Hi, let's check out this amazing bit of kit I've had sitting in my bunker for like ever. This is a Keithley 515a Mega M bridge. It's a wheat stone bridge and you might have heard me mention uh, that term in the previous video that I did linkedin up here. If you haven't seen it where we resolve that, uh, resistor cube problem and check this out.

this is just an amazing bit of kit. It dates from uh, the late 1960s, early 1970s. I'm not sure when this one was actually manufactured, but look at this. We've got not six, but seven decades here with a range multiplier for measuring high resistance values I.e in the mega Ohm region and gig Ohms and Terra Ohm region.

Even now, wheat stone bridges were very common before, like modern multimeters were available and you could actually measure um quite precisely, capacitors, inductors, and resistance values. You can get Lcr breaches. This one is a resistance bridge only, and it's only designed for high values like you can probably do down in like the hundreds of kilo Ohms range. but it's really designed for like high amp.

I mean, check out this. This is ten to the power of five, so that's a hundred K range. Then you got your one mega range, ten to the power of six, and then you got ten to the power of nine which is your one gig range. and then you've got ten to the power of 12 which is, uh, Terra Ohms range.

And then you've got a seven decade resistance box here. Uh, which basically lets you, um, figure out and match using the Wheatstone Bridge method which will go into and explain, then match your device under test. And because we're dealing with really high value resistors. That's why it has this shielded drop down door here with a tri-axial and Bnc connector down in there and if you've seen my video, I'll link it in.

Uh, if you haven't seen it, it's really good on the Keithley Electrometer and how triaxial connectors work and how the art of measuring extremely low currents works. and that's what you need for something like this. You really need this shielded box tri-axial connectors which have uh, ground and guard connections in there as well. because it when you're dealing with extremely large resistors like Gig Ohms and Terra Ohms, then uh, you need like a really properly shielded and properly designed uh test fixture and this has it built in and it's got calibration modes and there is the null meter for you null meter fanboys I know you're out there and then you can actually uh, set the null range and stuff and you can set the bridge voltage.

It can do internal or external voltages up to a thousand volts and there's your specs of this bad boy. I mean, they call it standard deviation, but uh, it's basically specification from uh, basically 0.01 percent. Even right up to the tear, Ohms range is still talking one and a half percent. Not too shabby, huh? But as you can see on the uh hi, once you get to the really extreme you know hundreds, you know, tens to hundreds of gig ohm range.

And the Terra Ohm range you really need do need. Uh, the high voltage high bridge voltage in order to get actual sufficient current that you can actually measure. Now I don't think anyone would actually be using something like this anymore because it is like 50 years old. Although there are still uses for wheat stone bridges not only in like strain gauges and other measurement topology like that, but also in high precision metrology applications.

And this is where you would have found this bitter kit. you wouldn't have had it in your ordinary lab. I think this one did actually come from a metrology lab. A measurement lab.

All right. let's see, this bad boy still works power, internal boing and our meters gone all the way. No magic smoke and this is a bit more complicated than your, uh, traditional Lcr uh, bridge. So uh yeah, let's go.

Zero check and well, there we go. So we can caught. can we? Yeah, yeah, we can course adjust that. There you go.

Yeah, so I can. I can zero that so that's working. So yeah, and we dial that in. oh closer, closer, closer so we get near zero.

All right there we go. there we go. hold the tongue at the right angle, don't want any of that parallax error rubbish. And uh, there you go.

Where's your uncle? We have the instructions here as a Star Wars crawl. Um, there you go. So yeah, it helps to, uh, read the instructions. but hey, I did actually do.

um, step two there. So let's go for step three Standardizing. So set function switch to standardize and then multiplier to 10 to the six and then set it to exactly 10 point. which is what that is.

Um, that's sorry to assume that's supposed to light up. But yeah, there we go. 10.00 and we'll set that to read position and we need to bring it to exactly null on here on our standardized function. So yep, yep, Yep, there we go.

So it's somewhere in there. There we go. So it's somewhere like that. There we go.

Oh, that's bang on. Oh yeah, beauty. Ah, you're gonna get some parallax error, but trust me that is. bang on.

And then we have to set it to zero, check and then over to calibrate like this. and we adjust the 10 to 6 calibration potentiometer which is down here so I'll put that back to read and we need to. All right. Whoa.

that's that's. a bit twitchy. That's twitchy. Definitely tongue at the right angle for this back a little bit.

Whoa. Jeez, that's tough. Oh no, I took pressure off it. Ah, that's going to have to be good enough for Australia.

Now ordinarily we'd have to go through and calibrate all the different ranges here going all the way up. But um, we're only going to measure on the uh, mega ohm range that we just calibrated. So yeah, I'm just not going to go through and bother do the rest next. So we're going to, uh, try and measure this whirlwind.

200 meg. That's not 200 milli. it's a 10. Um, a high voltage.

Um, a high ohmic resistor. So 200 meg. Let's um, see if we can dial this in. Okay, so I just adjusted the ground point over here.

These are all the same. Uh, ground point? This is the input and this is an external tri-axial input. So we just used a regular input here and uh, see if we can do that. So I'll close that up and there is a micro switch down here so that will actually now enable it.

So let's measure this sucker. So let's put it back to operate like this. I'm going to have 10 volts here for our Wheatstone voltage and whoop. and we're over.

Okay, so now we have to dial in. Okay, what? We are near to where we what we think it is 200 meg. So we're on the Megome range. so this is times 100 so we have to go to 200 there.

Like that, I'll dial up the sensitivity a bit more like that and so right. So we dial in exactly 200 what we think it is and it's on this side. so that's actually on the low side. So we have to increase it a bit and you'll notice that we can bring that into the middle so it's somewhere you notice it's gone to the i can turn the increase the sensitivity so you can see how it goes either side of the center there.

So that means it's somewhere between 230 meg and 220 meg. So what we'll do is, we'll just increase that 220 ah, 225 meg or thereabouts. And you know the others. we can dial in as well.

But really, uh, you know you're getting down to sort of, oh, oh, is it drifted a little bit? Well, 224 Meg. something like that on maximum sensitivity. Anyway, there you go. So the other ones really aren't relevant here.

We could probably go up in voltage and we can get some extra range here. So let's actually go all the way up to 100 volts, shall we? And at 100 volts, it's actually, uh, quite quite different. It's oh, it's somewhere between 200 210 point tongue at the right angle 210.4 or thereabouts. There you go.

So that's so the higher up in the voltage we go, the more uh, current that we can get. Well, actually, I can go up a precision here. and yeah, I wasn't even on maximum sensitivity. There we go.

Oh yeah, nah, it's it's 210.3 mega ohms. And that's how you measure using a bridge. So we've got actual resistors in here into the actual value of 210.3 meg ohms. and we can dial that in even further.

Um, and then if our device under test on the other side of the Wheatstone Bridge is exactly the same value, it knows out the current like that. If it's either, if it's you know more or less it goes either side. Neat huh? Back in the old days of multimeters, this is how you actually measured resistances. So anyway, there you have it.

and it does have some satisfying relays in there. Listen to this clunk. I love them in the center zero needle. It's great.

And I can actually measure this here in the lab using my, uh, modern Keithley seven and a half digit meter here. I don't know the accuracy of this, uh for this particular range, but it's showing 233. Uh, Meg Ohms there. And yeah, like I'm keeping short leads here because if you put long leads on this and put your hands near it, you're going to come and gutsa.

But there you go. 233 So I don't know which one's what. Uh, you know, maybe could have calibrated the um, other one a bit better. Maybe he's drifted? I don't know.

Look, it's ancient right? But as you can see, we dialed in and you remember it also changed with voltage. It was actually at 10 volts, it was like 220 odd meg. Then it dropped down to about 210 meg at 100 volts. So you know this is measuring much lower than 10..

But there you go. That's what we get in. I mean, you know you can do like all these digits are just Bs, right? They're just just there for show. Really, it's nowhere near the accuracy that matches that resolution.

But there you go. So we'll do a brief look at how Wheat Stone bridges work on the whiteboard here. Now, it was actually invented by Samuel Christie here, but uh, you know, Wheat Stone took it and developed it further. and he's the one who got all the glory.

anyway. Charles Wheatstone. Um, so it's a bridge. uh, configuration.

which of course you'll know from like a diode bridge. Uh, configuration. And it's basically just four resistors like this. This is all it is with a voltmeter in the center, or an electrometer in this case, because we're measuring extremely low currents with this Keithley device, but basically a voltmeter in the middle.

So I've got a Dc voltage source here and you can see that we've basically got two resistor dividers, one here. So this is one point on the divider, and here's the other point over here. and you can see that by inspection as we've talked about in previous videos. If the ratio of this divider here is the same as the ratio of this divider here, then the voltage difference between these two points is precisely zero.

They're equipotential nodes, as I've mentioned in that resistor cube video. So our device under test our dut is this resistor here, which we put on our terminals and then on the other branch over here we've got our adjustable decade resistors. So this is a Six decade jobby and yeah, put your tongue at the right angle and you twiddle these until you actually read off exactly the same value that matches this. and of course these values up here these will be your range resistors and you can actually uh so this will be like have a switch in there with multiple ranges and this one here can have like little adjustable trimmer in there as well to sort of like tweak all the ranges and that they were the calibration controls that we saw on the Uh front panel there.

So once you've actually uh, calibrated this bridge and you can do that very precisely, then you can actually read off the dials assuming that these are very precise resistors, but also you can trim these as well. You can calibrate them as accurately as you want to. Then you can just dial off and read your device under test. When you when that meter gets to zero like that, it means there's zero voltage difference between here.

And the great thing about the Wheatstone Bridge is that you can measure incredibly accurate stuff. And the great thing about the bridge configuration as opposed to just like a single voltage divider, you could do a voltage divider, and just read off this node here relative to ground point here and with a you know, a precision volt meter and stuff like that. but the bridge configuration allows you to get much greater accuracy. and as I said in this particular case, this is actually an electrometer in here.

So you'll have an amplifier like this and you can have different range resistors also in here like this and you can select uh, that was that uh, sensitivity adjustment that we were playing around with. So we've basically got an amplifier with an electrometer in there so you can even measure more precise values close down to zero volts. And you can use the Wheatstone Bridge configuration for not only measuring resistors, but as I showed before, inductors, capacitors, and you can put other devices in here like light dependent resistors. You can measure you know light intensity and but one of the big applications these days for wheat stone bridges.

They don't really use them except for really. As I mentioned, really precise metrology, uh, measurements. uh these days. But one of the common uses for this is in strain gauges.

uh, like inside a load cell or something like that that measures uh, like tension or uh, like weight scales and stuff like that. And they use the bridge configuration because you can get extreme accuracy instead of just using one branch. So instead of using just one arm like this, you use two arms and then you can actually measure the difference. when like a strain gauge.

for example, you might put a flat resistive strain gauge on like a metal bar, and then you can detect when it's actually bending. and then you can get a voltage measure the voltage difference out of here. So you're typically in, like a weight scale or a strain gauge. You'll actually put a very precise amplifier in here, and then you can sample that digitally and that zero check function we're playing around with on the front panel.

that's just simply a switch which shorts out these two terminals so you can trim it to measure precisely zero right in the center. And just as an aside, because we're measuring mega Ohms and gig ohms and Tera Ohms here, we're using an electrometer. Very small currents we're talking about. Then the grounding matters.

So here's the diagram four that's actually used in the Keithley manual. and you'll see that there's basically a shield around here. There's a guard shield like this, and it includes the voltmeter. Like this includes the electrometer.

so all of that is shielded. It doesn't matter. All this on this other branch over here. it can be outside the shield, but all of this uh, high impedance node stuff.

You really want that inside a guarded shield. So that's why you use a three terminal tri-axial connector. So you might have like chassis mains earth out here like this. but inside.

this is actually our grounded guard terminal and that eliminates leakage and other interference issues from your precise electrometer here. And you've seen that in my Keithley Pico Ami to tear down as well. I'll link that one in too. But anyway, Wheatstone Bridges, They're very cool devices and they're still used today, even though not really for any mainstream measurement.

As I said, really precise metrology stuff. You can get incredible precision with these things, but you've got to take the time to set them up, calibrate them, do everything else. But once you do that, yeah, you can get way better than you know, almost any modern instrumentation. So let's take this bad boy apart and see what's inside.

for those playing along at home. Made in the United States of America, Cleveland, Ohio and I actually checked the address in the original manual From this from like 1970 is still the Keithley address. Today they're still in the same building. I wonder if it's still the same phone number and everything.

Anyway, what we've got in the back, We've got the external input. That's that. uh, you know if you want to feed up to a thousand volts because it's only got 100 volts internal. It's an accessory outlet so you can power other um, main stuff with it and a good old-fashioned switching? uh rubbish.

Got a good old-fashioned transformer with uh voltage selection and you might think uh, given the vintage of this uh designed in 68 and first like sold in like early 1970s, you might think that this is like valve. Um, you know it might have some vowels in it, but no, this is the fancy pantsy modern A model uh, which is all transistor. They did have the non-a version before this, uh, which dates back to i think around 1960, 1961. and yeah, that used valves or Jfets with pilot lights as they're affectionately known.

But uh yeah, this will be all, uh, all modern transistor stuff. One thing I'm expecting in this is a lot of space. I mean, how many rack unit high is this? Have you ever seen anything that big? Um, it's just absolutely enormous. But yeah.

anyway, oh no, no, we're gonna have to break. gonna have to break the cow seal. Oh, it's a Greek tragedy. All right, let's have a squeeze in here and ta-dah oh so high.

My cameras are not up high enough, but there you go. Oh look at that. Yep, it's mostly empty space. Hi.

So just check out the beautiful wire looming inside this thing. Very nice indeed. And of course anything that needs to is shielded. There's very careful guard stuff, but look, check out this that's actually an open air high voltage relay and you can see the coax going off the bottom here.

This actually goes off to, uh, the high voltage, um external uh connector on the rear of it. So yeah, they've just like stuck it in the open there. I had to get my weird ass Yankee um, Allen key set out so that, uh, yeah, this is like a 5 fourths whatever that means. All right, but it fits.

Oh, there's the back of it, isn't it? Beautiful. I should have actually done this teardown in 4k. but uh, yeah, sorry about that. Too late now really.

And there's your inputs down here. and check out how they've just got it flapping around in the breeze. Here Here is the input here and it's going up. like just completely out in the open like this.

But of course it's in a shielded box, so no worries. But they've kept it away from absolutely everything else goes up into this shielded box up the top here. and then it's just incredible. And here's the tri-axial input here.

You'll notice that the input, of course goes over to the duplicate input over here, but then this resistor here connects the ground of this over to the center guard of the external connection like this. and then of course, this is uh, connected. and in the main part of the Uh tri-axial connector is connected to the Uh Mains ground chassis and then the ground over here, the mains earth. that actually goes over to the terminals.

Uh, all the way along here, you can see just the bus bar connecting all those. So we were actually using a single-ended earth measurement before, but if we wanted to do higher values that are more critical, we could have actually used the tri-axial connector with its proper guard connection. And they all run back, including the guard connection into the wiring loom. But because it's all inside, like you know, you don't need to shield any of this because it's inside a shielded box.

There's our trimmer, uh, calibration pots on the front, and uh, our range resistors. And anyway, these, oh god, I'm going to lift the tripod up. These wires come up here and they bugger off. They bugger off.

They bugger off. Right along here. They go along this arm and then they go into finally, the back of this shielded box that we've got here. So this is all of our measurement stuff.

Uh, inside this box, here, and then down here. Uh, there's our mains power supply. We'll have a quick look. Well, nope.

I thought a mains power supply would be in there. But no, it's just the voltage selection. So no, there's the mains power supply hidden away in plain sight there on the top. Because this is so tall, it's hard to like see inside this thing.

So anyway, look at that. isn't that. that just looks beautiful, doesn't it? And Mallory, oh all the Mallory fanboys go wild. Look at that cap.

It's a beauty. Still works after all this time. Of course it does. Made in the Usa, that is a very nice old-school power supply, isn't it? I really like it.

We've got some Rca jobbies down here. Look at those Oh Bobby Dazzlers. No day code on them though. Uh, not that plastic package.

Rubbish. Metal can all the way. That's our main transformer. It looks like like a modern switching or a semi-modern switching transformer.

but it's not. This is a purely linear jobby and it does look like it is two-stage because this can go up to 100 volts. This is probably like given the high volt out. We've got another bridge in here, so we've got one bridge over there.

another one here which is a secondary one. Um, for like the lower voltage stuff over here. That's a few wafer switch aficionados. There you go, they still looking really good nick after all these years.

we still don't know how many years that is though. But uh yeah. Anyway, these are all the uh decade range resistors. So this is where all your precision resistors are.

There's your switches and resistors for your uh voltage range and as you can see, they're only like one percent Job is. but you know, 4.02 K there? you know they they dialed that one in. Ha. I'm here all week.

Check it out. On the outside of this box. look at this. They've got a uh reel.

This is an external the coil they wanted for the relay they wanted to keep outside of the box so it doesn't interfere with any of the stuff in size. Nice attention to detail and that would basically be a read switch in the center of that. and then that's just a coil around the outside to activate the reed switch inside. Check out the axial cap in there with its own bracket.

Beautiful! So there's all our decade resistors with the highest values up there. You can see them. They're the Uh glass tube ones. and then they've got a Daven brand.

I don't think I've ever heard of Daven Precision resistors. Anyway, these are 0.01 percent Jobbies and they get smaller and smaller. smaller as they go up. But uh, yeah, that's where all the precision and once you get to the other end.

Um, these ones are only like, uh, one percent Jobbies. They don't have to be anything special because they're so far down the chain that, uh, you really don't need the high tolerance. You really need the high tolerance on the upper end resistors. You can really see those glass, uh, encapsulated higher value resistors that we've got on our top uh, x 100 decade there.

Very similar to the whirlwind one that we actually uh tested. But yeah, they're very nice. I mean, you know, like if you didn't want to use this thing anymore, you would gut them. for the precision resistors, they'd still be good.

Uh, you know. In fact, the stability probably goes up with age. Uh, you know, they're probably still good. Like 45 50 years later.

One thing I really love this is actually the potentiometer. Um, that we're adjusting the null adjust potentiometer. Um, and you'll see maybe down in there the um, how the shaft it's got. It's just an irregular like slot cut into it.

and then they've got the little um, shafty thing on there. Let's see if I can rotate that. Check it out. They've actually got a pin which goes in there and just rotates that from.

So they've got the wafer switch on the bottom and then it just got the center goes through. That's that's beautiful. Ah, thing of beauty. It's a joy forever.

Look inside our can here. These are our range selection resistors. Look at that. Oh, you've got the uh, high value glass tube jobbies that is just gorgeous isn't it? But you can see that they're only like one percent.

Our job is they don't need to be hugely accurate. It's the decade switches which are the ones that have to be really accurate. But uh, jeez. Look at this.

look. It's just. it's just beautiful. Look at.

it's all interconnected. Look at this with penetrators. Those white things look like penetrators, but there's not actually anything connected to them, so that's interesting. Although they do penetrate, there's nothing connected to the top side.

They're just using those as as connection points for the resistors on there. But um, yeah. look all of our proper star ground in everything else, right? or star guarding. This would probably be the guard most likely go back to the guard terminal and then they've got that and that goes off to Uh down here, which is our function selection switch.

So that's where we were selecting uh, the operate mode, the calibrate mode and whatnot. and there's our read switch in there. look beautiful. They've put some tape on that, I'm not sure exactly why, but um, yeah.

Anyway, they're able to get the They because the coil that coils on the other side here. you can just see it and they get uh, the magnetization to come through on these screws by the looks of it. and that's what activates the reed switch internally so that they don't have to bring so that they i don't have to have that magnetic coil inside the box. and uh, B they don't have to have the read switch actually penetrating going outside the box because it's obviously a high impedance node.

and well, it's it's it does. look guard switching. Actually, I'm not sure if this is actually the guard terminal. whether or not this is actually the measure could be the measurement terminal.

I'm not sure. Anyway, you'd have to look at the schematic diagram linked down below. by the way. Full service manual, all the parts, all the schematics, everything like they used to.

You'll notice that there's hardly any circuitry in here at all. There's a couple of, uh, discrete transistors there on the teflon standoffs, but you can see these penetrators here and these screws hold on a can that's actually on the back side of the case. and that's where the like amplifier circuitry must be, because it's not inside here. So linking the service, menu and schematic for this down below and I'll whack it up here.

Uh, briefly. you can see that there's actually not much in this. It's just a wheat stone bridge. you know, a high voltage source.

um, and that's you know, and some amplifier stuff to get your, uh, you know, your null uh ranges and stuff like that. But apart from that, um, it's incredibly simple. and as you can see, it's mostly empty space, so they certainly didn't need to make it uh, that big. but it was made for a particular market.

As I said, this is not designed to be like a desktop instrument or anything like that, but there's really a lot of art that's gone into this, especially in terms of, like, like, we could really spend hours looking at how all the gardening system works and uh, stuff like that. and just the layout of the wiring and the uh, shielding and the guardian to prevent leakage and all sorts of other stuff interfering with incredibly low current measurements. which is what you get when you try to measure, you know, Meg? Ohms gig. Ohms terror Ohm value resistors and you take it for granted these days.

But you know, back then? um, this was pretty much the only way to do it. So that's a very unusual old-school bitter kit and I hope you really like that. Um, I'll put some high-res photos over on my Eevblog flickr account. They're linked in down below if you want to see a bit more uh, detail and zoom in and stuff.

but this is absolutely fascinating. Had this one sitting around for a while. I'm glad I finally got around to it. It's a thing of beauty joy forever.

If you liked it, and if you liked it, give it a big thumbs up. And as always, discuss down below. catch you next time.