Hi. This is just going to be a quick follow-up video to my previous one on the HP 3457a multimeter and you notice that I mentioned the 3478a multimeter that I was replacing with the 34588 and I I Just realized I thought I had done a video tearing down uh, this 34788 but I hadn't So I thought I'd just uh uh, take the covers off and give you a quick look around and see how very similar it is to the 34 Uh 57a And really, um, the performance of this thing is almost identical. Not too far off the Uh 3457 A. So if you don't need the uh, fancy math, uh, functions or anything like that, this is an excellent and Oh, and if you don't need the extra Um, six and a half extra digit resolution, the 6 and 1 half digits and this is an excellent 5 1/2 digigit multimeter and you can pick them up for quite, uh, reasonable prices if you look around in pretty good condition, so highly recommend it.

Uh as a Uh Precision 5 1/2 digigit multimeters, but just a brief look at the back here. it's got almost identical Uh inputs to the front, the four wire switchable. You can switch the terminals from the front to the rear, which can be quite handy. Doesn't have the current on the rear terminal, but certainly does have the two and four wire Ohms input.

It's got Gpib interface of course you can set the address and this one is not Auto line frequency sensing like the Uh 3457a which automatically when it Powers up detects the line frequency and uh uh takes care of that. This one, you have to actually switch it between 50 or 60 Herz there standard IC Mains input external trigger out and the case comes off this thing quite easily. There's just two screws at back plus Uh one on the bottom and it just slides off and single board construction basically apart from the switching board here to switch the terminals from the front to the back, but it's a a very similar Uh design be this is a previous design to the 3457a. it actually uh predates it, but you can see the similarities between the two and they use lots of common parts as well as we'll see.

And it's got a nice Uh extension bar up here for the main switch and there's the main switch tucked in there very nicely. uh, heat shrunk down in there. They've got just a Uh regulator, a linear I Believe it's a linear regulator, it's got a HP part number on it. just put some wires over to here like this that would be the regulator that's powering all of this uh, digital stuff all around here.

the main processor. there's the backup battery which we'll talk about and uh, they're just using that for heat sinking on the side. I mean I Don't think this thing uh uses much power particularly so that's why they can get away with the linear Regulators inside this which are of course much quieter than uh having switching regulators and that's why they need less shielding and stuff like that inside. But there's our Main's Transformer there it is fused on the back down there.

We got our Gpib cable going across and they basically split this thing into the Uh just like they did in the 3457a. They split it into the digital or the Uh processing and display section up here and then electrically isolated from the analog or measurement part down in the bottom. here. you should be able to see that split pretty clearly from I'll show you more detail in here, but it basically splits around like that.

There's this uh Shield here, which you take a look at and you can see that there's no traces and the ground planes are separated between these two halves here. These are obviously our two Opto couplers, so it's got that single serial line in and out actually connecting the two sections. just like the 34 uh 57a. And there you go.

And this section of course has its own Uh Power its own uh Transformer tap on there. completely isolated goes into some linear voltage Regulators here plus - 12 or 15 volts or something like that and a 5V regulator as well Powers All this circuitry in the measurement section and you can see the two Uh Opto isolators there. They've got the HP part number I won't bother looking it up. Some people in The Forum have posted uh, uh links to, uh, cross references of all the part numbers to their real things, but they'll be just, you know, like 4N 25s or some sort of similar, uh type of coupler.

It is an 8 Pin uh dip package though. Now curiously, the service manual I've got for the 3478a um, doesn't show these two devices. It actually shows uh two Transformers instead actually. um, you know Transformer coupling, uh, the serial signals from one side to the other.

But that's not what's going on here. So there's um, clearly some design differences between these units. So yeah, if you got any any idea where you can clarify that, please let me know. And yes, lots of those HP part numbers all the way through this.

Probably half the chips in here have HP part numbers on them. Now you'll notice in here that there's another actuator arm going up there like that, which uh, mounts on goes to this Riser board which has presumably a very high quality uh, low, EMF uh. switch up here, which basically switches all four of the terminals from the rear. They go along the bottom of the case down in there.

they're not Shield at all. They don't really need to be because there's not a lot of, you know, really high frequency, uh switching stuff. really just spewing out noise in this thing so it's you. Know it's not going to be too bad at all, and more than good enough obviously for the performance of this fairly Precision 5 and 1/2 digit meter.

but there's the four terminals on the back there and they just switch those through either. The front terminals are also wide through, similarly front and the back. Very simple, but it works and you'll notice the identical Uh switchin input switch in hybrid here uh, that's used in its big brother we saw in the previous video and there's lots of similarities. Of course, these uh Cotto high quality Cotto relays down in here for the Uh switch in.

there's not nearly as many of them as there in is in uh, its big brother, so uh, presumably less uh, self testing and stuff like that. but it still does have self test and switching capabilities. So really high quality relays, High quality Parts As far as the I can see and the measurement process is a bit different, instead of an 8051, It's actually an 80 Intel 8049. Most of the chips in here are Uh 1990.

So this unit was Um built in late 1990 or there about, so maybe early 1991 and you'll notice exactly the same um, hybrid uh, multi slope analog to digital converter chip. or it's actually the Uh logic for the analog to digital converter. It's not an actual analog to digital converter as you know it, it's the external um, the external integration capacitor around the outside. The external switching and stuff like that is done outside, This is just the logic for a multi slope analog to digital converter, which is a quite a neat way to do it.

And of course, the voltage reference. Absolutely identical linear technology. It's got the HP part number, but as we found out, that is a a Linear technology lm3 999 so exactly the same voltage reference. I presume it's the same grade that's used in its big brother, so the specs are going to be very similar, if not identical to its big brother.

I Think they are slightly different in some respects, but not by a huge margin. That's why if you don't need the extra digit resolution and you don't need the math functions, this Um 3478a is a really great Precision meter. Now there's some M Talk about this moldy slow analog to digital converter technique on the Uh Forum so it's well worth reading and it is a very interesting technique. It it basically um, it's you know, not too dissimilar.

I Guess to uh your traditional Uh dual slope integration and it basically measures the Uh the time period. So it's basically a really precise timer instead of being a more traditional analog to digital Uh converter. As such, like a flash analog to digital converter. Etc It's based on precision timing measurements of charging and discharging a reference capacitor to known reference voltages and you can get very precise and by using a few tricks, very quick.

Way to uh, actually sample. That's why these things can have you know hundreds of samples per second and you can see the one nanofarad reference cap down there. c410 I believe it is. and yeah, it's plus - 10% like its actual value does not matter.

Once again, it's the stability of that cap capacitor with temperature. so that would be a low. A very stable low Tempco uh Capacitor: Some sort of, you know, poly propop, or something like that. Some sort of uh Precision capacitor would have been carefully chosen for this application.

and of course it's a dead giveaway. Uh, that it's the reference cap because look at its physical size and proportions. For a simple what a 1 nanofarad cap, you know you could have just used a, you know, a crappy bypass cap like this if was just doing bypass application. So it's a dead giveaway.

That You Don't See caps like that unless unless they're used for precision applications like this one. And there's uh, differences in the main processor section over here. the main processor is actually an Intel um 8039 and if we go over to here, we can see an Intel Uh 8 291a Gpib controller and this looks like the original Uh Panasonic battery in this thing and it's still measuring uh 3.49 volt so still probably have some life left. But there's been a bit of discussion about these batteries on the EV log forum and how they can possibly have depending on the type of very flat discharge characteristic and not being able to determine uh, you know how much life is left just based on the measured voltage.

So if you do get this sort of gear, um, it's often it's not a bad idea to actually replace the batteries. But as I mentioned in the previous video, this battery holds all of the Uh calibration data in the volatile memory. So if you simply just desolder this thing and wiring a new one, um, there's no internal capacitor on there to keep the Uh charge on this thing. um, while you replace the battery.

So if you simply just take it out and replace it, you're going to lose all that calibration data and your meter is useless. You got to send it away and pay the same money or more again as what you paid for it to get the thing properly traceably calibrated. So really, when you're uh, replacing these batteries, you need to be very careful because the negative uh terminal on here which is up here is going to be Main's Earth referenced if you use your soldering iron. For example, even if you power this thing on, if you leave the power on when you actually solder it and it might be okay, the soldering iron goes on.

The negative: termal over here: H it's all ground Re Main's Earth reference You're not going to do any uh uh damage at all, but if you then place your iron on this positive terminal over here bang, you've just shorted it out to Main's Earth and directly back to the negative terminal, you shorted out the battery and you could potentially um, if you're not careful, lose your calibration constant. So what you need to do is actually, um, well, the safest way to do it is to um, solder another battery in parallel or diode or it in there. um, I would actually solder it on. Some people say oh, you can just connect it in there and you know, with alligator clips or something like that.

but if you get a dodgy connection, eh, you can lose your data I wouldn't risk it I would actually go to the effort to solder another battery in there, possibly diode or with this one so that you can, uh, keep that contents alive while you safely desolder this one with the Main's uh, power cord disconnected so that you, uh, don't create any uh Earth Loop shorts in the thing and short out the battery and therefore your cont sense. So just be careful. And the manual for this one says the battery is supposed to be a Lithium sulfur dioxide one, but uh, it's clearly not because they would be. well and it says it on there as well.

plus 3 Vols But this one's actually um, you know, 3.45 almost 3.5 Vol So clearly they've uh, put a different type in there now this Shield here is, uh, rather interesting. You notice that's actually connected down to a trace down there and it's connect. It's bent like that and sort of Shield it's a sort of attempting to Shield some of the digital circuits you over here you not. It's right near the Crystal and that's probably the reason why it's doing it.

probably trying to Shield it from one side to the other and you'll notice that that Trace actually goes around here and if you follow, it might not be easy to get on camera, but it goes down to this brown wire here which then jumps over to the LCD um, a point on the uh, well on the uh keypad board Bo down there and then it goes off through the case. try and get it and then it uh actually comes and it's connected onto the Earth Chazzy down here. and of course that then that Earth is then connected over to the Main's Earth terminal all the way over here. So um, yeah, that Shield is Main's Earth referenced but theyve actually got they've got to the trouble to sort of flow that around the board there.

you can see that Trace snaking its way around like that so they're isolating they they're trying to Shield all of this part of it from the measurement part of the circuit. so I'm not not sure the the you know the advantage of just that. Shield Like that it's not Like it's you know, a can covering all of the digital circuit or any or anything like that. So I'm not sure if it's actual Effectiveness Um, but it's obviously strategically located I Don't think it's an accident that the main Crystal and there's main processor and it's probably you know a fair bit of that's probably around here is probably the highest frequency uh, stuff that we've actually got happening inside this multimeter.

So they just I don't know they've decided to add that in. So there you have it. There's a classic HP 3478a Just a quick look inside this thing. it's well worth.

Um, just downloading the service manual and uh, having to read and start reading about how the multi slope uh converter works and other stuff and they're quite fascinating devices. very well engineered. Um, as I said I Highly recommend picking one up if you're in the market for a Precision 5, 1 12 digigit multimeter. If you can get it for a reasonable price on eBay by all means, do so.

So if you want to discuss it, jump on over to the EV blog. Forum Catch you next time.