adjusted for this reason (I'd have thought?), the channels have different errors, so I assumed it was just ageing.I tried cooling it and the "calibration" doesn't move much (about 0.1 of a division), the position does move about 0.3 of a division though (so when we reposition the wave the overall height has changed a little)I wont be able to test the -1.1kv test point for the CRT though.TY

author: Ian.M
author_rank: Super Contributor
timestamp: November 05, 2022, 01:38:08 pm
content: General rule:Don't Twiddle what you cant measure!If the other supply and CRT voltages are in spec. odds are the -1.1 kV will be as well. If they aren't, fixing their problems, or minor adjustments to bring them into spec. if no faults can be found, will probably mean it will also now be in spec. *DO* *NOT* touch any adjustments that directly affect it without a meter that can measure it.

author: py-bb
author_rank: Regular Contributor
timestamp: November 05, 2022, 02:09:21 pm
content: Yeah my plan is still to follow the manual for channel calibration.The goal is to be able to read voltages off the screen without having to try and add 23% in my head

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 05, 2022, 05:48:53 pm
content: You did not say if your reading that was off was an AC signal (sine) or the DC levelIf you have the instructions and follow them, you should be able to get good DC values.Sometimes it is very difficult to get accurate sine wave calibration.and old scopes can have high tolerance values for these readings.So do not get disappointed if you can only get close.A general rule is that old scopes are 80% of value at their rated highest frequency. They usually can go higher but the amplitude is not very accurate at all.

author: py-bb
author_rank: Regular Contributor
timestamp: November 05, 2022, 11:37:52 pm
content: Quote from: Wallace Gasiewicz on November 05, 2022, 05:48:53 pmNot entirely sure what the difference is there? Without specifying a frequency....Generally if you get like a battery and a multimeter for example the rule holds true (~20% undervaluied on the screen) - I work with lots of +3.3v UARTs and obviously when nothing is transmitting we get a steady +3.3v ish - this is off by about the same (measured with a multimeter)I assume that being older than me (by a decade or so) it's just drifted in that time. It seems to "under-value" stuff by ~20%I tried peak of mains (with a x10 probe - it's rated for 400v pk-pk and I'm UK) and sure enough it's under by about 20% both relative to ground and done-by-eye of full wave.