34401A Historical question

[Johnboy]

This HP/Agilent/Keysight model has been discussed in dozens (if not hundreds) of threads on this forum. While I've discovered that others have indeed been similarly baffled, I've been unable to find a definitive answer to this:

What is the deal with the black negative connections on HP and Agilent 34401A's?

The earlier models made by both manufacturers seem to have only red connectors. Why would later models by both HP and Agilent feature black for negative? Is there a possibility that some of the HP models were Agilent branded? Were the black connectors a random occurrence ("Stan, we've run out of the reds; toss me a couple of those black ones from over in that pile by the wastebasket")? Or what?

In case anyone wonders why I'd be concerned with something so trivial, I recently acquired a dirt-cheap specimen of the breed. It is completely stripped down. No bumpers, no handle, no display window, no serial. No mention of Hewlett-Packard or Agilent on the rear of the chassis (no, I haven't cracked it open yet). It has black negative connectors, but other than that I have very little to go on as far as making positive identifications.

Thought I'd ask whether anyone had ever managed to shed any light on this issue. I called Keysight, but the gentleman I spoke to in their technical department did not have much information at hand (unsurprisingly, as these units are obviously discontinued, and model notes were probably not archived throughout the changes the company went through over the decades).

[coromonadalix]

I think  highvoltage member had an answer fir this

Unless i'm mistaken, correct me if im wrong

There is 2 versions of the front display pcb, the "older"  was with the red jacks only, using different ic's than the later one with red and black jacks ?

[Mr. Scram]

I think those terminals were changed from red to black near the end of the HP era. They remained this way after that, unless I'm mistaken. Adding colour coding increases front panel legibility and should reduce the potential for error. It should also be noted subtle changes have been made to this model throughout its life, like changing the blue button to a more grey colour around or at the same time.

Production moving from the US to Malaysia may also have played a role, as I've seen units produced in either.

[HighVoltage]

I had an original HP 34401A from 1999 and it had already the red and black colored terminal blocks.
Johnboy is right, it was changed by the end of the HP branding and continued to the end of production like this.

Also, I bought an older model 34401A from 1993 and it had also black and red terminals and I was under the believe it must be a newer model, when I bought it. Well, someone did replace the terminal block with a newer model. So, be careful when you see them on ebay with a new terminal block!


 

[LapTop006]

And of course the branding is only on the display bezel, and a replacement bezel may have a newer (or older) brand confusing things further.

[joeqsmith]

And of course the branding is only on the display bezel, and a replacement bezel may have a newer (or older) brand confusing things further.

I bought mine brand new from HP when they came out.  Mine is branded on the back of the chassis as well in two places, the case and serial number. 

[LapTop006]

Oh sure, I probably should have said "easily visible" branding or something like that.

[Johnboy]

I bought an older model 34401A from 1993 and it had also black and red terminals and I was under the believe it must be a newer model, when I bought it. Well, someone did replace the terminal block with a newer model. So, be careful when you see them on ebay with a new terminal block!

And of course the branding is only on the display bezel, and a replacement bezel may have a newer (or older) brand confusing things further.

The obverse is also apparently the case. Here are links to two units which appear to be HP's confused with, or masquerading as, Agilents. Note that they have only red connectors.

https://www.ebay.com/itm/HP-Agilent-34401A-Digital-Multimeter-6-Digit-Tested-Spot-on-Very-clean/323493128681?hash=item4b51b169e9:g:nSkAAOSw64tbmB~y:rk:1:pf:0

https://www.ebay.com/itm/Hp-Agilent-Keysight-34401A-6-DMM-w-5519A-test-leads-WARRANTY-and-Cal-cert/202246552170?epid=14020075319&hash=item2f16d57a6a:g:SLMAAOSwVkNamFeW:rk:4:pf:0

Here is one with window claiming Keysight-era model while possessing a telltale rectangular blue "shift" button:

https://www.ebay.com/itm/KEYSIGHT-AGILENT-HP-34401A-6-1-2-DIGIT-MULTIMETER-CALIBRATED-TO-NIST/223144223111?epid=710140765&hash=item33f46ea187:g:QxcAAOSw3IdZihOP:rk:46:pf:0

And finally, two pictures of the model I have here. Note that there is no HP or Agilent logo. If this is a later HP model, as the grey shift button would lead me to believe, why is there no logo above the missing serial number? Can anyone else verify having something similar?

The naked display is possibly a better bet for identification, as there have been several users here who have replaced theirs.

[HighVoltage]

The serial number of your instrument can be read out remotely and then you can check the Keysight website to find out when the instrument was first sold / delivered.

Some offers can be deceiving
In Europe, older HP units sell for around 300 Euro
Newer Keysight branded units sell for sometimes 650 Euro
So, some sellers try to "upgrade" their items for a 10$ new window and fake China made bumpers and handles.

You never really know what you are in for until you buy.

I have bought many of these instruments over the years also for some of my customers to be integrated in to test stands and it is almost always a surprise, if you buy them used.

   

[Johnboy]

The serial number of your instrument can be read out remotely and then you can check the Keysight website to find out when the instrument was first sold / delivered.

This is something I had read about while parsing all those old threads about this model. My understanding, though, is that the serial can only be read in this manner if the previous owner had themselves gone to the trouble of entering the serial number remotely in the first place. IIRC, Keysight finally made it a point to program the serials in at the factory, but the HP's and Agilent's will only show a zero in the info string. I don't have an RS-232 to USB adapter at the moment but they're inexpensive enough to pick up to take a shot at it, and someone has helpfully posted a link to the old software, so I've got a fairly good chance at seeing which firmware is installed. That might be useful in determining when it was manufactured if I can't find anything else.

I will take the time this weekend to open it up and take a look at what might be hiding inside, but I've got a couple of advance guesses. One of them is that this unit rolled off the line at the very end of the HP branding. It is still USA made, but doesn't have the HP logo, so maybe it was a transitional production period where they might not much longer going to be sold as HP models, but they weren't yet committing to hard-branding them as Agilent either. That seems a little far-fetched, I guess, but I think it's also possible that they might've only put "Hewlett-Packard" on the serial number sticker at this point for other reasons.

When I bought it I was still unaware that there are so many "Frankenstein" units on the used market. It wouldn't have deterred me from picking this one up for what I paid for it, but it's sobering to realize just how many of these out there are not what they appear to be.   

[Mr. Scram]

Your unit in the pictures looks exactly like an Agilent branded US made unit I've seen which I'm fairly sure is original.

[Johnboy]

Thank you for weighing in with that. Thanks to all of you, actually. I've realized that what was intended to be a question about the manufacturing run of these things has kind of turned into something else that might better have its own thread in the repair section  I kind of derailed the thread by asking about my specific model.

But before I actually crack this thing open I'd like to share with you how I got here.
There are so many HP's for sale/auction on eBay with what appears to be Agilent branding that I started to wonder whether Agilent had started its manufacture of the 34401A in the same way HP did; that is, with all red terminals. To make this a little less confusing, here's an erroneous chronology to illustrate how I had mistakenly assumed things went (and looking at this now, it seems even stranger to me that I would've come up with this reasoning):

HP begins manufacturing in USA, all red connectors
HP introduces black connectors for negative terminals / changes color of shift key from blue to grey
Agilent begins manufacture in Malaysia, all red connectors
Agilent apes HP's earlier design change, implementing black connectors for negative terminals

And this was, again, dead wrong, as Mr. Scram, HighVoltage, and others have helpfully pointed out. Even the tech at Keysight I spoke to made the likely identification of my unit as an Agilent, sight unseen, on the basis of the presence of the black.