TNP #49 - Keysight 34420A 7.5 Digit Nano Volt / Micro Ohm Meter Teardown, Repair & Verification

In this quick episode Shahriar repairs an HP 34420A. This 7.5 Digit meter is specialized for making low voltage & low resistance measurements. In 4W mode, it can provide excellent resistance measurements in the micro-Ohm range.
This unit does not make any measurements on Channel 1 while Channel 2 reports the correct readings. An overview of the front-end schematics points to the likely culprit: the solid-state relays responsible for channel switching. After replacing the components the instruments returns to normal. Despite its age and the this repair, voltage and resistance measurements are remarkably close to their ideal values as compared with voltage & resistance calibrator instruments.
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[@BartKus]

TSP such a badass he didn't even bother measuring those optical relay outputs to verify voltage isn't making it through them. He just DIVINED his way through the repair! Amazing. :)

[@redmist4963]

I'd say an intelligent deduction. Chances of a jfet which is isolated from everything and has been working within the datasheet parameters all its life il less likely to fail than a solid state switch which is in direct contact with low and high voltages.

[@EEVblog]

I'm triggered by 1.2346V

[@donepearce]

I recently bought a 3478A multimeter, advertised as non-working. This was a result of having no push rods for the switches. Half an hour in Solidworks and I had a couple of files I could 3D-print, and now that is fine. It turns out that all the elctrolytics have been changed, as has the battery. It has what looks like a decent cal, but I'm not 100% positive. I shall keep on looking, but I may try to find a cal centre that is not too expensive.

[@mikeiver]

All calibration is expensive!

[@SatyajitRoy2048]

This is a paradise for measuring instruments, folks!

[@ghlscitel6714]

Quick and clean fix. Congrats!

[@OctavMandru]

The characteristics of the SS relays probably has to be calibrated out. Anyway, great repair, impatient man :)

[@campbellmorrison8540]

I've never seen those SS switches before LH1510 are a very neat device but I'm still surprised that can be used in such a sensitive low level front end

[@Connexelectronic]

Great video

[@mikeiver]

I was fortunate enough to keep one of these from being scrapped after a project was over. All I can say is that it is a ridiculous device! More accuracy than I deserve and it is the reference I use for all my other meters. Not looking forward to the cost of calibration though.

[@xDevscom_EE]

Calibration of these nV meters is not as straightforward as one might think. Lucky enough I know a nearby lab just capable of handling that task! 😅 Broken optoswitches is a very common fault in these 34420As.

[@pahom2]

TSP casually comparing two 7.5 digits ohm meters

[@cosmolittle1395]

Please, more repair videos, especially really difficult RF and microwave ones

[@paulhawkins6415]

Did Shahriar get a PhD in electronics just so he could collect ALL the test instruments :)

[@simontay4851]

Pretty much, yes.

[@glasslinger]

Aw shucks! Only 7 digits! :)

[@ryebis]

Awwwww such an anti climax. I was hoping for something more interesting :) Nice score.

[@ivolol]

Is that 7 and a half digits with only LM399? And don't you have at least one 3458A to verify voltage measurements with?

[@simontay4851]

Why did the solid state relay fail? If its solid state, it should fail short, not open. Its basically an opto triac, isn't it?

[@motherjoon]

You're in Marco Rep's turf

[@xDevscom_EE]

Marco is yet to get 34420A :)

[@Kosmonooit]

Shahriar is on another level, realm even

[@nickstanley5064]

But he did tho? Check his latest video on building nanovolt scanners.

[@MrMersh-ts7jl]

Off topic, but in the background on your oscilloscope there's a blue square cover on what I'm assuming is the trigger port. What is that? I have seen them in other people's labs on other oscilloscopes. Thanks for everything you do. And two videos in 2 days is most excellent dude!

[@pipatron]

Looks like a 3D-printed dustcap.

[@Thesignalpath]

I think you are referring to the dust cap.

[@MrMersh-ts7jl]

@@Thesignalpath hahahha. Just amazing. Thank you.

[@firasgh871]

هل من اممكن ان تحصل على اسلاك من الذهب فقط

[@changjiangliu5951]

Nice repair. Can you measure the injection current of this HP nanovoltmeter? This can be done by using an Oscilloscope. I recently got a 2182A nanovoltmeter, but i am not quite happy about its injection current although Keithley claims 2182A has one order of magnitude smaller injection current (pump out current) than any other nanovoltmeter/micro ohmmeter (obviously referring to 34420A).