Hi today it's going to be vintage tear down repair, troubleshooting, and calibration time. Check out what I've got! It's a vintage Keithly 480 Pico Amer Fantastic instrument. It dates from uh, about 1979, although the Uh manual the Keithly manual for this thing which you can download which has all the full schematics and uh, calibration and servicing information in it um, was last printed in 1990. So I think this thing had a pretty darn long life and you know I'm into uh, small currents and stuff like that.

I've got a whole range of uh Keithly gear over here in including Pico Amer sources and uh, stuff like that. So I thought this is a nice little match for that. Sure I've got my micro current to measure low currents, but this little Keithly unit? nice score I thought I got it for 65 bucks on eBay um and these are pretty rare to find in Australia there are not so rare in the US of course and that's probably about the go price for these things. So I thought I'd snap it up now.

it was actually advertised as um, you know it, it would fire up. it would, uh, you know, power up but untested. fair enough. It's pretty hard to, uh, test um, this thing unless you've got the proper instruments to do it.

So um, yeah, it the it showed a picture of something on the display and I thought okay, it'll leads Powers up Beauty but I got it, plug the thing in and W nothing. nothing on the display so that's a bit of a bummer, but that means we can do a repair and troubleshooting video so let's give it a go. Once again, this one will be a real time repair. Um so I have no idea if um, this thing is actually repairable, it should be.

They're pretty simple inside. there's not much um in them so I'm be surprised if I couldn't uh fix this thing. It could be a very simple thing. could be a blown fuse I don't know.

No, we'll find out. Let's go. and here it is. It's very old school.

It's got the um, oldfashioned, uh, ganged push button range switches here and um, it's it's very simple. It's got a single uh BNC input. It's got the zero adjust pot down here on the front panel which is really nice and it has ranges from 1 milliamp full scale all the way down to 1 nanoamp uh, full scale there. so with 3 and A2 digit uh display there as you can see behind the red ppcs there it uh can uh has a resolution of about one Pico amp which is really quite nice.

Very handy unit for measuring low currents and it is a feedback uh operational type. So the burden voltage on this thing is incredibly low. this one's spec to about 250 microv volts burden voltage. Now it's got a tilting bail here and if we have a look on the back it's got uh selectable 240 and 10.

it's already set to Uh 240 Vol so you know, presumably. Um, that's not the reason why it is blowing. It's got an Australian plug on it and uh, well, yeah, we'll find out. It's got a couple of banana uh jacks on, um, the output here.

and uh. the good thing about this is that it's the output um directly from that feedback operational amplifier without going through any additional circuitry. So um, you can get the direct output from the feedback app, which is really quite nice. There's no indication of uh, a date on this thing, but we do have a uh operating instructions on the back I Rather like that, That's quite neat.

Why can't all units, uh, do this? and uh, it tells you down here. Input Burden Voltage: There it is. Um, Input Ver burden voltage is 200 microv volts or less for an on- range reading when zero is properly adjusted so the burden voltage is incredibly low. Um, it specs aren't too bad at all.

Um, half a per to 8% plus a few three or four digits there. Not too bad at all. So I Rather like it. Um, of course, on the 1 milliamp range you're only going to get Um.

maximum input voltage is uh, 20 volts and you'll note up the top here that it is designed to have a battery uh, pack in it as well I Have no idea if this one, um's got one based on the weight of it. It's very light so I'm assuming this one doesn't have the battery um option in it. So um, yeah, trust me. I've powered this up and we get absolutely nothing on the display so it's troubleshooting time.

First thing we're going to do is uh, check the Main's plug to see if like there's a Main's fuse blown and I assume like there's a Main's fuse inside. There's not one on the back panel so we'll just measure the resistance and bingo Yes, Um, this is quite common. um to when you're measuring primaries of Transformers you can see the multimeter skipping like that. That's the auto ranging being confused by the massive inductance in the Transformer.

But there you go. So yep, we're measuring the primary of the Transformer there and it looks like oh, look, look the the display just popped up. Did you see that? I'm sure it did. Yeah.

Look, the display is popping on. how the hell is it doing that? There must be some stored charge in there. couldn't couldn't be getting it from the multimeter. That's that's bizarre.

I Didn't notice that before. No looks like it's it's not going to do that anymore. I Hope you got that on camera there. But anyway, um, the Uh primary is um in intact and the primary is in there regardless of the power switch.

So this is effectively a soft uh, power switch on this secondary of the Transformer Ah, you're not going to believe it, man. I Cannot cop a break. Look at this. it's working I Swear it wasn't working before the thing has decided to work.

All of a sudden, a man massively disappointed I was hoping this would be a troubleshooting video, but maybe there is something. Maybe there's an intermittent um issue there. So yeah, um, we may have to. Well, we're definitely going to crack it open and uh, have a look.

but H that's a that's a real bummer. And back in the Glory Days of Keith Of course, before the evil Daner group took them over. um, it was made in the good old United States of America Sure, it brings a tear to the eye and to's some Yanks out there, but yeah, you won't catch me singing the Star Spangle Banner so we'll whip those out. Looks like they go into post into the other side.

Very old school for these instrument cases. Very typical and still used today for this style of instrument case. And one thing to note you don't see this very often is this right angle cable clamp here. um Main's input cable clamp on the bottom of the case instead of the back panel I Have no idea why they've done that.

That's weird. but anyway. oh, let's put it up this way. screws are out and let's crack it open.

Taada there it is. Piece of cake. A lovely and that is wonderfully wonderfully old school. I Love it! Check out all the square uh traces on the PCB there.

Beautiful. Look at this. Crystal look at the size of that sucker. that's we'll go in and have a look at that that looks it's 100 khz, 100 khz Crystal look at the size of it.

Um I'll see if we can get a date code off one of these uh chips and see uh, see what the uh build data this thing is. There's no data on there. there's a Cal inspection sticker, all of the um, all of the stuff is under the shielded can. of course.

um, this is all just the display and power supply stuff outside here. but all the um, the feedback amplifier and everything's inside here. One interesting thing to note is that the BNC input here. Check.

Check this out. They've got a shield there, right? They got a shield over the BNC and then they've got a little what looks like an unshielded, um, it's not quite a coax. It looks like just a single core wire. uh, with some uh, tubing on it coming out there and going down into the can down under there.

So that's I You know. I Don't know why they've bothered to, uh, do that? Why they've taken that outside of the cannon. All that sort of stuff. Weird.

Well, certainly no shortage of uh test points here. Here's our power supply. We've got plus - 15 there, plus 5 Vol test points We can, uh, check those out and um, yeah, we just got some axial filter capacitors. our Transformer is up here.

We've got an internal fuse which isn't blown of course because it's working and we've got a battery and line switch here which allows us to select the battery module which presumably plugs into this this connector here so we don't have the battery module on there and looks like we got a bridge rectifier there two more uh rectifier diodes there, so they may be getting another tap on the Transformer. You can see multiple Uh Taps on the secondary side of the Transformer there trim pot. There's another couple of trim pots down inside the Uh can down here, so I'm not exactly uh sure what they're designed to do I'll have to read the Uh calibration information for that and we have some date codes here folks. We got a 4,000 series of 4011 Classic 91 42nd Week 1991 So and this one's directly sold it in so that, um, you know it was at least manufactured 9192 so it's a relatively uh, recent unit.

probably as recent as you can get these things I would guess and uh, this um, I chip over here is a 93 one, but it's in a socket I'm not sure, um why we've got a socket there. We've got another 93 one down here. so I assume they haven't been replaced and those IC sockets were Factory fitted and we've got uh, curiously, a blank socket down here. so I don't know what that one's for.

but uh, there you go. So definitely, um, early 90s unit. beautiful and it's a REV revision F PCB And check out the shielding spring that they've got here. Um, the Curious Thing about that um is.

well, it's designed to M to something. but inside the lid, there's no um, you know, shielding on the upper part of the case for it to M to so I don't know. maybe uh, something to do with the charger board but I can't see it so don't know what's going on there. Now let's take a look at the data sheet for these two puppies here.

We got an ICL 71, CO3 and an ICL 8052 and as it turns out, they're actually a pair of. they're a matched pair of devices Tada and what they are is a Precision, 4 and 1 half digigit ad converter and display driver. It doesn't tell you that, but that's what it is as we'll see and apparently was pretty darn state-ofthe-art It's designed 4 and a half digit accuracy. Um, uh.

two 200 molts to 2 volts? Uh, full scale capability Auto zero Auto polarity as you'd expect of a dual slope uh conversion. uh unit like this, typically less than two microvolts peak-to peak noise. Um, you know, accuracy guaranteed. A plusus one count over the entire full scale range.

Guaranteed zero reading, so a pretty darn nice chip for its day. I Like it. Um. use of these chip pairs eliminates clock feed through problems and avoids critical board layout.

Woohoo! Beautiful stuff. And it's also Uh does a three and a half digit mode which is what? Um, it's used here and you can get up to 30 readings per second to do that. And I Love hear how they're toting their horn. Almost ideal.

Differential linearity and time proven dual slope conversion. Ah, love it. It's got a medium quality reference in it, Not a high quality reference. It's a medium quality reference.

40 PPM Yeah, not that, uh terrific. but um, more than good enough for this. 5 Pico amp uh input current down there and it's a dual chip solution and they've got them in the way data sheet which is, um, quite unusual. Usually usually they'll have Um separate data sheets for each one.

But and here's the block diagram for the two chip solution. and you can see the Um 71 C03 with the Uh red outline here like this and the Um 8052 is that one there. So the 8052 just contains a few Uh buffers and the in integration amp. And you need the Um external integration capacitor and Uh various external components there internal Uh voltage reference and the main Um ICL 71 CO3 Uh contains the switching and the zeroc crossing detector and all the multiplexer multiplexing and latching and Counting solution for that and you'll notice that it's not a seven segment display.

um output Here it's a four-digit uh BCD um output. So you need a BCD to seven segment uh decoder. so we should find that chip elsewhere in the design probably on the front panel there. and uh, as you can see, it can drive a 4 and 1 half digit display.

but in this case we're only driving 3 and 1/2 digits and there's the PIN to strap it to 4 and 1/2 or 3 and 1/2 digit mode there. And as you can see, there's not much to it but it's A. It was a very Precision device for its day and if we check out the bottom of the board here, here's this uh shielding tab. Again, it's all the one piece of uh, bent metal so we don't know what it's going to on the top.

There doesn't appear to go into anything but the bottom Here of course is going to a big shielding plate on the bottom of the case there and and uh yeah, not much doing there. Um, check out the big star ground point up here. Look at that nice kind of looks like almost a Flying Spaghetti Monster I See a vision? Look at that Beautiful. um I Love all the square traces and everything like that.

It looks like it hasn't been reworked. Really, it looks like it is, um all Factory solded down here. There's quite a bit of flux residue on the hand sold for the Transformer but that not uncommon even these days. We've got some uh guard traces around there, little guard rings around these.

Um, and they're those uh uh test points that we saw before through the main cover. So we'll whip that uh metal cover off later and we'll take a look. And on the front panel there there's our BCD to 7 segment Dcota a Um Cd4511 Absolute Classic. Uh, over here we have a National Semiconductor DS 75 492 and that is a um uh hex.

uh Moss display driver. so that's just uh, driving. um, the heavy current on the digits and there's that Crystal 100.00 Kertz I Love it! I Haven't seen one that big in a long time. So as I said, it's a real bummer that this thing's working I Was hoping to, uh, do a troubleshooting thing on this, but uh.

anyway, let's give it a go. Let's uh, measure these rails I don't know where a ground is, but uh, presumably I don't know the Can the can here or the can of the crystal. Let's give it a go. So our 5V rail there it is.

5 5.02 Not a problem -5 Yep, + 15. Not a problem. They're our three power rails. so this thing's just hunky dory.

Um I Have no idea why it wasn't working before. Um, it's really weird. Very strange and there's that rather unusual input can I Just took the Uh Shield off there and it just you know, they've gone to a lot of trouble to actually design that so it wedges in there and like just over the BNC and there it is. There's a single solid core cable going out there over into the shielded can.

Weird. Uh, why? they just didn't run that on the PCB with a ground Trace over it. I've got. uh, no idea, but but you know.

look they've done the zeros here like the there's a zero adjust pot and they've just got those running right over there. Don't know why they didn't do that with the Uh. With that, why they went to all that trouble. Hm.

Weird, Like you know, like you would have ordinarily just designed this input can, designed the circuitry, laid out the board so the input can like extended over this input. BNC and this zero pot here. That's how I would have designed it. Now, as you may seen before, it looks like we're not getting a zero reading on some of these ranges.

I Mean that's the Uh one nanoamp range there. so we're getting like five Pico amps there with no input and others is showing is showing zero. Not a problem, but we're getting five on that as well. So it looks like we're getting five on those alternate ranges there and we'll probably get five on this one based on that.

Not there we go. So that one, that. one, and that range, we're getting five. So um, and if we do the zero adjust button here, let's put that in zero adjust.

Let's tweak that down, shall we? Let's go down to the lowest range I Haven't read the manual, but it's okay. I've got my tongue at the right angle and give that a little tweak and we're down to zero there and let's try that again. There we go. Nice.

Okay, everything's fine. I say we just get my um key current Source out and uh, whack some current into this thing and see if it's spot on. Hopefully it won't be cuz then we'll have to go through the calibration procedure. All right.

What I've got is my Uh Keith Le 261 Pico amp current source so it's a perfect match for this thing. I've also got the uh uh, 225 current source. so uh, if we need to go to higher currents for the milliamp ranges, this can't uh do that. This starts from uh 10 to the minus5 so it starts from 10 microamps.

Um, full scale down to uh, you know, Fto amps. It can go well. this this knob is Dicky it's completely dodgy. Ah, there we go n ruined.

Got to really tighten. Maybe flatten the shaft out on this thing and do that. but uh. anyway, trust me, this thing does actually go down to a minimum of 10 the power of - 12 I 10 Pico amps full scale and you can adjust that because this shows the uh where the decimal point is.

Then we're talking about. um, you know, 10 Fto amps resolution on this thing. It's really quite good and more than good enough for um, testing the range of this thing, which is one, uh, Nano um amp full scale. Anyway, what I've got this thing on is the 10 Micro amp range.

So it's 10. microamps here and I've got it on the uh uh, 10 microamp range. here and there it is 9.99 You saw it before it was 10. So you know something's drifted somewhere a little bit.

but it's basically spot on and we should be able to tweak that. There we go. Look at that. just dial it in.

We're only one least significant digit out between these two units. Absolutely incredible. We can dial that up to exactly 10. There we go.

we can tweak it. It's hard to read that uh uh, lead display I think at least on the screen here. so I might have to up the current on that thing I might just change the resistor Network there or something like that just to make it a bit brighter. Maybe it's a bit washed out, but yeah, look at that.

I can just dial in that digit there. Fantastic. I Love it! So that's bloody spot on unfortunately and you can see it blinking over range there. and I just noticed something that I shouldn't be doing.

There's the Main's cord there right next to my Um lead. That's probably not the best idea. so let's get that completely away from there. And so what we're going to do now is we'll just try out the negative polarity.

I can just swap the leads of course or I can just use my negative switch here and bang. That's you know, we're only talking. It's changed by two least significant digits so it's basically spot on. Plusus one.

I Love it. Um, and of course we go down. We get a flashing over range there there and uh, so that is working. a treat.

So let's change this. uh, range down. and if we go up a range, there we go. 9.9 Not a problem.

we should get nine. yep or 10. I Was expecting there. So ah man, this thing spot on.

Not going to do any troubleshooting, not going to do any repair. Looks like we're not going to do any calibration either. Bummer. And if we go up to the 100 microamp range, yeah, that's as, uh, basically as high as we can.

Uh, go on this. um, uh. 261 current Source But look at that. I mean that's we're talking.

Look at that. That's just ridiculous. Down to the 1 micro amp range we're absolutely bang on. So let's keep going.

Excuse the crude adjustment here. What are we on? Now we're on the 100 nanoamp range. Bang on. Absolutely.

bang on. A That's just that's just filthy. That really is unbelievable. Ah, it's obscene.

There we go. and we're now down on the 10 nanoamp range. If you remember, there's all the ranges there there. they are one one nanoamp through to 1 milliamp.

haven't tested the 1 milliamp one yet I'll have to get my uh, other current Source out to do that. but there you go. we're bang on. that's 10 nanoamp range.

Not a problem and there we go. We're down to well one Nano amp and we're picking up some noise here. you can see it's jumping around. I mean what I've got here is I've just got a shielded B See, you can see it.

You can see it changing as I play around with that. I mean we're right down in the noise here. Um, if you want to do uh, good low current measurements, keithly have the um I think it's called the low Current measurement handbook or low. You know, something like that.

um I'll link it in here and it is one of the industry standard reads on uh um, current measurement like this I mean you know I've done a little bit of twisting the wire there just to keep it low. but really, you know I mean um, this probably isn't going to cut it as you can see and if we go back up a range to 10 nanoamps there, you can see that it's basically spot on. and then if I start to handle that, you can see it starting to kick in there. So really getting Tri electric effects in the cable and all sorts of stuff there? so really, you don't want to touch it? Hands off, keep it as short.

There's a whole art to uh doing this. Keep it, you know, double shielded boxes and all sorts of um, weird and wonderful techniques which uh, will no doubt be in the Keithly uh, measurement handbook. So check that out. But if we go right down to the lowest range there, then we adjust that it is it is bang on if I don't touch that cable at all.

If I get anywhere near that cable bang, it's just going all around the place. But this thing seems to be in perfect calibration. It's absolutely bang on. um I'd be a fool to even attempt to, uh, touch this thing.

Let's go to switch it to negative mode. so we're getting minus one Nano amp. Ah, near enough. Not going to complain about that.

I'd really have to uh, probe it all properly and spend hours around to try and get that right. But there you go. positive and negative. and if we switch up range there, it's bang on one negative and positive.

Brilliant bang on one nanoamp. Do you believe it? Look at that. I can just dial that in and I can probably dial this one in if I don't get my hands near it. Look at that! Great stuff! And I have to test out the milliamp range.

So I've got my other Keithly current Source here, which is a 225 which covers a uh, larger range and doesn't go as low. so it goes all the way from 99.9 milliamps here. So 100 milliamps basically all the way down to 99.9 nanoamp. So this one has a resolution of 100 Pico amp.

so not nearly as low as the Uh current Source we had before. So with these two instruments, I can cover practically. Um, and with my other power supplies I can cover practically um, everything from 10 Fto amps, um, all the way up to you know, amps crazy many, many orders of magnitude. And with this one, you can adjust the Um maximum output voltage anywhere from 10 volts right up to 100 volts.

And also it's got the Uh. it's got the positive and negative switch as well and it's also got an output filter. so let's hook this thing up all right. So I got this set to 1 milliamp here.

1.00 Milliamp. We've obviously got an extra digit over here and look at this. We are bang on folks. Absolutely.

Bang Hang on I Love it So we can dial in. Look at that. that's pornographic. Really.

We can just dial in those digits and it matches precisely. Fantastic. So if we switch that down to micro range once again, we're bang on. So these two units match.

I mean I Obviously keep them in cow to calibrate my microcurrents. Oh there we go. Couple of least significant digits out there. Whoop-de-doo folks.

and of course, um, this thing and we're not even near full scale on this thing where the accuracy is the Uh is is the best on this thing. We're right down at 1.0 Z So let's go down there. we go a a couple of least significant digits out there. Oh what a bummer, huh? So 9 99.9 Let's wind the wick up.

There we go: 99.9 Ah That's obscene and let's just try the negative there. Switch it around and we almost forgot to have a look under the metal can there. but look at this beautiful uh Point too. hand soldering with the Uh turrets there completely surrounded by the ground plane on top there.

Beautiful. so they've just gone completely point to point. We got uh, metal cans here. Beautiful.

but by far the most interesting thing in this is look how they're doing the range switching here. I mean I I Don't know if these Um switches are actually connected up to anything. looks like there are some traces going down there, but look at that. These things push on these gold uh Leaf contacts here which then push on that gold pin like that.

It's just beautiful. I They've gone to a lot of effort there to switch to ensure that they um switch very low noise. they just switching part of the circuit there. It's interesting though, that they don't do it to the lowest two ranges down here.

There's none of that switching at all on those lowest two ranges where you think it would be, um, absolutely critical to uh, do that thing. So I have to look at the Uh schematic for that, but that is. that is just lovely. They've deemed that they have to go to that effort to get the Uh to get the signal.

Integrity The low noise on this thing instead of using, you know, the crummy switches inside these gang switches. I Mean, no matter how good you manufacture these switches, they're probably going to be pretty crusty. So they've gone for a beautiful, you know, um, very, probably a very heavily plated. Gold Leaf Um, contact there onto another heavily plated gold pin.

Beautiful. And let's just go for the money shot there. Ah, look at that. Oh could play with that all day.

And if we go and have a look at our schematic here. I'll link it in down below. And by the way, if you want to check out the Uh manual for this thing which has the full schematic and theory of operation and all sorts of stuff, it'll make a great bedtime reading. I'm sure.

But here we go. We've got some switching down here and that's the decimal point switching. so that's all the digital stuff. So that's what those Um Crummy Gang switches will be used for.

is just the digital part of the Uh decimal point switching of course. but all of that beautiful Um low noise Gold Leaf Contact There is all part of the feedback amplifier. and here's the feedback amplifier. here.

There's the Uh Zero adjust, uh pot there. it's Um, by the way, you would do this with a you know, a real Topspec Fet input Um Op amp these days, but they actually used a Uh Jfet Um input front end with matched Uh Jfets and incidentally, these two resistors. they got an asterisk next to them. They if you look at the notes, they're actually Um selected at the factory to match the Uh transistors there.

Here you go, you can see the Uh switch contacts in the feedback path there for the feedback resistors. There they are there and there. your gold leaf contacts and they have to be incredibly reliable. Incredibly low noise contacts when you're talking about an instrument of this caliber.

And if you noticed before, we had had four of those gold spring contacts. and there's the four contacts. And they've also got some of the other range switches here which is switching some more non-critical stuff. and you can see that the Uh 10 nanoamp range and the 100 nanoamp range there isn't actually Um Switched at all as we noticed Um inside the unit.

So we bring the unit back over here again. You'll notice that the there you go. We've got our four Spring Leaf contacts and the these and the two lowest ranges aren't switched in there at all with those gold leaf contacts because um, they are fixed across the feedback path and then the others are put in parallel. So as you can see, there's not much to these things.

It literally is just a feedback amplifier. and uh, if you used a modern Um Op amp in there I mean this thing was designed in, you know, uh, the late '70s uh, they just weren't around then so they had to, you know, hand match these jfed inputs. But you can just get a Um fed input Um Op Amp a really low bias current, you know, Precision Opamp these days just to do that. put a feedback resistor in there, some Um low pass Uh filtering caps on there, and Bobs your uncle and there's the output Um terminals on the output, banana jacks on the back panel so you can access directly the output of the feedback amplifier and that um, you know I won't go in the Fuor feedback Uh amps here I may have even done it uh, before.

but um, basically it converts current into voltage on the output with effectively only the Uh difference between the offset voltage between the inputs to the Op amp Um, which is your burden voltage on the input. So this is how you can get incredibly low burden voltage. Unlike my microcurrent one which just uses a traditional shunt resistor, this one is a feedback amplifier which works differently so you can get even lower burden voltage than my microcurrent. And by the way, just as an aside, if you're playing around with uh, very, uh, you know, uh, low current Precision circuits like this, just be careful how you are.

handle this or try to avoid handling them if at all possible. because your hands. If you you know if your hands aren't clean or even if you've just uh, washed them, they can still have um oils and stuff which can leave residues on critical parts of the circuit. cuz we're talking about, you know, 100 megga ohm resistors here.

that one's 99 Meg And you know, if you start getting in there and you get all sorts of dirt and residue and you know all sorts of other gunk in there, it can, um, you know, upset the calibration of this things. So just be careful. So of course it turns out I was wrong about these being um, access uh, points. It seemed a bit weird that they were very deep down in there.

They're just, um, little plastic holders to keep those posts in place. um so that that those contact switches don't bend. and Just for kicks. let's uh, make sure the burden voltage is less than the 200 microvolts claim.

So uh, what I've got is I'm feeding in 1 milliamp here and um I've got a banana um to uh BNC Jack here so that we can get in here with our meter and probe it now I'm going to use my Agilant u1272a cuz it's got the 50 molt uh range so it can. The resolution can go all the way down to 1 microvolt. Fantastic! And as you can see, it's pretty darn close to zero there. And let's get in there and probe this sucker.

Let me try and apply some pressure. You got to be careful here. But yeah, we're definitely under the 200 microvolts there. We're only about 110 microv volts.

love it! and just a double check on a lower range there. I've got the 100 nanoamp range there and uh, we're only about 80 odd microvolts. And because I'm sure there will be people who will ask how does it compare to my microcurrent here? Well here it is. I've got 100 microamps in there and we're getting 99.96% bang on Perfect! This should be 99.9 nanoamps there or 99.9 molts cuz it's 1 molt per nanoamp range on my microcurrent.

So there you go. Everything's well within spec and if you don't have a calibrated current Source like I do you can, uh, easily test this Um, in fact, this is probably the recommended uh method to um actually calibrate these because uh, you can easily get um High Precision voltage and resistance standards. So I've got my Um MV 106 DC voltage standard you've seen here before. way Overkill I mean the Keithly 480 P meter.

You know he only rated to like 0.5% and this sucker is a couple of orders of magnitude better than that. So um I've also got my resistance standard here, which you've um seen before which is basically just a Uh 50 PPM resistor in a box. I've got a 10K one and a 1K one. Um, usually you would uh use a much higher value than this for Um testing the lower current ranges, but this is the best.

Um, this is the best resistor I've got I've got larger resistance uh values, but they're you know, a few percent or something like that. they're certainly not Precision so we're talking about. you know, 0.005% Um, accurate resistor in a box. here.

you can buy those for about 20 bucks or something like that from Digi key. Yes, you can pay 20 bucks for one resistor, but it's a pretty darn Schmick one. So um, I've got it hooked up here. We're on the Uh 10vt range, but I'm outputting 1vt here 1vt on 10K we're going to get a 100.

Oh there it is. 100 microamps. We are absolutely bang on to the Le significant digit. Of course if I take that up to 10 volts on the Uh voltage standard here.

Oh, one least significant digit out at 10 milliamps there you go. And of course, if you, um, you know you've got to be careful what you're doing here. you got to take into account the burden voltage. We've already measured that 250 microvolts, so it's um, insignificant Here, it's actually 100 microvolts.

Uh, the spec is two 200 micro volts, so you know it's down in the noise here. Now you might think that we're um, simply able to, uh, reduce the voltage here, and um, measure the lower current ranges. but that's not really the case. You can see it's um, slightly out here.

So I've got uh, 10 molts there over my 10K which is going to be Uh 1 microamp there and you can see we're out. But I know it's not out. It's because the Um offset voltage now becomes a very significant proportion or the burden, uh, offset voltage in this thing becomes a very significant proportion of our Um of our you know of our calibration setup here. So that's why the manual for this thing will recommend minimum Um input impedances for this thing.

Uh, for Um for these various ranges. But what that essentially translates to is um, not necessarily a minimum input, uh Source impedance, but um, a minimum Um input voltage Essentially so that the burden voltage of this thing doesn't matter. Now, look, if we go even lower like we'll we'll go up one there. so we'll go up one and see it gets closer there as we go up.

So we're 10 microamps there, but we'll get further out as we go down. So let's drop that down even further. Zero and one there. Now we're even.

We're way out. Okay, we're just. you know we're We're just completely and utterly gone. If I put that to one one molt.

There we go. We're completely out. And if we take a look at this Dave CAD drawing here, we can see exactly what's going on here. We've got our MV 106 voltage standard generating our test voltage here.

We've got our 10K series resistor going into our feedback amplifier here. Now at the moment, let's just ignore the feedback resistance here and we've got that, uh, measured um, Vos or offset or burden voltage there of around 100 microvolts. Let's just you know, round it to 100 microvolts. it's going to change per range and all that sort of stuff.

But let's just take that as a value. So let's have the 1vt that we had before 1 Vols minus 100 microvolts / by 10K because that's what's um, flowing into this. um, this feedback amplifier here gives us 99.99 microamps and we were measuring. Bang on.

So you know the error is in the Vos error here is in significant. In this case where we had the 1 volt and we were generating 100 microamps is pretty darn close. But then if we drop our test voltage here from the MV 106 to 10 MTS then we can see our 100 m microvolt offset. Voltage becomes very significant.

You do the math here and it's 990 microamp. so we'll actually measure that and we should get roughly that figure. And then if we drop it even further, we're going to ridiculously low Um voltage here. 1 molt minus 100 microvolts.

Um, of course that's going to have a very significant Um error or a 10% error there of 90 microamp. So let's actually measure that. So let's go up here. and we've got it set to one volt here.

and we're getting our 100 microamps as we saw before. Spot on. because um, in theory, we should actually expect 99.99 microamps, but because, um, that value is one, uh digit. Um, better than the resolution we've got here.

Eh, it's You know it's insignificant, especially when you consider the accuracy of this thing so, or the intended accuracy of this thing. So it's insignificant. But if we wind that down to 10 MTS here I'm on the 100 molt range change we're generating 10 ms, you'll notice that we were expecting what were we expecting before. We were expecting 990 Micr.

And there you go. We're getting reasonably close to that and but our error is going to get um, significantly larger as you'll see in a second Now and if we switch down to our 10 molt range, we'll generate 1 molt. There you go, we're getting that 90 microamps, which we expect, but aha, well, you know, reasonably close to within a ballark. But let's switch down this range and see what happens.

100 nanoamps. Look at this. We're measuring 20 like that. so our error is very hugely significant.

You know it's it's almost now. Pointless it. It just reads gibberish. Now, why is it doing that? And here's the answer: I've added an additional Dave CAD drawing here with a formula which now becomes very, very significant based on our source resistance.

RS So I basically relabeled the 10K resistor RS that's our source resistance. Um, our feedback resistor here is RFB for feedback and I've redrawn the Uh Vos as a voltage source here, which is a better which is a more common representation of it. but it's the same thing. It's that 100 microvolts, but that 100 microvolts we measured way before is not a fixed value.

It's actually multiplied by this term here, which is RFB plus RS / RS. So let's take uh, the example of the Um: the 100 microamp range that's got a 10K feedback resistor. What happens if you plug 10K and 10K into this formula here? this term here becomes a value of two. So the V at 100 microvolts gets multiplied by two.

and then if you change the range again, let's say you jump to the 100 nanoamp range, then RFB The feedback resistor is actually a 1 Mega resistor and you can look up these values on the schematic for yourself and I recommend you go do that. Um, have the schematic here as you follow along in fact, So then the term becomes huge and that V just goes completely out the window. So that's why we're reading absolute gibberish as we go as we switch down those ranges CU As we switch down the ranges to you know, 100 nanoamp, 10 nanoamp, 1 nanoamp, this RFB gets much much larger and this term becomes much much larger and Vos just goes out the window. and we're only got a 1 Molt Source here and Vos is way bigger than that.

Well, in theory, and you just read absolute gibberish, it just doesn't work. That's why if you read the manual for this thing, it will specify a minimum RS or Source resistance value here. Uh, based on whatever range it is you're measuring. Now the manual actually says for a 10K Source resistance here, which is what we're using ignoring the source resistance of the MV 106 for a minute, then the lowest range we can use is the 10 microamp range.

If we go any lower than that, it just you know the error term becomes too significant and if you really want to go into it and you can read the Uh manual for this, there's an additional voltage source in here which is the VN which is the noise source which is going to depend on your series capacitance as well as well as your feedback capacitance in here. like this, and all sorts of stuff like that and it starts to become very, very complicated with lots of traps for young players. If you're measuring um, very low values of current like this, um, there's a real art to measuring this sort of stuff and knowing where all your error terms and things like that are. So I won't go into details on that.

It's in the Um, some of it's in the Um U some of it's in the the manual for this thing if you want to read it, it's very interesting and of course our simple little dumbass. um, you know, like I've got just wires just hanging loose over here? It's you know, it's pretty pathetic actually. So um, you know this isn't the way to do it. As I said before, you've really got to um, uh, you know, uh, do like have uh Jewel shielded uh, boxes and shielded leads and you know all sorts of you know, great quality contacts and stuff like that if you're really going down to, uh, very low levels of current like in the order of under 100.

Nan Once you get under, you know that sort of micro amp figure, sort of. Those sort of things start becoming quite significant and you've really got to know what you're doing. Maybe I'll do another video on that. You know all that sort of stuff of really accurately measuring, uh, low value resistances.

But yeah, you've got to have Um Precision high value resistors in, you know, double shielded boxes and they've got to be isolated with minimum amounts of capacitance and all sorts of stuff can really get tricky. Quite a fascinating topic though. And here's what I'm talking about in terms of the uh, double shielded Uh test fixture here. Now, you can see that the Um outer case here is actually connected to the Um Earth.

That's you can see the Earth uh symbol there. It's connected to the Earth of the Um DC voltage calibrator over here. And of course we've got our low and high in our sense lines in internal you You might also um Shield this circuitry internally from The Shield which is non- Mains referenced to the KE Le 480 over here. So you'd have the internal Precision resistors.

They recommend 10K 10, Meg and 100 Megan based on those three and the individual test voltage over here. you can generate all the required Uh currents, but that's what you would do. You would put this inside an earth shielded box over here and you'll notice that it's only connected to to Main's Earth over here because the Keithly is not Main's Earth referenced on the input and then the internal ground. You might Shield that internally as well.

If you're going really low, probably not need double shielding probably not needed in this particular case, but if you had another instrument that was going even lower than this one, then that would be important. So there you have it. I Hope you enjoyed that little uh, tear down and uh little look at uh, calibration checking this uh, very nice. Keithly 480 Pico Amer and if you can pick up one of these puppies I Highly recommend it.

Um, you know I wouldn't pay more than you know 100 uh bucks for one? uh for sure. but they're a really nice bit of kit for measuring low currents and uh, it'll be a nice addition to the lab here. I think and it was bang on I can't believe it. so yeah, sorry about that I Still don't know what was initially wrong with this thing cuz it definitely was not working when I first plugged it in.

it wasn't working at home when I first got it and then I uh bought it to the lab here and it didn't work either. but all of a sudden bang. so I don't know maybe there was a Dicky contact in the uh switch or uh, something like that and after a couple of uh goes, it just uh, self cleaned or something like that. That is the only thing I can think of.

so yeah, sorry about that. I was hoping to get a troubleshooting and repair video and ah, Murphy gets you every time you hope for a fire and you don't bloody well get one. even in um, the fire of the uh uh, circuit itself, the unit itself, or the calibration. I Was hoping maybe we could, uh, you know, tweak a few more pots and actually go through the calibration procedure.

but it's bang on so certainly not going to touch it. So anyway, if you want to discuss it, jump on over to the Eev blog forum and if you like it, please give it a big thumbs up. Catch you next time.