my 3456A - Page 1
Forum section: electronics / general

author: mikeg
author_rank: Newbie
timestamp: December 29, 2017, 04:51:32 pm
content: I have to gush.The last time it was cal'd was 8 years ago.The supplies failed and it sat for a couple years.(life has gotten in the way)I have now rebuilt the supplies and had it cal'd. It's still in spec! They didn't touch anything, didn't have to.I'm stunned. I love my meter.

author: MisterDiodes
author_rank: Frequent Contributor
timestamp: December 29, 2017, 06:43:28 pm
content: Congratulations! That story is typical of what we see also. We've got one that hasn't had the cal pots touched for decades, and it's still perfectly fine and in spec. So much for pots getting a bad rap for being unstable...High quality trimmer pots used correctly can work very well.We've got several of those units and in terms of long term stability and very low drift - they are spot on. They might drift only slightly more than our best 3458a's, and sometimes right on par at about 1ppm per year or less.If you can afford the larger space and the lower read speed, these are a fantastic tool for a rugged, quiet and stable 6.5 dig instrument. Very budget friendly and the cost to calibrate one is modest. The display is a nice rugged, easy to read LED that doesn't crap out like the VFD's on the 3458a's.In terms of drift and stability a working, aged 3456a will run circles around the newer meters - to the point the Keysight quit trying to sell us the newer 6.5 / 7.5 digit DMMs (that will typically drift more in 3 hours than our 3456a's drift in 30 or 90 days or a year).There are other uses for the newer units of course - but if you're not in a hurry and need stable, low drift measures then 3456a is a reliable, lower cost way to do that. Newer meters are smaller of course and get you faster read speeds, and a more exciting display. But that might not matter in the end application.We've found that for our needs the 3456a is a much more valuable tool that provides a very, very good benefit to cost ratio in a 6.5 digit bench meter. It's one of the most profitable pieces of equipment on the line when you need a 6.5 dig meter.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 30, 2017, 09:15:32 pm
content: My 3455A is basically the same. Years and years with only about 1 ppm drift. I've always said an antique with good performance and a stable history is better than something new and unknown. IMO, the only downside to these meters is people don't have that much bench space anymore. I built my bench with this size instrument specifically in mind.

author: jeremynichols
author_rank: Newbie
timestamp: December 31, 2017, 04:33:13 pm
content: What are folks using to log data from the 3455, 3456 and similar old beasts? For my 5370 counter I have TimeLab but there doesn't seem to be anything similar for voltmeters. I have a Prologix GPIB-Ethernet controller that I can talk to from my PC but my programming experience ended with AppleSoft BASIC so I'd rather get something ready-made. I don't need anything fancy, just record the values and a time/date stamp that I can save as a text file and later pull into Excel. Anything out there?Jeremy

author: borghese
author_rank: Regular Contributor
timestamp: December 31, 2017, 05:24:08 pm
content: Put the 3456 in talk only; reset; set manually the voltmeter and from the Timelab acquire menu select "acquire from counter in talk-only". That's all.Happy new year

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: December 31, 2017, 08:19:23 pm
content: Jeremy,EZGPIB might be of interest to you. It will run on Windows using PASCAL. You can also use a Raspberry Pi and python.There are plenty of people here that can help. All you need is to decide on your OS and get the manuals with the GPIB commands. You may even find examples in them designed to run on HP BASIC.If the commands are similar to the older 3458A, then that will make it easier.

author: e61_phil
author_rank: Frequent Contributor
timestamp: December 31, 2017, 09:16:30 pm
content: If you don't want to use a raspberry pi you can simply run python on Windows. pyvisa works very well with NI-VISA and also with the Keysight Connection Expert.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 31, 2017, 09:19:39 pm
content: I don't usually log, just periodic checks against three Fluke 731s. When I do log something, I use a Prologix GPIB adapter and programs I've written in Powerbasic.

author: alm
author_rank: Super Contributor
timestamp: January 01, 2018, 01:47:22 am
content: Quote from: ManateeMafia on December 31, 2017, 08:19:23 pmNope, the 3455A and 3456A commands are not at all similar to the 3458A commands. Many early instruments, including the HP 3455A and 3456A, used what is sometimes called 'R2D2' language. These are commands that usually exist of a letter and a number. For example, R1 might refer to a range setting, and F1 to a function. Often this was directly mapped to front panel buttons. On the HP 3455A, the left most range button is the 0.1 V range (in DCV mode), so R1 corresponds to 0.1 V. On the 3456A, however, R1 refers to auto-ranging, and R2 is the command for the lowest manual range. This means that software has to support that exact model of DMM, there are no universal standard commands.With the 3457A and 3458A, HP tried to come up with a common language that would be similar across different instruments. Not long after, IEEE 488.2 was released, which introduced SCPI. This is the language that is used to this day and is semi-standardized across instruments (don't expect to be able to switch manufacturer without any changes, but likely 95% of your code will be compatible as long as you are not using unique features).Then there is theIVIstandard, which is an abstraction that drivers can implement so software can treat the instrument as a generic DMM, rather than caring about specifics. I'm not aware of IVI drivers for the HP 3455A/3456A, however.

author: precaud
author_rank: Frequent Contributor
timestamp: January 01, 2018, 03:32:03 pm
content: Congrats on reviving your 3456A. A very nice meter , indeed.In a similar vein, regarding the long-term accuracy and reliability of that generation of HP instruments, is the 3466A DMM that I bought new in 1980 ($995 for a 4.5-digit DVM with 100kHz TRMS AC !). 18 months in, the unit failed and was sent in for repair. Warranty had expired, but HP replaced the internals at no charge and sent cal data back with it. (These guys also knew how to create brand loyalty.) 35 years later, with daily use, that instrument is still in cal and has been completely problem free. It is on all day every work day, my go-to meter for normal measurements. It is by far the most stable and reliable piece of test gear I've ever purchased!I have a 3456A that I repaired and would love to have it on the bench, but unfortunately I didn't plan my workspace shelving to accommodate such deep instruments... and so I use a 3457A. But if the 3456A had better low-Ohms resolution and repeatability (when measuring in the milliOhms range) I would find a way to make it fit! (IMO that is a weak spot of all 6.5-digit DMM's from that era.)

author: guenthert
author_rank: Frequent Contributor
timestamp: January 01, 2018, 03:58:45 pm
content: Quote from: MisterDiodes on December 29, 2017, 06:43:28 pmMy understanding was, that those (or any) pots become problematic when moved for the first time after a long period of sitting still. They're not unstable per se, but might fall apart or unable to provide good contact when moved. I'm afraid to turn them of my 3456a, even though I know that the DC gain is a little high. If I need it precise, I use the built-in calculator to correct the displayed value

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: January 01, 2018, 08:37:28 pm
content: I have to disagree with you there, the Bourns 3250 is the only MIL qualified trimmer on the market and it is much more stable than anybody else's. Yes it isn't cheap but if you search around you can find them for around $12 and sometimes even less. Forget the mainline distributors like Mouser, Newark and so forth, they're much higher on markup.guenthert: A high quality WW pot will stabilize again after sitting at the same spot for a long time, just run the wiper back and forth a few times, this will 'clean' up any bit of whatever might be around it, most of these trimmers are well sealed and use an alloy which does not oxidize significantly with time. Other trimmer types are not so stable, particularly the plastic elements, also watch out for the additional TCR of the wiper contact, it can be far more than the wiper element's TCR. Another problematic source is current levels in the wiper.I have not had any problems with readjusting the trimmers in my 3456A after they had sat for years untouched, if there is any signs of jumpiness when the wiper is moved just run the wiper back and forth a few times until it settles down. In rare cases, you may end up having to replace it but I haven't had that happen very often. Even the more common WW trimmers have many thousands of rotation life so even an old instrument is unlikely to have had their trimmers tweaked very many times.

author: e61_phil
author_rank: Frequent Contributor
timestamp: January 01, 2018, 08:42:36 pm
content: Quote from: DiligentMinds.com on January 01, 2018, 07:35:27 pmNo, there is a small cover in the frontpanel. There is no need to open the instrument for trimming.

author: MisterDiodes
author_rank: Frequent Contributor
timestamp: January 02, 2018, 01:00:25 am
content: On our 3456a's that get adjusted now and then, there is never a problem doing the adjustment with the cal pots. Just bring the pot to where about you want it, then swing the wiper back and forth the area where you want it...I'll give it a few "cleaning wipes" if you see any jitters - you'll know when you've got the new wiper contact area cleaned up if it hasn't been adjusted in ages.Then give it the final nudge trim, remove the tool and stand back, wait a minute or two and it should be OK. You might not touch that pot again for a years or decades. They work fine for decades IF they are higher quality pots - forget the cheap plastic pots.DigilentMinds: There is a small dust cover on the front of a 3456a already for the cal pots access - no need to remove the covers. The meter doesn't really care if that cover is in place or not during cal. So you don't really have to remove any of the main instrument covers to do a cal. The pots are positioned that way on purpose.We build trimmers into some Vref also and I can vouch for the Mr. Pettis' recommendation on the Bourns PWW pots - these are excellent pots, run around $12 or $15 each and give you an excellent method of trimming a circuit without adding digital noise. They also come with a natural non-volatile memory and are much, much quieter than any digital-tuned, diffused resistor array in a DAC (for instance) - AND pots don't contaminate a pure analog system with digital hash.Bourns has some excellent literature on how to design a trimmer pot circuit correctly - a good read if you're not familiar with the procedure - the common mistakes we see with young players is not considering what happens when the wiper is adjusted into the max / min stops, and building a circuit with a "dry" wiper contact (not enough current flow - usually you want around 20~40uA minimum current on the wiper):https://www.bourns.com/pdfs/onlinepotentiometerhandbook.pdfWatch out on Vishay pot specs also - First they will claim some low TC for the pot, and then realize the TC of the wiper will be 50 or 100ppm.HINT: Design your trimmer circuit correctly and the trim pot TC won't have a huge effect... Lots of older equipment works this way and will often beat the newer stuff on stability, noise and TC.

author: rigrunner
author_rank: Frequent Contributor
timestamp: January 02, 2018, 01:46:14 am
content: Quote from: MisterDiodes on January 02, 2018, 01:00:25 amJust a side note to this based on my sample of 4 - 2 of each.The fanless 3456a are seemingly ambivalent to the cal cover being in place irrespective of ambient temperature. My earlier models with fan show a 2-4ppm shift with the cal cover off if the room temperature is ~16C or lower.

author: mikeg
author_rank: Newbie
timestamp: January 03, 2018, 06:37:14 pm
content: I don't log data with mine. I just want a nice 6.5 digit. I bought it 8 years ago and had it cal'd then.I've also had the 3457 and the 34970A. I'd like to have another logger for data collection and the 3457 I never liked. I don't know what it is about the 3457 but it and I never got along.I also have a 5328 counter and the next item to buy is a 3325 generator. I bought a 19 inch rack, it'll all go in there along with a Tek scope.

author: Echo88
author_rank: Frequent Contributor
timestamp: January 10, 2018, 10:21:25 pm
content: Since ive needed another reasonable good DMM for my reference-drift-scanner-project i first thought about getting a Keithley 2000, but then was reminded about the kind words of MisterDiodes about the HP 3456A. After seeing one on ebay from Italy and throwing money at ebay, letting it warm up and connecting it to my Fluke 5440B:Makes me all tingly inside.

author: e61_phil
author_rank: Frequent Contributor
timestamp: January 10, 2018, 10:27:14 pm
content: And what about 10V?

author: Echo88
author_rank: Frequent Contributor
timestamp: January 10, 2018, 10:44:05 pm
content: Errors checked against a parallel connected 3458A (calibrated a year ago) after ACAL, all in positive range.Range/Error:100mV Range / +1µV1V Range / +6µV10V Range / +100µV100V Range / -1700µV

author: AG7CK
author_rank: Regular Contributor
timestamp: January 10, 2018, 10:57:15 pm
content: I got this hopefully fine machine from a scientific instruments surplus dealer in the USA. Due to the seller insisting on a decent packing, the box is too big for Priority Mail International. So it was sent as USPS Global Express Guaranteed GXG®. Hence the shipping cost was 3 times the price of the instrument. Still I feel privileged to be able to buy such a fine machine with its excellent service manual ...Could anyone imagine the story of the anti-tampering stickers: Time period, color, why put stickers on case screws etc.?

author: lowimpedance
author_rank: Super Contributor
timestamp: January 11, 2018, 01:25:48 am
content: Looks like HP did the last cal on it , nice to have intact HP stickers. It means no numpty has been inside messing with it.And that's also the reason for the stickers when this sort of equipment was in serious Lab use where its calibration history was important.Remember after repairs it would be sent for re cal. and broken stickers showed the unit was not to be used until they were replaced at the Cal Lab.

author: AG7CK
author_rank: Regular Contributor
timestamp: January 11, 2018, 02:03:26 am
content: Thank you, lowimpedance.That's about what I was thinking. The front sticker is obvious because it hides the pots/switch. And the back/side stickers add a layer of "confidence" to the instrument's condition and calibration by also showing that the case has not been opened (for inspection or repair or whatever) after the calibration.Very good.

author: MisterDiodes
author_rank: Frequent Contributor
timestamp: January 11, 2018, 02:55:47 am
content: Echo88 - Looks like yours is ready to go to work! Great news!

author: BU508A
author_rank: Super Contributor
timestamp: January 11, 2018, 07:28:36 am
content: Quote from: Echo88 on January 10, 2018, 10:21:25 pmAh, you've bought that one.I got me a Prema 6040 S, but it is a bit sick, have to repair it first.