EEVblog #485 - Agilent TrueVolt 34461A Multimeter Teardown

Inside the new Agilent TrueVolt 34461A 6.5digit bench multimeter. A replacement for the venerable 34401A.

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[JohnHungerfordJr]

Dave please do me a favor to turn on CC. I’m deaf need to read CC which helps me understand while you’re show and tell. Sometimes not all videos you setup are automatically turn on. There are millions of deaf and hearing impaired are learning from your greatest experts. Please support us that we love you, Dave. My picture hand signs means “I love You”. Cheers, AlpineJWH

[ronaldlijs]

Hi Dave, nice tear down, I fixed my HP34401A myself, works like a treat now. Great meter and can't wait to buy this new version...! Keep them coming!

[EEVblog]

I usually let it process into full HD before releasing, but it was already delayed enough, so left it public overnight and it went live before finishing. Still not monetised either, so I lose out on that.

[EarthRealmLiuKang]

c621 @32:14 next to the 10 bit ADC might be the cap you are looking for. Didn't look it up because it's hard to see the part

[mikeselectricstuff]

Text is already on the forum in 34461A thread!

[floodo]

whoa, this has got to be the most detailed teardown that i've ever seen!

[davidjereb]

Half an hour ago you asked me what I consider current flowing "through". I answered that an electron should be able to enter on one side and leave on the other. Only in an electrical current do you have charge that's actually moving. When you are talking displacement currents, you are talking about the change of displacement field over time through an area. Charge isn't moving (at least not between the cap plates).

[EEVblog]

No you are not correct. Current still "flows through" the capacitor, it's just a different kind of current. There is electric current, and displacement current at work in this circuit. They come under the same umbrella.

[davidjereb]

I am currently studying electrical engineering at university. I have used Maxwell's equations very often, there is no way around them.
One equation derived from the main equations (Ampere's law and is I = dQ/dt. Current the differential of charge over time. You have a current flowing onto one plate and that plate builds up a charge. That charge makes an electric field that attracts an opposite charge to the other plate. If charge flows to the other plate, you have current.

[EEVblog]

So that constant current through the resistor magically flows into the opamp input? Or off into the ether? Or is gobbled up by the loch ness current monster?
I won't play semantics.

[ChristofferViken]

About the re-cal, I'm sure that is probably a "better safe than sorry" thing. It _may_ mess with calibration. More importantly, if you need to work with certified calibration on everything then safe is probably better.
So I would probably have written "voids calibration" if i were to write the manual; but it may be relevant.

[EEVblog]

Congrats, you just used the word current 3 times in trying to explain how current *doesn't* flow through a capacitor. Current flows on the plate, current flows off the plate, the current is equal on both sides - but it *doesn't* flow through! :->

[EEVblog]

Yes, shooting this one was delayed, then it was a 2GB upload so took a long time.

[EEVblog]

Kirchhoff's current law also applies. That alone is enough definition for anyone to use such the term. How can you say my use of the term is perfectly fine and commonly accepted, but then also say there is no EE definition?

[MDNukemJr]

If I'm not mistaken, it would be fairly atypical for the developers to leave the WinCE command shell running on the COM port, hence the lack of response. They may have never even used it; in the past, I've used COM for bootloader and startup messages, then a separate mechanism to access an embedded telnet server during development (which is subsequently disabled during production).

[nearmitech]

Hi Dave, An excellent teardown video.

I have two questions for you.

(1) As you look into the multimeter just behind the front panel there is next to the large capacitor a small 3 legged component with the numbers 8DCNG inscribed on it. Next to it are the letters ERC. Can you please help me identify this component.

(2) The second question I have is regarding a problem I am facing with my multimeter. When we power it on, majority of the times the self test fails and therefore prevents any further action. The LAN seems to be defective however the rear USB is functioning alright. The front USB has some issues because of which I am unable to update the frimware. That said, there are those rare times when the self test passes and the multimeter works beautifully (until powered off). This leaves me to believe that some timing related issue in the start up sequence is playing up.

With Keysight's assistance over a video call, I tried to see if I could do a firmware update with little luck. I am given to understand that repairs of the multimeter by Kesight could involve one of two things -
(a) replacement of the front panel board alone and
(b) if that does not work replacing both boards. I am also given to understand that both these options could cost up to 40000 or 90000 Indian rupees depending on how many boards they have to replace. That seems steep when the price of a new multimeter is just over (and comes with a three year warranty).

Is there any more advice you could provide me with to try and fix this problem?

[opelize]

What does the spark gap do? (labeled E101 on the board)

[davidjereb]

I'd say this is kind of a debatable area. Electronics enthusiasts/engineers often speak of current flowing through capacitors, but from a physicists point of view, it's a whole different story. Current is flowing onto one plate of the capacitor and an equal current is flowing off the other plate, so charges on both plates are always equal (the signs are obviously opposite). Because you have equal currents on both sides, people (including me) often say through, but that's not technically true.

[JanuS]

i have no idea what all these chips do but i still watch these videos

[francistheodorecatte]

if it's like any other soft off/on power supply, the standby circuit probably pulls ~5-10mA +/-2mA. it's heavily dependent on how conscious of the standby power draw the EE that designed the supply was.