EEVblog #427 - HP 3478A Multimeter Teardown

Inside the HP 3478A Multimeter
A quick follow-up to the previous 3457A teardown video.

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[richaw]

I was one of the engineers that designed the HP3478A. Battery: The battery in my 30-year-old prototype measures 3.011V. The design life minimum was 10 years, probably at maximum storage temperature. Shield: At least in the prototype, a small shield was needed to calm down the AC RMS converter. In initial production, it was just a large disc capacitor. Inside the micro, there is another digit which was pretty good and could have been displayed. I wish I'd left that test code in my prototype...

[JosipMiller]

I am proud user of this beauty. I inherited this instrument from my retired and then departed colleague who sadly passed away in 2014. There were quite a few beautiful instruments and tools all maintained and calibrated nicely to this day. There was also 3465A 4 and 1/2 digits which required massive cleanup of switches and works perfectly now. 3478A we calibrated in local calibration facility and looking this calibration report was really impressive. There was almost nothing to calibrate in 2018. on instrument from 1990. Only correction was made somewhere in current measurement. Original battery is still inside. What I forgot is to replace filter capacitors which I intend to do before next calibration as well as replacing the battery using isolated soldering iron and backup battery. It has 4-wire resistance measurement which was extremely useful during development of sensitive temperature measurement equipment. Very often I make my own check-ups with voltage reference but proper calibration is needed anyway.

[NCHLAS]

I really enjoy watching your videos. I love the fact you're very smart and will admit you don't know something and ask people to tell you the things you don't know.

[seamonkeysy]

I got a HP3478A, the battery of mine still appears to work fine but I made an arduino GPIB adapter to grab the calibration data just in case. Maybe the previous owner of it replaced the battery though

[ChipGuy]

Thanks for the video, it make me realize that i need to use one of these 2Ah Lithium primary batteries instead of these 290mAh CR2032 cells :) Love them teardown vids :)

[GabrielRotar]

Learn something new every day I guess, Cheers mate.

[yoramstein]

hi does anybody hasthe link to the cross refferance guide to HP's parts?

[lynxgroupblog]

Dave : remeber those gas powered soldering irons, they do wonders in this bettery replacement ?

[CassegrainSweden]

Regarding exchanging the battery, I would use an isolation transformer feeding the unit on which to exchange the battery. Gives no galvanic connection to the soldering iron.

[PaulTurner_Haizo]

Yes, they certainly could be toroids, the service manual that shows how simple to do that is. but the board layout doesn't match. Maybe it is indeed a later opto-isolated model?

[MrFreeElectron]

slope error means the rundown failed to close. check the integrator and comparator.
most likely the comparator is shot.

[GabrielRotar]

is that voltage reference a bit loose on the board or just it's casing ?

[stevenspmd]

@EEVblog just picked one up for $110 usd but shipping by UPS was a kick in the nuts

[adkharu]

hey Dave.
i purchased one of these units from a US seller and it appears to be an identical model. i would set the dip switches the same because i live in Australia. did you need to replace the fuse? also, the service manual says to internally change the Line Select Jumper. is this necessary? thanks for your time.
-ak

[BryceSchroeder]

For me the critical difference between the 3478A and the 3457A was that the latter was specified to measure AC current at 100 kHz and the former wasn't. It's so hard to find any instrument at a reasonable price that is specified for current beyond 10 kHz... the 3458A is another that is, and Fluke has one that _might_ be, but the spec sheet was ambiguous. I wonder if it's just a specification / testing issue or if that current measuring shunt really is a huge bandwidth hit? (All of them advertise 100 kHz or so AC volts bandwidth.)

[Razor]

Couldn't you use a butane soldering iron to desolder the battery while the unit is powered on?

[frankbuss]

The forum link doesn't work. Looks like it expects a trailing slash. Might be worth to fix the forum software, so that you don't have to fix all links in all your videos.

[RandyLott]

Is it typical for one person to design the PCB, or is it divided between a few people? Thanks Dave!

[vink]

This might be a silly question, but what is the benefit of having that extension bar from the front panel to the power switch, as opposed to having the power switch sitting just behind the front panel? Is it to do with keeping wiring emitting a 50/60Hz signal to a minimum?

[blizniak]

Dave, tell us something about Swerleins algorithm !