Date   

Re: SUCCESS! The "sick" 475A is now the "fixed" 475A!

Tom Lee
 

Jeff,

If you ever wish to examine the spectral content of a signal, you need an SA. Using a scope's presentation of a sinusoid, you'd be hard-pressed to resolve distortions below a percent, and you'd only do that well for certain types of distortion, and if your eyeballs and neural net are pretty damn well calibrated. A properly designed spectrum analyzer can resolve distortion levels in the ppm range (and still better ones exist). Better still, you see the nature of the distortion -- e.g., is the distortion dominated by a certain harmonic or group of harmonics? Is it not distortion at all, but interference or power supply noise creeping into the system? Etc.

So, depending on context, either a scope (for volts vs. time) or a spectrum analyzer (for volts vs. frequency) may be the right tool. If you have both, you've got both domains covered.

I should mention that some digital scopes have an FFT capability, giving you something that looks like what a spectrum analyzer would present. But there are many tradeoffs in trying to make a scope also act as a decent spectrum analyzer, so most scopes are not decent SAs. But if the software comes for free, one cannot complain too much, as long as one is aware that the instrument may present a screenful of lies.

-- Cheers,
Tom


--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 12/7/2020 23:54, Jeff Dutky wrote:
Tom,

I've had a look at the TinySA home page (www.tinysa.org/wiki), watched several of the recommended videos, and I'm sold.

I've just got one question: aside from using it as a signal generator, what would I DO with a spectrum analyzer?

-- Jeff Dutky




Re: Can capacitors

 

See also the earlier discussion on Multi section electrolytic capacitors:

https://groups.io/g/TekScopes/topic/multi_section_electrolytic/78408639

Regards,
John


Re: SUCCESS! The "sick" 475A is now the "fixed" 475A!

 

Tom,

I've had a look at the TinySA home page (www.tinysa.org/wiki), watched several of the recommended videos, and I'm sold.

I've just got one question: aside from using it as a signal generator, what would I DO with a spectrum analyzer?

-- Jeff Dutky


Re: Tek blue (and gray) paint

 

Dave Casey wrote:

I would think UPS Ground would take it, just as they do Lithium batteries.
I would have thought that too, and the seller seemed to expect that as well, but his impression was that they were refusing to ship aerosol cans during the pandemic. Whatever the case I will find out this week, as I plan to call both UPS and FedEx to verify what their current policies are.

-- Jeff Dutky


Re: Tek blue (and gray) paint

Dave Casey
 

I would think UPS Ground would take it, just as they do Lithium batteries.

Dave Casey

On Tue, Dec 8, 2020 at 12:28 AM Jeff Dutky <jeff.dutky@gmail.com> wrote:

I ordered some of the spray paint from the eBay seller, but he had to
cancel my order today because he can't find a shipper who will handle
aerosol spray paint. I was a little worried about this when I made the
order. I know that USPS won't handle such things, and I was worried that
UPS and FedEx might balk (which, apparently they have).

I am going to investigate UPS ground and FedEx this week, to see if there
is any way to ship these items, but the seller didn't sound optimistic.

-- Jeff Dutky






Re: Tek blue (and gray) paint

 

I ordered some of the spray paint from the eBay seller, but he had to cancel my order today because he can't find a shipper who will handle aerosol spray paint. I was a little worried about this when I made the order. I know that USPS won't handle such things, and I was worried that UPS and FedEx might balk (which, apparently they have).

I am going to investigate UPS ground and FedEx this week, to see if there is any way to ship these items, but the seller didn't sound optimistic.

-- Jeff Dutky


Re: SUCCESS! The "sick" 475A is now the "fixed" 475A!

 

Tom, Raymond, Harvey, and John,

This is an absolutely amazing wealth of information. I've already copied it into a notebook page, and will be going through it carefully over the next week (and trying to cross reference it to the 475A calibration process). This sort of information needs to be collected in one place and presented as a coherent alternative to the Tek-specified process (and, yes, I'm volunteering to do that).

Thanks for all this wonderful commentary and information. It almost sounds like I rolling my own calibration fixtures isn't nearly as challenging as I had feared it would be (probably much less challenging than buying the vintage equipment and restoring it to correct working order).

I've got a significantly-more-than-ten-dollar pulse generator (http://www.leobodnar.com/shop/index.php?main_page=product_info&cPath=124&products_id=295&zenid=2cd5390de5b54cabb48f8ed903b80480) but I'm kind of excited to build the rest of the kit (I really need to get my feet wet with transistors, and I just finished reading about ECL for exactly this kind of project).

-- Jeff Dutky


Re: Anyone need 561 parts?

Bruce Atwood
 

I'm rewinding the HV XFMR in my 561 but would not mind a backup plan. How is the T600 in yours? As an aside, does yours have a 4.3 Ohm resister in series with the 1 turn secondary for the filament of the HV rectifier? Every schematic I see has one but there is no trace of it in my 561.

Fully OT: My 561 is almost all that is left of ITI Electronics (I also have their GR impedance bridge, a real marvel). ITI did TVs for bars and hotels in the late '40 and '50. My dad, Horace, W2SXW, was founder and president. One of the few small firms founded in the 40s that made it to the 80s. Dad told me of the IRE show where Tektronix showed of their first calibrated sweep, triggered sweep, calibrated gain scope where the assembled engineers all agreed it was a marvel but wondered who the hell would pay $ks for a 'scope. He was designing 'scopes for Allen B DuMont the time, later doing TVs for him.

cheers


Re: CRT static charge

Richard Knoppow
 

I wonder if the blue filter has a static filter on it or in it. Also, there are laundry softening tissues that go in the dryer that have anti-static stuff in them Wiping one on a meter face or CRT will often get rid of static charge. The tissues are often called anti-cling.
   If you have an ohm meter that will read very high resistance try it on the two filters to see if there is any difference in surface conduction.

On 12/7/2020 10:29 AM, Dave Peterson via groups.io wrote:
Something interesting noted on restored parts scope: when powered up the CRT seems to create a static charge crackling noise.

That is, my perception is that the sound is coming from the face of the CRT. This scope currently has a clear shield on it. My existing 465 with a blue shield does not crackle when started. Every component of the face of the scope has been freshly cleaned, so is presumably very dry and free of any oils from handling. The screen shield wiped with a glasses cloth you get from the optometrist to get rid of the static cling. Otherwise lots of dust would obviously accumulate. So presumably the shield has been discharged. The scope is properly grounded, including the ground plug on the face of the scope.

It sounds like fresh laundry being separated, but much quieter. It barely audible, but it's there.

Is this normal? I don't ever recall it from my previous experiences, but it's so innocuous sounding that I'm sure it wouldn't arouse much suspicion otherwise. I'm just very attentive to every sound, and very wary of anything with a scope of completely unknown origin and state. Could it be an indication of anything untoward? Haven't gone through a calibration yet. Getting ready to do so.

Dave



--
Richard Knoppow
dickburk@ix.netcom.com
WB6KBL


Re: SUCCESS! The "sick" 475A is now the "fixed" 475A!

John Gord
 

For relatively slow (Hz to 10s of kHz) square waves of known amplitude, you can use some paralleled beefy CMOS buffers powered by a precision supply feeding a relatively low impedance multi-tap voltage divider. Output levels can be adjusted at DC with a DMM, then the buffers can be driven from a convenient function generator (or crystal controlled synthesizer if you want accruate timing). This is limited to 5v max or so with most fast CMOS, but 4000B series can be supplied with 10v or more if desired. (The 4000B series is not as beefy, though.)
--John Gord

On Mon, Dec 7, 2020 at 06:53 PM, Tom Lee wrote:


Hi Raymond,

You're not wrong about the need for lower frequency square waves if you
want to perform the full set of cal procedures. You'd have to supplement
the tinySA with a pulse generator, as you've noted. Luckily, the
performance requirements are pretty lax -- the frequency doesn't have to
be all that precise, and it's easy to produce good, clean waveforms over
the relevant frequency range. Homebrewing it would be entirely feasible,
and I wouldn't be surprised if there's a $10 pulse gen kit on fleabay.

I don't remember the frequencies needed for a 475, but ISTR numbers
between 1kHz and 50kHz. But when I homebrewed mine back in the day, I
used some TTL dividers driving a PNP (I used 2N3906s, I'm sure, since I
had a mountain of them) emitter-coupled output stage to feed a 50-ohm
termination. That stage cleans up the less-than-pristine TTL output to
provide a well-behaved square wave.

I guess you could even square up the 100kHz output of the tinySA to
drive something similar, and you'd be good to go.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 12/7/2020 18:29, Raymond Domp Frank wrote:
On Tue, Dec 8, 2020 at 02:53 AM, Tom Lee wrote:

from 100kHz to 360MHz (beyond that you get square waves)
Nice! Just an SMA-to-BNC and 50 Ohm feedthrough needed. I'd have to think
for which Tek 465/475 adjustments exactly, except for horizontal timing as you
describe and observing BW, if the TinySA produces something enough like a sine
wave, which I guess it does between 100 kHz and >300 MHz. For adjusting the
high-impedance CH1 and CH2 step attenuators, a frequency much lower than the
TinySA's minimum (@100 kHz) is necessary. For edge adjustment, using the
square wave could possibly have been nice but 360 MHz (the minimum square wave
frequency) won't make it through to the vertical amp in any useful way.

Am I wrong and/or missing something?

Raymond





Re: SUCCESS! The "sick" 475A is now the "fixed" 475A!

 

Tom,
Re. equipment (generators and passives) needed (I only checked for the 465):
- For adjusting the Hi-Z step attenuators' frequency behavior, the 1 MHz CH1 and CH2 inputs need to see a 20 pF capacitance. A 20 pF "normalizer" is needed with many people using their "reference" 10x passive probe, hoping it's set to about 20 pF or at least away from max or min capacitance. Source is a 1 kHz square wave generator. As you say, very lax requirements there.
Since adjustments need to be made up to the 5 V/div setting, a relatively high-amplitude, "good-looking" square wave is needed, especially when using a 10x probe. Normally, the high-amplitude output of a PG506 is used, with fixed attenuators as needed. Wave front (HF) and flat top (LF) are adjusted this way.
- A 50 kHz sine wave is used for determining the base deflection against which the BW is measured. I'm sure that 100 kHz would be no problem so 100 kHz and >=100 MHz from the TinySA may be used. Voltages up to 2 Vpp, so no problem for a TinySA. Just an SMA-to-BNC cable and 50 Ohm feed-through needed.

Raymond


Anyone need 561 parts?

WB4IUY
 

I salvaged some bits from an old 561 to restore my 564B, have a good bit of stuff left. wb4iuy@gmail.com

Dave WB4IUY


Re: SUCCESS! The "sick" 475A is now the "fixed" 475A!

Dave Peterson
 

The first 465 I got was horribly, unusably, out of "cal". CH1 level disagreed with CH2 by something like 30%. At this point I had NO lab equipment beyond a DMM and some test leads. I was able to get a free function generator app on my cell phone and set levels that agreed with the output. CH2 agreed out of the box, CH1 needed adjustment. Is it to any kind of spec? No. Is it better than it was? Sure.

Time base was out of agreement too. It said my line voltage was running at 67Hz. I don't think so. Again, using the free FG app I got the scope within reason. I was also able to get trace rotation and geometry improved. And the line frequency is sensible now too. For 99-bucks I got a two channel 60MHz FG/counter with a plethora of waveforms, modulation, sweep, VCO, sync in, etc.

I have a long way to go before my bench is to my standards. But I don't mind making incremental steps. It's the journey.

Dave


Re: Tek blue (and gray) paint

victor.silva
 

I have some cans of Tek Blue that matches the 24xx series perfectly.
I posted a message it before.

I wonder if these can still be purchase given the information on the label?

It is Krylon OMNI-PAK MasterBlend EZ TOUCH <- This is the generic can for custom paints.

Specific blend is on a label which reads:

L61 Opex Lacquer
L61XXL42-4383
TEK BLUE
09/26/05


Re: SUCCESS! The "sick" 475A is now the "fixed" 475A!

Tom Lee
 

Hi Raymond,

You're not wrong about the need for lower frequency square waves if you want to perform the full set of cal procedures. You'd have to supplement the tinySA with a pulse generator, as you've noted. Luckily, the performance requirements are pretty lax -- the frequency doesn't have to be all that precise, and it's easy to produce good, clean waveforms over the relevant frequency range. Homebrewing it would be entirely feasible, and I wouldn't be surprised if there's a $10 pulse gen kit on fleabay.

I don't remember the frequencies needed for a 475, but ISTR numbers between 1kHz and 50kHz. But when I homebrewed mine back in the day, I used some TTL dividers driving a PNP (I used 2N3906s, I'm sure, since I had a mountain of them) emitter-coupled output stage to feed a 50-ohm termination. That stage cleans up the less-than-pristine TTL output to provide a well-behaved square wave.

I guess you could even square up the 100kHz output of the tinySA to drive something similar, and you'd be good to go.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 12/7/2020 18:29, Raymond Domp Frank wrote:
On Tue, Dec 8, 2020 at 02:53 AM, Tom Lee wrote:

from 100kHz to 360MHz (beyond that you get square waves)
Nice! Just an SMA-to-BNC and 50 Ohm feedthrough needed. I'd have to think for which Tek 465/475 adjustments exactly, except for horizontal timing as you describe and observing BW, if the TinySA produces something enough like a sine wave, which I guess it does between 100 kHz and >300 MHz. For adjusting the high-impedance CH1 and CH2 step attenuators, a frequency much lower than the TinySA's minimum (@100 kHz) is necessary. For edge adjustment, using the square wave could possibly have been nice but 360 MHz (the minimum square wave frequency) won't make it through to the vertical amp in any useful way.

Am I wrong and/or missing something?

Raymond




Re: SUCCESS! The "sick" 475A is now the "fixed" 475A!

Harvey White
 

Most oscilloscopes of that era are analog only, and as such, you measure things against a trace on a graticule, by eye.

I remember somewhere where Tek said that 2-3% was the best you could get in such measurements.

Go to digital, and while the timebases are digital, they may be derived from a non adjustable frequency source.

Your average time interval calibrator, when adjusted against a known frequency standard (WWV has been around for ages) ought to be more accurate in terms of absolutes than the scope.

That leaves amplitude and frequency response.

There are 10 volt references on EBAY (and equivalent) which can be  used to adjust a DMM/DVM.  With some compromises those could be used to adjust the scope's Y axis.

Should time response be needed, some very accurate risetime generators can be had.

You should be able to get reasonably close for a home lab.

Sometimes the purpose of calibration on a scope is to make sure that if you measure thing A on scope #1, it'll be the same on scope #2.

Harvey

On 12/7/2020 7:16 PM, Dave Peterson via groups.io wrote:
This thread is validating my suspicions. I was surprised to accidentally come across a cal service website offering calibration for a 465 while looking for what the waveform on TP1478 (or something like that) was supposed to look like. Part of the Z-axis compensation adjustment. As Jeff noted, they're available, for more $$ than I payed for the scope.

I'm sure I could adjust my scopes as well as, if not better, than a dedicated shop. But I know the difference is that I don't have certified standards to validate against. But then my balancing thought is what you're saying about 0.1% - how many significant digits can one achieve with these things. Trying to adjust a 465 pot to less that 1% is just not possible. Doesn't stop me from trying. Wink. But if I have two or three instruments agreeing to 1% I have good confidence that my adjustments are valid. But that's not really what "calibrated" means.

Again, the issue circles back to what one is trying to sell. If I sold as "calibrated" I wouldn't do that without: A) charging for it, and B) providing the paperwork proving it. I like the term "performance verified". Or otherwise functionally and performance verified or validated. I'll have to investigate what eBay calls "Refurbished". Then again I'm probably holding myself to a standard that other sellers may not. I gotta be me.

I've got a ways to go before I'll feel comfortable reselling a scope as refurbished and verified. I also don't see myself selling one until I am comfortable saying that. If I wasn't comfortable selling it as validated I'd have to sell it as parts. Which I'm also not thrilled with. I don't see a market for selling a "calibrated" 465 vintage scope. I'd leave that to the buyer. If they need that, then it's part of their operating expense anyway.

Thanks for your inputs. Enlightening, educational, and entertaining, as always.Dave

On Monday, December 7, 2020, 03:30:32 PM PST, demianm_1 via groups.io <demianm_1=yahoo.com@groups.io> wrote:
My experience with cal labs has not been encouraging. Mostly they want to run instruments through the automated cal procedure. Anything else will be expensive (labor). And its really whether the instrument is within tolerance (go/nogo). Its not adjusted to perfect.  If adjustment/repairs are needed the rate can be astonishing and keep in mind on these older instruments the repairs are not 10 minute fuse replacements. They are usually really difficult things to figure out.

However for a scope a time mark generator and a leveled signal generator would cover most tasks. CRTs are not .1% instruments so no need to get that involved. I picked up a Ballantine 6130 time mark generator and a Tek 191 leveled generator (to 100 MHz) pretty reasonably. With those you could say "performance verified" instead of "calibrated".









Re: SUCCESS! The "sick" 475A is now the "fixed" 475A!

 

On Tue, Dec 8, 2020 at 02:53 AM, Tom Lee wrote:


from 100kHz to 360MHz (beyond that you get square waves)
Nice! Just an SMA-to-BNC and 50 Ohm feedthrough needed. I'd have to think for which Tek 465/475 adjustments exactly, except for horizontal timing as you describe and observing BW, if the TinySA produces something enough like a sine wave, which I guess it does between 100 kHz and >300 MHz. For adjusting the high-impedance CH1 and CH2 step attenuators, a frequency much lower than the TinySA's minimum (@100 kHz) is necessary. For edge adjustment, using the square wave could possibly have been nice but 360 MHz (the minimum square wave frequency) won't make it through to the vertical amp in any useful way.

Am I wrong and/or missing something?

Raymond


Re: SUCCESS! The "sick" 475A is now the "fixed" 475A!

 

On Tue, Dec 8, 2020 at 02:06 AM, Dave Peterson wrote:

Hi Dave,
Please find my comments embedded, after your text.


I bought a cheap function generator that I wouldn't take home to meet my
parents. Much less brag about here.

However, it is a modern piece of technology that I think can outperform my old
465 well enough to tell it how to dance. Modern run of the mill digital
electronics are running at several orders of magnitude higher frequencies than
this old scope.
Not too fast:
A cheap function generator
- generating signals up to and beyond 100 MHz (465 BW) and
- with a predictable/calibrated amplitude?

I'm currently using it to generate square waves to use in place of a time mark
generator. From what I've seen so far from the calibration procedure, and I've
done a little tweaking on my existing scope with an even more embarrassing
source of square waves (a free function generator app on my cell phone), the
primary purpose is to provide a time standard to adjust sweep timing and also
geometry edges for rotation and "Geometry, R1442" vertical alignment with the
graticule. So giving you clean square edges for aligning the X and Y axis of
your trace. The 465 can just make out the vertical edges of around 10-20ns.
Plenty good for 465 edges.
Not for (rising and falling) edges: the 465's spec is 3.5 ns.
Adjusting geometry affects vertical and horizontal amplitude calibration, mostly at the edges.
Trace rotation is gravity- and therefore also orientation-dependent. That's why it is easily adjusted from the front of the 'scope.

Other entries in the calibration procedure use the time mark generator to
calibrate (check and adjust) the B-sweep. I'm not clear on the process yet.
I'm sure there are others. What I've seen of time mark generator output look
like pulses more than square waves. But the effect is the same - regular
vertical edges. The cheap FG will do pulses too. Anything actually.
As long as the distance between rising or falling edges of the FG (IOW the period) are accurate to within at least 1%, that's ok.

The point being, I think a decently competent square wave generator can
suffice as a stand-in for the time mark generator.
Very limited at best for horizontal calibration of a 465's (and 475's) high-speed horizontal timing (1 - 50 ns).

Standards for level and
time are subject to suspicion. But again, my sneaking suspicion is that the
cheap function generator is within the tolerances of what the 465 can measure.
I disagree on several points, see above.

I continue to contemplate how to validate it's time and levels.
It's perfectly possible to use DC voltages, checked with your DMM, for adjustment of vertical deflection (sensitivity), IOW "levels".

But this is far better than nothing, for a lot less $$.
If you intend to expand your collection and perform calibration and adjustment (can't help myself), consider hunting for a PG506 and TG501 at least. Prices have gone down - and so has availability...

Calibrated? No. Tuned up? Hell yes!
Raymond


Re: SUCCESS! The "sick" 475A is now the "fixed" 475A!

Tom Lee
 

Hi Jeff,

A time mark generator just provides some blips that have known time spacings to guide the tweaking of the scope's timebase. In my student days, I just cobbled an equivalent together out of some castoff TTL crystal oscillator modules and some 74LS90 dividers. Any crystal oscillator will have much tighter accuracy than you need. Even the crappy ones used as CPU clocks will be correct to the ~100ppm level, as compared to your scope's ~1% timebase accuracy. Version 1.0 didn't even use oscillators of round-numbered frequencies; they were just whatever I could salvage from other ewaste. Made tweaking a little more math-intensive ("how many nanoseconds is one period of 3.579545MHz?"), but it was still easy to get the job done for the lower speeds. For higher speeds I used injection-locking techniques. A bit more involved, but also not beyond the skills of a DIYer.

But wait, there's more! As for a leveled sig gen, the tinySA (less than $50 for the original, even less for clones) can be used as a sig gen and does a good job from 100kHz to 360MHz (beyond that you get square waves). The tinySA's output amplitude control has a 31dB range that covers most of what you would need. And as a bonus, the whole thing is crystal controlled, so you can dispense with the time mark generator and just use this thing for both purposes. It's a floor wax and a dessert topping, as an old SNL advert once boasted.

--Tom


--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 12/7/2020 16:39, Jeff Dutky wrote:
demianm_1 wrote:
for a scope a time mark generator and a leveled signal generator would cover most tasks
Yes, it's the leveled signal generator that seems like the most challenging thing to roll my own. I've read that the time mark generator can be substituted with a fast pulse generator (which I have) though I don't claim to understand how the time mark generator is used or how the pulse generator could stand in for it. At some point I will sit down, read the entire calibration process, and read the manuals for the relevant calibration equipment, just so I can make a somewhat more informed decision about this.

You can get a full set of 500 series plugins for calibrating a 475 on eBay for about $500, though there's no guarantee that they are complete or completely functional. I could pay for calibration for a couple scopes with that same money. I suppose I could buy the calibration equipment, do any repairs that appear necessary, and then pay to have THEM calibrated, and then I'd be set to do my own calibrations. I feel, however, that that would only be worthwhile if I were going to be doing this as something other than a hobby.

-- Jeff Dutky




Re: SUCCESS! The "sick" 475A is now the "fixed" 475A!

 

On Tue, Dec 8, 2020 at 01:16 AM, Dave Peterson wrote:


I'm sure I could adjust my scopes as well as, if not better, than a dedicated
shop. But I know the difference is that I don't have certified standards to
validate against. But then my balancing thought is what you're saying about
0.1% - how many significant digits can one achieve with these things. Trying
to adjust a 465 pot to less that 1% is just not possible. Doesn't stop me from
trying. Wink. But if I have two or three instruments agreeing to 1% I have
good confidence that my adjustments are valid. But that's not really what
"calibrated" means.

Again, the issue circles back to what one is trying to sell. If I sold as
"calibrated" I wouldn't do that without: A) charging for it, and B) providing
the paperwork proving it. I like the term "performance verified". Or otherwise
functionally and performance verified or validated. I'll have to investigate
what eBay calls "Refurbished". Then again I'm probably holding myself to a
standard that other sellers may not. I gotta be me.

I've got a ways to go before I'll feel comfortable reselling a scope as
refurbished and verified. I also don't see myself selling one until I am
comfortable saying that. If I wasn't comfortable selling it as validated I'd
have to sell it as parts. Which I'm also not thrilled with. I don't see a
market for selling a "calibrated" 465 vintage scope. I'd leave that to the
buyer. If they need that, then it's part of their operating expense anyway.
Hi Dave,
I've considered responding re. calibration for a while but decided to wait what others would say - call me lazy.
So far, I haven't seen mentioned what I wanted to say:

In a strict sense, "calibration" means verifying against a standard, nothing more. For electronic instruments, the standard would be the specification of the instrument to be calibrated (DUT), *not* the specification of the instrument one is using to verify the DUT with.

A DUT is considered "in spec" or "calibrated" if it performs within its published specifications (all of them). IOW, if the published spec for vertical sensitivity of a 'scope is +/- 2% and it performs within +/- 1.5%, it is *within spec* in that respect (or parameter, or feature, whatever you like to call it). Since there are only two possibilities: *within spec* or *not* within spec, if it performs within say +/- 0.01%, it's not "more within spec" and in a sense, not even "better calibrated". It's just more precise.

While (or after, whatever you call) restoring, many hobbyists tend to start adjusting things like the low-voltage power supplies, because they have a pretty good DVM - but nothing else.
It's perfectly possible that the instrument was within spec ("calibrated") before that but after adjusting, no longer is, and they have no way to check or adjust because they'd need different instruments.

Many calibration labs distinguish between "calibrating" and "adjusting", with very different pricing. For many people (and in normal speak) calibrating means the same as adjusting. Strictly speaking this isn't so, as I explained above. You need to be aware of that.
It would seem to make sense to try and adjust close to perfect but that's not a requirement to be "in calibration".

Naturally, adjusting as much as possible precisely in the middle (aiming at +/- 0%) seems to make sense to allow as much drift as possible until you lose calibration status but even that's *not* true in all cases. It may depend on temperature behavior or aging for instance. If e.g. a particular model quartz oscillator is known to age toward becoming slower, it may make sense to adjust it a bit higher than exactly +/- 0%.

As another example, it may make very little sense to adjust within say 0.1% where the spec says +/- 2%, because the parameter could drift outside of that within minutes to say .3% (because of temperature variations or so) but stay within calibration (= spec) for a year because the drift would only be say +/- 1% over a specified calibration interval.

BTW, I don't think it makes sense to calibrate or even adjust a healthy 465/475 twice a year, as was suggested, unless it lives in an unhealthy or very unstable environment, because it's not expected to drift out of spec within such a relatively short period. Checking (calibrating!) a restored instruments initially after a few months may make sense.

Note: The drift that is seen with many old components, like carbon composite resistors, far exceeds normal drift that the original calibration interval is based on.

Raymond

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