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Uli · 04-23-2021, 04:26 PM

Hi technicians,

because of the HP-IL connection (and because it is a wonderful device) I have bought a HP-3468A in very good condition.
It works with 230VAC, 50Hz. Selftest is okay. But... after switch it on, without measuring, in VDC mode, display shows +2,86...VDC. In VAC mode it shows 0,001 VAC. If I check the voltage of a new AA battery it shows 1,62 VDC. AC voltage from the net is shown with 231 VAC.

So, measuring seems to be ok. But I am amazed about the +2,86 ... VDC without any measuring device.
Is that normal? Do you have any idea?

Uli

Dave Frederickson · 04-23-2021, 05:44 PM

Try shorting the inputs.

Uli · 04-23-2021, 06:03 PM

(04-23-2021 05:44 PM)Dave Frederickson Wrote: Try shorting the inputs.

Yes, when I shorting the inputs display shows 0,000005 VDC. When I disconnect the shorting it shows again +2,86.... VDC.

Dave Frederickson · 04-23-2021, 08:48 PM

Mine does that, too. Pretty sure it's normal.

BobVA · 04-24-2021, 02:00 AM

This is normal. What you're seeing is noise induced in the probe leads from (mostly) nearby AC power lines. The sample time of the HP3468A is fast enough to snapshot this (lower sample times average it out to near zero) so you see random values.

Dave Jones did a deep dive on this topic:

Uli · 04-24-2021, 12:31 PM

(04-24-2021 02:00 AM)BobVA Wrote: This is normal. What you're seeing is noise induced in the probe leads from (mostly) nearby AC power lines. The sample time of the HP3468A is fast enough to snapshot this (lower sample times average it out to near zero) so you see random values.

Dave Jones did a deep dive on this topic:

Thank you for this idea and for the video link that I did not know before. That might be the reason. But ... there is always a but ... my HP-3468A shows +2,86 VDC without any measuring device. Without measuring cables plugged in. And I have carried the HP-3468A into my garden to test it outside. Same result. Now I have to read the manual to learn how to reduce the sample rate, to see what will happen.

Uli · 04-24-2021, 12:32 PM

(04-23-2021 08:48 PM)Dave Frederickson Wrote: Mine does that, too. Pretty sure it's normal.

Thank you for that. That give me hope, that my HP-3468A is okay.

Dave Frederickson · 04-24-2021, 02:01 PM

(04-24-2021 12:32 PM)Uli Wrote:
(04-23-2021 08:48 PM)Dave Frederickson Wrote: Mine does that, too. Pretty sure it's normal.

Thank you for that. That give me hope, that my HP-3468A is okay.

Good News & Bad News:

Bad News - Your 3468 is likely out of calibration. It'll still work but the readings will probably not be accurate. And, the CAL RAM battery could be on it's last leg as well.

Good News - You can save and restore the calibration factors to facilitate changing the battery.
https://www.hpmuseum.org/forum/thread-80...l#pid71014

Dave

Uli · (This post was last modified: 04-24-2021 05:13 PM by Uli.)

(04-24-2021 02:01 PM)Dave Frederickson Wrote:
(04-24-2021 12:32 PM)Uli Wrote: Thank you for that. That give me hope, that my HP-3468A is okay.

Good News & Bad News:

Bad News - Your 3468 is likely out of calibration. It'll still work but the readings will probably not be accurate. And, the CAL RAM battery could be on it's last leg as well.

Good News - You can save and restore the calibration factors to facilitate changing the battery.
https://www.hpmuseum.org/forum/thread-80...l#pid71014

Dave

I have checked the battery. It has 3,0 VDC. The calibration procedure is to heavy for me. I am a mechanical engineer , in other words a hammer thrower. I am not able to manage it. And how could it help to save the actual calibration data if these are not okay? I know a company in Germany that has calibrated an old FLUKE for me. I will ask them for help.
Thanks a lot for your help. Now I understand that something is wrong with my HP-3468A.
PS.: I have also asked the seller, an Italian company for electronically devices for help. Their answer: When no signal is applied it is normal for the display to show strange values.

Dave Frederickson · 04-24-2021, 05:02 PM

(04-24-2021 04:34 PM)Uli Wrote:
(04-24-2021 02:01 PM)Dave Frederickson Wrote: Good News & Bad News:

Bad News - Your 3468 is likely out of calibration. It'll still work but the readings will probably not be accurate. And, the CAL RAM battery could be on it's last leg as well.

Good News - You can save and restore the calibration factors to facilitate changing the battery.
https://www.hpmuseum.org/forum/thread-80...l#pid71014

Dave

I have checked the battery. It has 3,0 VDC.

The perfect time to backup your calibration constants.

Uli · 04-25-2021, 05:11 AM

(04-24-2021 05:02 PM)Dave Frederickson Wrote:
(04-24-2021 04:34 PM)Uli Wrote: I have checked the battery. It has 3,0 VDC.

The perfect time to backup your calibration constants.

I hope , I don't stress you. Can I change the battery by connecting the HP-3468A to 230VAC and making the soldering with an butan powered soldering device to avoid electrical connecting to earth? All this in hope that I don't delete the calibration, if there is still any.

Paul Berger (Canada) · 04-25-2021, 02:28 PM

(04-25-2021 05:11 AM)Uli Wrote:
(04-24-2021 05:02 PM)Dave Frederickson Wrote: The perfect time to backup your calibration constants.

I hope , I don't stress you. Can I change the battery by connecting the HP-3468A to 230VAC and making the soldering with an butan powered soldering device to avoid electrical connecting to earth? All this in hope that I don't delete the calibration, if there is still any.

That would work but is vulnerable to a power outage, I have used the method suggested by Katie Wasserman in this thread http://www.hpmuseum.org/cgi-sys/cgiwrap/...ead=176914 to replace the batteries in a 3468 and two 3421s.

Paul.

Dave Frederickson · 04-25-2021, 02:56 PM

(04-25-2021 05:11 AM)Uli Wrote:
(04-24-2021 05:02 PM)Dave Frederickson Wrote: The perfect time to backup your calibration constants.

I hope , I don't stress you. Can I change the battery by connecting the HP-3468A to 230VAC and making the soldering with an butan powered soldering device to avoid electrical connecting to earth? All this in hope that I don't delete the calibration, if there is still any.

Yes, and that method is documented on some other websites.

Once we find a better way of doing things we usually discard the old method. You pointed out the nifty HP-IL connection. Use it.

Dave

twoweims · 04-25-2021, 04:22 PM

This seems like a reasonable method as well...

https://www.youtube.com/watch?v=e-itiJSftzs&t=1510s

Dave Frederickson · 04-25-2021, 06:10 PM

(04-25-2021 04:22 PM)twoweims Wrote: This seems like a reasonable method as well...

shttps://www.youtube.com/watch?v=e-itiJSftzs&t=1510s

The battery replacement part starts a little earlier, at around 12:10.
https://www.youtube.com/watch?v=e-itiJSftzs&t=1210
This is the same as the Katie Wasserman method mentioned by Paul.

Again, when do you replace the battery?

Uli · 04-26-2021, 05:56 AM

(04-25-2021 06:10 PM)Dave Frederickson Wrote:
(04-25-2021 04:22 PM)twoweims Wrote: This seems like a reasonable method as well...

shttps://www.youtube.com/watch?v=e-itiJSftzs&t=1510s

The battery replacement part starts a little earlier, at around 12:10.
https://www.youtube.com/watch?v=e-itiJSftzs&t=1210
This is the same as the Katie Wasserman method mentioned by Paul.

Again, when do you replace the battery?

Very good video. I will make it with the 2 x AA in parallel. Ordering a new Lithium 3V is in progress. Many thanks for your help. I know what I have to do.

jklsadf · 05-02-2021, 03:30 PM

What you're seeing is completely normal. Many bench multimeters have an extremely high input impedance (>1Gohm), unlike handheld multimeters (which usually have a 10Mohm input impedance).

The 3468 has an input impedance > 10^10 ohms in the 0.3V and 3V range (input impedance falls to 10Mohm in the 30V and 300V ranges), so what you're seeing is normal.

Garth Wilson · 05-02-2021, 06:43 PM

I can imagine the problem being caused by slight leakage between pads that should not be connected. A steady DC reading will not be caused by AC noise in the area as someone suggested. Did someone get in there with a soldering iron and water-soluble flux at some point? Water-soluble fluxes have been a disaster. They are corrosive if you don't clean them off. But just as bad for our company's sensitive analog circuits, water-soluble fluxes penetrate the pores in the PC board where it's not masked, and they produce conductive paths, even if the board appears to be clean. And further, those conductive paths don't act like a quiet resistor, but make a noise like frying bacon in the audio signals we deal with. I always specify in PC-board manufacturing instructions that the holes in my soldermask files are not to be opened up any bigger, and that any pair of adjacent pads absolutely must have soldermask running between them (unless they're connected already anyway). This is more of a flux mask than a solder mask. The first time I encountered the problem 30 years ago, we had just changed assembly houses for a particular board—a rather simple one with no soldermask—and the new assembly house was using water-soluble flux because of the EPA making life difficult for them otherwise. A technician who was testing the product before shipping to customers ran into a problem right away and couldn't figure it out. He asked me for help. It didn't make sense. I had him remove various components from a net until there was nothing at all left on it, and still there was maybe 50K between that trace and another. I told him, "Then there's only one possibility left. Take your solvent and really scrub that thing. When you think you have it super clean, do it again." That fixed it.

Dave Frederickson · (This post was last modified: 05-02-2021 06:56 PM by Dave Frederickson.)

(05-02-2021 06:43 PM)Garth Wilson Wrote: I can imagine the problem being caused by slight leakage between pads that should not be connected. A steady DC reading will not be caused by AC noise in the area as someone suggested. Did someone get in there with a soldering iron and water-soluble flux at some point? Water-soluble fluxes have been a disaster. They are corrosive if you don't clean them off. But just as bad for our company's sensitive analog circuits, water-soluble fluxes penetrate the pores in the PC board where it's not masked, and they produce conductive paths, even if the board appears to be clean. And further, those conductive paths don't act like a quiet resistor, but make a noise like frying bacon in the audio signals we deal with. I always specify in PC-board manufacturing instructions that the holes in my soldermask files are not to be opened up any bigger, and that any pair of adjacent pads absolutely must have soldermask running between them (unless they're connected already anyway). This is more of a flux mask than a solder mask. The first time I encountered the problem 30 years ago, we had just changed assembly houses for a particular board—a rather simple one with no soldermask—and the new assembly house was using water-soluble flux because of the EPA making life difficult for them otherwise. A technician who was testing the product before shipping to customers ran into a problem right away and couldn't figure it out. He asked me for help. It didn't make sense. I had him remove various components from a net until there was nothing at all left on it, and still there was maybe 50K between that trace and another. I told him, "Then there's only one possibility left. Take your solvent and really scrub that thing. When you think you have it super clean, do it again." That fixed it.

The reading isn't steady, it fluctuates like the signal from an unconnected scope probe. This isn't a problem, it's predicted behavior.

Garth Wilson · 05-02-2021, 07:16 PM

(05-02-2021 06:55 PM)Dave Frederickson Wrote: The reading isn't steady, it fluctuates like the signal from an unconnected scope probe. This isn't a problem, it's predicted behavior.

The noise I mentioned being caused by water-soluble flux would also cause an unsteady reading. The head post indicated a steady reading of 2.86V though.