Entry Level Metrology

[PixieDust]

Hi Everyone,

I have slowly been buying electronics stuff over the years with a view to get into part time hobby activities within the next year or two. I purchased an old Tektronix 2431L two years ago and a HP 3478A last year. I replaced the RIFA capacitors in both of them (just did the 3478A today) and now they are just sitting on my desk waiting for their time. I would really like to figure out their calibration state to see whether they are working as intended or not and whether I can rely on them. What should I do to figure this out?

I got calibration estimates for the oscilloscope ranging from $300 to $1400 AUD. I haven't asked around how much it would cost to get the multimeter calibrated but I would imagine I would get similar quotes. I purchased each unit for a lot less than the lowest $300 quote and don't fancy paying this much every 2-3 years (definitely don't want to calibrate yearly).

The way I see it, to check the multimeter, you need 3 things. DC voltage source, an AC voltage source and some resistors.

Based on this video:

https://youtu.be/uCUIXasdtDk?t=1312

There exists a resistor standard by Vishay, but I haven't found it on sale anywhere, so no idea how much it costs or even if they have them on sale anywhere.

In this video:

https://youtu.be/e-itiJSftzs?t=1520

There is an example of a DC standard and he uses a Variac with a reference of a brand new Keysight bench multimeter for AC testing. I don't have access to a calibrated unit for comparative testing.

I found:

https://dmmcheckplus.com/
https://voltagestandard.com/

Researching those units now. Anyone have any feedback on them?

As for the oscilloscope, I don't know how to test this other than using the included calibrator on the front panel. In that respect it seems to be ok. Self testing returns PASS for everything.

So will dig into those.

So looking for suggestions on the best way to run these two devices through their paces.

I'm not doing anything related to rocket science so my needs will unlikely to be super demanding. So overall I think I'm mostly worried whether these devices are in the right ballpark or not. I think for my needs this will be good enough but the more price I can test things the better.

All suggestions welcome!

EDIT: Found these two videos including one by Dave:

[bdunham7]

Sending an (old, generic) oscilloscope out for calibration is usually only done when some regulation or written procedure demands it.  It is pointless for the normal user. The front panel calibrator is not relevant--it is for compensating the probes.  If you want to do a check and adjustment, which is what 'calibration' should really be called in this case, you just need a decent signal generator or two, a DC PSU and a DMM that is not broken to get started.  The reasoning here is that the scope is like an analog VOM--it is only accurate to 2-3% at the very best both vertically and horizontally.  Calibration is making sure it isn't broken and adjusting it a bit here and there to get the picture as good as possible.  I haven't seen that exact model, but on those later-model CRTs, most of the actual manual adjustments will be in the CRT section ad the rest will be automated.  So I would keep the money in pocket and look at acquiring what you need to test it.  Also, read the manual and figure out how the memory is retained.  I think it is EAROM, which is battery-free, but I'm not sure.

As for the HP3478A, if the calibration memory battery has not gone bad, then it is likely still pretty accurate except perhaps for the low-ohms range.  So the first order of business with that, IMO, would be to read up on how to carefully replace that battery without disturbing the memory.  It is a bit tricky and there are a couple of ways to do it, but one common way is using an isolated soldering iron while the unit is powered up.  You can also use an auxiliary battery with separate leads.  Great care is needed no matter how you do it.  Once that is handled, get something like the DMM Check just to verify that it hasn't lost its memory, which would cause an error big enough to notice.  If you want to fully calibrate it, you need a lot more than a single source. Read the manual to see what all the required stimuli are.  And definitely don't bother calibrating it, at home or by sending it out, without a new battery in place.

[PixieDust]

Thanks, I had a feeling that sending these things out for calibration wouldn't be the best decision. I will look into the service manuals to see what's needed.

I have backed up the calibration data for the 3478A using an Arduino GPIB project which I found on these forums.

When I was replacing the caps today, it looked like the battery may have been replaced previously. I saw definite repairs on other parts of the meter but the battery looks like it may have been touched also but I'm not 100% sure, the work looked almost factory. Looked off by like 10% or so from the rest of the meter's workmanship. Given that the unit isn't pristine, it made me wonder whether something may be amiss with it.

[guenthert]

Entry Level Metrology?   I can't think of anything better than to link to http://conradhoffman.com/mini_metro_lab.html.  Mr. Hoffman is here around from time to time.

[David Hess]

Oscilloscopes are not really precision devices which require calibration, and I sure would not pay someone to calibrate a Tektronix 2431L, and doubly so because the calibration of a 2431L can be done by the user with a minimum of equipment.  It would be more cost effective to buy a new low end DSO or the equipment to calibrate the 2431L like a function generator, multimeter, and cables and adapters.

[PixieDust]

Entry Level Metrology?   I can't think of anything better than to link to http://conradhoffman.com/mini_metro_lab.html.  Mr. Hoffman is here around from time to time.

Have started reading through the articles thanks!

Oscilloscopes are not really precision devices which require calibration, and I sure would not pay someone to calibrate a Tektronix 2431L, and doubly so because the calibration of a 2431L can be done by the user with a minimum of equipment.  It would be more cost effective to buy a new low end DSO or the equipment to calibrate the 2431L like a function generator, multimeter, and cables and adapters.

Got it, thanks!

[james_s]

Calibration is typically something you do if you work for a ISO certified company. It's less about the instrument being accurate than it is about the instrument having a paper trail that indicates it is as accurate as the paperwork says it is. You can check the multimeter against a voltage reference or compare it with another multimeter, it's unlikely any two meters will be inaccurate by the same amount. Relatively high precision resistors are modestly priced if you want to check the resistance range.

In my experience older A-list equipment is usually either still within specified tolerances or something is broken and it's way off.

[PixieDust]

Got it, thanks James.