Keysight DMM 34465A Service Note -  ACV Problem - Page 1
Forum section: electronics / testgear

author: subliti
author_rank: Contributor
timestamp: March 27, 2019, 06:22:02 pm
content: HiI have some questions about this problem. The service note (http://literature.cdn.keysight.com/litweb/pdf/34465A-02.pdf) states thatQuoteQuoteQuoteSo what does this mean exactly...Does it mean that while having a firmware version below 2.17ANDdoing extensive changing from othermeasurement functions to ACV causes a component to heat up and "fail" ?So i conclude that having a firmware version below 2.17 and ACV mode was never used before , one is not affected by this issue and everything is ok ? Ofc the potential issue itself would remain, but if i upgrade to firmware 2.17 or greater, the issue is eliminated ?Is my understanding of this correct ?Also..Quote1V what ? Vpp or rms ?Thanks

author: Keysight DanielBogdanoff
author_rank: Unknown
timestamp: March 27, 2019, 07:46:00 pm
content: I'm not certain this service note was intended to be public, but here are my thoughts.1: For definitive answers, call your local service center (link in my signature)2: My answers are "best effort" and not guaranteed to be 100% accurate. See #1 above3: I'd definitely upgrade your firmware. Not going to hurt4: I think 1Vpp will be fine. Either RMS or peak to peak should be effective. It's not this specific amplitude and frequency, it's more that this type of input will cause it to exhibit symptoms.5: Based on my experience with these service notes, if you haven't used ACV a LOT then you should be fine. I suspect it was something that popped up in manufacturing environments.

author: mr.fabe
author_rank: Regular Contributor
timestamp: March 27, 2019, 08:40:33 pm
content: I recently had my 34461A repaired by Keysight for this exact issue with the ACV. My unit had v1.0 and ACV measurements were all over the place. There is a component that overheats and causes the measurement failure. The fix is to replace the component and recalibrate the unit. Contact Keysight for guidance to see if your unit is one of the affected meters. You have until April 17th, 2019 if you need service on your meter. Not all units within the range of serial numbers are having this issue and a simple FW update will be fine. The service that I have experienced from Keysight has been the best I've seen in a long time from any company. From the initial call, followup call and proactive emails from Keysight, to the overnight courier hand delivery of my unit to me at home.Just so you know, the replacement component for my unit was not available at the time of repair so they swapped out the unit for a new one with current calibration docs and data.

author: subliti
author_rank: Contributor
timestamp: March 27, 2019, 09:55:11 pm
content: Thanks for sharing your experience @mr.fabeQuote from: Keysight DanielBogdanoff on March 27, 2019, 07:46:00 pmThanks for your fast answer. I have not used ACV at all, thats why im asking...because to me it sounds like that 2 conditions must be met before the fault shows up/develops:1: firmware version below 2.172: "extensive" changing from other measurement functions to ACVOnly point 1 is true for me, and as i said i never used ACV mode, so my assumption was that my DMM must be ok (since only extensive changing from other measurement functions to ACV causes this problem) and if i update to firmware version 2.17 im definitely ok and could switch to ACV mode all day whithout something happening. After reading the service note i switched it to ACV and tested it with a 1 Vpp 1Khz sine and read like 0.358 V if i remember correctly.So i wanted to ask here to go the easy way instead of going through the hassle contacting Keysight. But maybe it would be a good idea.I already updated to version 2.17 (yes i know there is a higher version available).

author: DaJMasta
author_rank: Super Contributor
timestamp: March 27, 2019, 09:59:16 pm
content: From the service note, it appears that older firmwares allowed the overheat to be a problem, so unless you reached that overheat threshold to cause excessive drift before the firmware revision that prevented that from happening (through whatever means they use), you shouldn't have any issue unless symptoms were already present.

author: subliti
author_rank: Contributor
timestamp: March 27, 2019, 10:10:40 pm
content: Quote from: DaJMasta on March 27, 2019, 09:59:16 pmYou explain it clearer and shorter than i could. Thanks.Thats what i also thought, since i never used it i should be safe. But i just wanted some opinions on that, because it would be a nightmare if i have to ship the DMM to Keysight.So the only thing that could be done to test if everything is ok is to apply a sine wave and see if the meassurement makes sense ?Well maybe i should have asked mr.fabe whatQuote from: mr.fabe on March 27, 2019, 08:40:33 pmreally ment.

author: mr.fabe
author_rank: Regular Contributor
timestamp: March 27, 2019, 10:40:39 pm
content: I normally don't measure ACV on my bench meter and when I was doing some testing on an AC power source, 30VAC was reading like 2VAC. I checked the AC measurements against two handhelds and confirmed that the 34461A was not measuring the AC voltages correctly. If you have the same issue, your measurement readings will be substantially off. Not even close.

author: KE5FX
author_rank: Super Contributor
timestamp: March 28, 2019, 12:06:53 am
content: Quote from: Keysight DanielBogdanoff on March 27, 2019, 07:46:00 pmEvery company has these service notes that their maintenance people use to remember where all the bodies are buried, but only you guys disclose them to customers, as far as I can see. That is why my DMM says "Keysight" on the front panel.(Well, actually, it says "Agilent," but the firmware says "Keysight," so...)

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: March 29, 2019, 12:41:18 am
content: I would presume it is RMS as this is the default readout under AC measurement.I agree with the comment about Keysight service, I have had nothing but superb service from them. You get what you paid for, it is definitely worth the premium.Quote from: subliti on March 27, 2019, 06:22:02 pm

author: subliti
author_rank: Contributor
timestamp: March 29, 2019, 10:34:36 pm
content: So a little update here..I send a mail to keysight asking about this problem and what to do and they only wrote that this problem is solved in firmware v. 2.17 and that i should update it and i would be fine. Tbh i expected a bit..moreBecause the problem as a whole is more than just updating the DMM.So i wrote again, explaining the problemand said that i was using a firmware below 2.17 the whole time and that the failure could have developed because of this, even though i didnt really used ACV in that time. I also asked if i could test it somehow to be 100% sure im not affected.I got an answer that it cant be tested and i should send the DMM in and they would replace the "faulty" component and then i can be 100% sure.Yeah, idk what to do. Maybe i should send it in, than i can be sure and if they open the unit and replace something i also get a calibration for free.I hesitate to send it to keysight because the shipping companys here treat everything like shit. They offered me to send it in or if i dont want they would come and take it. I dont know if this comes at an extra cost.

author: DaJMasta
author_rank: Super Contributor
timestamp: March 29, 2019, 11:14:49 pm
content: Don't bother, I don't know why updating the firmware seems like that unusual of a fix. It sounds to me like when using the AC measurement system a ton (it was discovered in production environments years after its release), the part eventually was damaged because of heat or voltage or some other factor. The more modern firmware prevents that system by manipulating the frontend/measurement system.Maybe it was an issue where 100% duty cycle would damage it eventually and the current firmware reduces that to a level that is sustainable (given the time to fault of the part, it was close)Maybe it was an issue where a larger voltage was being applied to it than should in fringe cases, so maybe it switches ranges better or manipulates signal conditioning to avoid that internal overload condition with the new firmwareMaybe it only happened with a high interior temperature and by ramping up fan control or reducing usage when warm is sufficient to avoid itThere are a lot of controllable parts and a lot of things being monitored in the unit, it seems perfectly feasible to me to think that you can prevent long term damage to a part by changing settings in firmware without effecting rated specs. Keep in mind this isn't like the input amp or any of the first-stage signal conditioning parts, it's specific to the AC measurement system regardless of the range, it's not just cooking a part in the frontend which is exposed and difficult to protect with software settings.My guess is replacing the component is actually just replacing the part with the same one that was there and then updating the firmware. No need to change the actual part or circuit if just running current software can prevent the damage.

author: TheSteve
author_rank: Unknown
timestamp: March 30, 2019, 12:30:48 am
content: Why not update the firmware and then measure a 1 volt 1 kHz AC signal - if it is correct you're fine, if its way off then U507 is damaged and need to be replaced. You can double check by opening and checking its temperature just like the service note suggests.

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: March 30, 2019, 02:04:58 pm
content: I agree with both two previous responses, especially when you don’t even use the AC measurement in the past and has a unreliable shipping service where you live.

author: mr.fabe
author_rank: Regular Contributor
timestamp: March 30, 2019, 06:05:09 pm
content: If it were me, I would send the unit sent to Keysight. Why have a current model DMM that is not fully functional? The benefit of trusting that your test instrument is performing accurately is one that shouldn't be overlooked. Calibration alone is worth more than the expense you would pay for transporting your unit to Keysight. If your meter is still on a firmware version lower than v2.17, it's probably over a year old and could possibly use a "tune-up".You have unit mid-April to decide if you decide to take advantage of the memo.

author: VK5RC
author_rank: Unknown
timestamp: March 31, 2019, 06:30:01 am
content: As a total aside, my very old 3458a's AC board decided to die, I bought a reconditioned one, but it needed calibration - not having the required $$$ AC-DC converters I used my agilent 33521 and my 34461, to do a very rough calibration. A few months later the 3458 headed off to Keysight Australia for a proper cal - that's right it passed!Mightily impressed.Rob

author: subliti
author_rank: Contributor
timestamp: April 04, 2019, 11:05:29 am
content: Excuse my late response, i had no access to my computer.Quote from: DaJMasta on March 29, 2019, 11:14:49 pmI also believe that, but since i was running fw <2.17 till a few days ago i was afraid of possible dmg, even though i actually never used ACV...yeah maybe one time i switched to it and thats it.Quote from: TheSteve on March 30, 2019, 12:30:48 amI dont have the guts to open my unit, maybe in a few years. But i tested it with a cheap wavegen. A 2.83 Vpp, 1kHz sine reads as 0.998 ACV on the 34465A and like 1.001 ACV on a cheap true rms handheld dmm, so i guess it must be ok.But all of this just leaves an unpleasant feeling in my guts. My decision to buy a keysight dmm and pay "big money" for it was because i could (and wanted to) trust it like no other thing.Ofc, trust is such a thing...their new psu 363xx line had these voltage spikes when plugged into mains, totally unacceptable, idk how such a basic thing could pass QC, but it did. I expect this from cheap stuff, but not from keysight. So yeah..mr.fabe said that while meassuring 30 VAC, it showd up as like 2 VAC, ofc thats easy to spot, but maybe the differences could be more subtle on other units. Maybe there are some other users out there that were affected by this bug like mr.fabe and could report here what they meassured.

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 04, 2019, 11:25:57 am
content: Quote from: subliti on April 04, 2019, 11:05:29 amSo if you did not use ACV up to now, and as the reading is in close accordance to another DMM, your unit is DEFINITELY fine.The described error was a very coarse mis-reading, but NOT a slight deviation.So there's really no reason to either have a bad guts feeling about that SN, and also about simply opening the instrument and have a look inside.Perhaps you've noticed, that the 34465A has no cal sticker or 'Guarantee Void' over the screws or across the case halves, in contrast to other manufacturers.That's due to the necessity to access the internal fuses in case of an over current.So you're literally invited by KS to open the 34465A, without any blame, and you can't hardly damage anything just by opening.Just don't power it up when open, and do not touch the PCB.Frank

author: subliti
author_rank: Contributor
timestamp: April 04, 2019, 09:57:07 pm
content: Yes i guess that you are right and tbh i also expected since the beginning that everything should be ok, but since i treat my 34465A like gold and im also not an expert and i also dont have the possibilities the test things to the extremes to verify if something is wrong, then everything that could be problematic i just start to raise my eyebrows and start to have a bad feeling.

author: JDubU
author_rank: Frequent Contributor
timestamp: April 05, 2019, 04:11:55 pm
content: Quote from: Dr. Frank on April 04, 2019, 11:25:57 amAlso, according to the service manual, the metal shield that covers the voltage reference area of the PCB cannot be removed (or even unscrewed) without affecting the calibration.Service manual, page 490:"CAUTION Do not turn the screw that fastens the shield in place, as this will affect the instrument's calibration."

author: mr.fabe
author_rank: Regular Contributor
timestamp: April 05, 2019, 10:14:36 pm
content: Dave also mentions about the shield removal affecting the calibration warning in his 34461A teardown video at 12:38. I heard a "Yeah, right"