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Old 29th December 2009, 10:32 PM   #1
dcsimon is offline dcsimon  United States
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Default Scope maintenance

Hi all, I purchased my first scope about two years ago (a Phillips PM3217). It seems to be a very capable scope and has worked well for most all my trivial needs. I recently went about finding a service manual after reading about the need to calibrate analog scopes for accuracy. After reading through the service manual I discovered that I would need several more pieces of test gear just to begin trying to calibrate my scope (I don't need extreme accuracy but having faith in my equipment has to be worth something).

My question is: how often does an analog scope such as mine need to be calibrated? I.E. should I go about purchasing a func generator and freq counter or try and borrow one for a one time calibration. I can hear the obvious answer already "Calibrate when it needs it", but in typical use cases, with maybe 10/15 hrs of use a week living in my non-climate controlled garage how high would the need to re-calibrate regularly be?

Please forgive me if I've overlooked these answers in my searching.
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Old 29th December 2009, 10:59 PM   #2
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I never calibrate mine.
If it started to give odd results I might look at it.

You could do some limited test using a meter to check AC and DC voltages.
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Old 29th December 2009, 11:35 PM   #3
dcsimon is offline dcsimon  United States
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wow, really? Even for a scope as old as mine? I felt sure that with the temperature fluctuation, the moving about, combined with age, these old scopes would fall out of spec rather quickly (one or two years?)
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Old 30th December 2009, 12:38 AM   #4
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Quote:
Originally Posted by dcsimon View Post
I.E. should I go about purchasing a func generator and freq counter or try and borrow one for a one time calibration. I can hear the obvious answer already "Calibrate when it needs it", but in typical use cases, with maybe 10/15 hrs of use a week living in my non-climate controlled garage how high would the need to re-calibrate regularly be?
I know this dilemma very well. "Calibrate when it needs it" seems an obvious answer to this, however, the problem is how can one know when calibrating is really 'needed'? When it`s already obvious that the measured results can`t be right, it`s not a measuring instrument anymore but just some kind of 'guesstimeter', calibrating is long overdue.
Less obvious errors might go unnoticed. This can be even worse than knowing that the measurement isn`t exactly right, as it could lead to false conclusions.

Unless You intend building up some kind of calibration lab yourself, I`d adwise against buying more gear only to get the scope calibrated because Your problems would just multiply. As You figured already yourself, usually You need a lot of instruments and special calibration fixtures to get this done properly. Those instruments and fixtures, of course, have to be calibrated as well.
You`d end up calibrating the calibration fixtures & instruments for the calibration fixtures & instruments which are needed for calibrating the scope......an endless and costly task.

If the scope is in good condition my advice therefore would be to get it once calibrated in a calibration lab and pay for it (provided that the costs for this are reasonable versus what is worth the instrument). In the commercial or scientific world measurement instruments get calibrated usually about once a year, not necessarily because it is 'needed' but just to verify that calibration is still valid and the instrument performs still inside the specifications. Well cared quality instruments like Tektronix, HP, Philips etc. in good condition can keep their calibration usually for a long time (years, sometimes even decades).
Hence for a hobbyist a one time calibration might be all You need for the next decade or so.

BTW:
Humidy is the among the worst You can do to an electronic instrument, therefore a 'non-climate controlled garage' is NOT a good place for storing quality measuring instruments (or any other electronic instrument for that matter). By any means do Yourself (and the scope) a favour and get it out from there into a dry place.
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Old 30th December 2009, 12:51 AM   #5
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The only thing which will need periodic adjustment is the trimmer capacitor on the scope probe. There is probably a node on the front of the scope with a square wave output and the probe trimmer capacitor is adjusted for best symmetry.
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Old 30th December 2009, 01:02 AM   #6
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If you have and use a scope, it's not unreasonable to have a signal generator and a frequency counter. I'm a big believer in frequent reality checks. If the scope agrees with the generator, or better yet, a counter, your confidence level goes up dramatically. You can also put a decent voltage reference chip in a box and measure it with a good DVM. If the scope agrees with that, and if you adjust the probes properly, your measurements should be pretty good. Does the scope by any chance have it's own signal to check probe compensation? Use that. A good tech will do a quick reality check between the equipment in use often, maybe every time a new measurement setup is done, or before anything really important. Once a year calibrations are nice, but you need to be more diligent than that to catch problems early when they're easy to fix.

CH
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Old 30th December 2009, 01:18 AM   #7
dcsimon is offline dcsimon  United States
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This is great info! There is indeed a cal terminal on the front panel, the spec is 2kHz @ 1.2v +/- %5. In the calibration procedure, this terminal is among the last to be adjusted (I assume due to the effects previous adjustments would have), the wave doesn't quite sit at 2kHz, though it is within the 5% spec. I guess the perfectionist in me wants to see that wave split those division lines down the middle. Is this unreasonable?

I do like the idea of "reality checks", in that if multiple instruments agree with one another then things are relatively good.
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Old 30th December 2009, 04:14 AM   #8
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Remember, until modern digital scopes, scopes were not the preferred device for precision quantitative measurements if there was anything else that could do the job. That's why we have triggered frequency counters and nice DVMs. HF AC amplitudes are often a bit iffy depending on strays, so a scope is/was quite useful even at 5% accuracy. I go through my old Tek boat anchor tube scopes every 5 years or so, and they stay plenty accurate. They have nice new digital scopes at work and people use all the built in measurement features about 98% of the time. I'd rather have a real counter, but the few digits they get on the scopes seem to be enough for most tasks.

CH
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Old 30th December 2009, 04:39 AM   #9
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Quote:
Originally Posted by Conrad Hoffman View Post
...I go through my old Tek boat anchor tube scopes every 5 years or so, and they stay plenty accurate....
I have a Tek 453 to which I've never done a thing except for running some No-Noise into a couple of the pots that were beginning to lose it. Aside from the built-in fixed sig gen and probe-trimming, are there other things to consider with this old beast?

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Old 30th December 2009, 07:11 AM   #10
Mooly is online now Mooly  United Kingdom
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Remember a scope measures voltage so use a DVM and DC PSU and make sure the trace moves the appropriate amount... easy

For horizontal timebase depending on your location the mains should be pretty accurate at 50 or 60 hz... OK for a checking a few lower ranges, for higher speeds use a crystal oscillator and divider.
Or... many multimeters have a frequency range, so use that and a signal generator.

You should be able to get it to "as accurate as you can see the trace" taking into account parallax errors.



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