HP/Agilent 34401A - Blown tantalum caps - what replacement? - Page 1
Forum section: electronics / testgear

author: amaschas
author_rank: Regular Contributor
timestamp: February 21, 2025, 11:02:38 pm
content: Hi all, I recently acquired an ebay 34401A that lured me into a false sense of security with voltage measurements more accurate than any of my current meters. About an hour into testing it however, I detected a burning smell coming from my bench and pulled the meter. Turns out two SMD tantalum caps on the board have blown. I think at this point I am going to replace all of the tantalums on the board and throw in the bulk electrolytics for good measure. For those who have done this before, do you recommend replacing the old tantalums with new tantalums?

author: dimmog
author_rank: Regular Contributor
timestamp: February 21, 2025, 11:08:30 pm
content: Quote from: amaschas on February 21, 2025, 11:02:38 pmIf you have a Facebook account, you can find the BOM in this post:https://www.facebook.com/share/p/18b8ECFqX3/from my 34401A Facebook group.

author: tautech
author_rank: Super Contributor
timestamp: February 21, 2025, 11:17:34 pm
content: Nothing wrong with modern Tants when used correctly with sufficient downrating.50% or more is advised....

author: Kean
author_rank: Unknown
timestamp: February 22, 2025, 12:18:10 am
content: Quote from: tautech on February 21, 2025, 11:17:34 pmI agree. Use higher voltage ratings if possible, whatever fits the footprint and your budget.Use decent quality parts, like AVX, Vishay or Kemet brands bought from DK/Mouser, not something from eBay/AliExpress.There are some fancy polymer parts, but without circuit analysis to check the usage there is a small chance you may introduce issues.

author: David Hess
author_rank: Super Contributor
timestamp: February 22, 2025, 12:32:10 am
content: I would use replacement solid tantalum capacitors, but with increased voltage derating if they were not already 2/3 or 1/2 voltage derated.If you do use aluminum electrolytic replacements, then they will need to be about 4 times the capacitance to achieve the same low impedance.

author: wraper
author_rank: Unknown
timestamp: February 22, 2025, 12:46:35 am
content: Replace bad ones, do not bother with the rest, it's not like they dry out over time. If you are really eager to replace more, then replace those of the same type which failed.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: February 22, 2025, 12:07:32 pm
content: from dimmog's facebook page, the BOM (according to Mit Mogylevski) seems to be:KEMET 47uf x 20V T495D476K020ATE150 2pcsKEMET 22uf x 35V T495D226M035ATE300 6pcsKEMET 1uf x 35V T495B105K035ATE1K5 6pcs (3 main board, 3 display board)KEMET 4.7uf x 16V T495B475K016ATE800 1pcs (possibly 2pcs)Chemi-Con 1000uf x 50V EKY-500ELL102ML25S 2pcsChemi-Con 2200uf x 25V EKZM250EC5222ML20S 1pcsChemi-Con 4700uf x 25V EKY-250ELL472MMP1S 1pcswhile i'm familiar with kemit, who is chemi-con? it would be useful if some kind soul could flash-out this list with part designators from the schematics, along with the actual voltage each one is likely to be exposed to. yes - i could find this info myself, but hopefully someone else already has the information recorded somewhere. i've not seen it all brought together before, and it could be useful.cheers,rob :-)

author: factory
author_rank: Super Contributor
timestamp: February 22, 2025, 12:37:43 pm
content: These are what I used, original parts are listed first, along with replacements from Kemet low leakage range;0180-4116 22uF 20V (15V across) x6 T491D226K020AS D, replacement = 35V T489D226K035ATE3000180-3744 4.7uF 10V (5V across) x1 T491B475K010AS B, replacement = 25V T489B475K025ATE1K00180-4228 47uF 10V (5V across) x2 T491D476M010AS D, replacement = 25V T489D476K025ATE2500180-3751 1uF 35V (18V across) x3 + x3 on display T491B105K035AS B, replacement = 35V T489B105K035ATE2K0I didn't actually bother to replace the ones of the display board, as they are under the display, I decided not to disturb it.You might also need some Zeners, two failed in mine due to the 20V tants on 15V rails shorting (only these 2 Zeners were replaced), I used some slightly tighter tolerance parts from Onsemi;1902-1541 3.3V BZX84C3V3 x6 = Onsemi BZX84B3V3LT1GThe electrolytics were fine, but I ended up replacing them too, these are what I used, I had the Nichicon UBTs in stock already, some are slightly taller than the original parts;0180-4433 16x25mm x2 = 1000uF 50V UBT1H102MHD (31.5mm)0180-4435 16x25mm x1 = 2200uF 25V UBT1E222MHD (31.5mm)0180-4434 18x35mm x1 = 4700uF 25V UBY1E472MHL1TN (16x25mm)David

author: Fungus
author_rank: Super Contributor
timestamp: February 22, 2025, 01:41:59 pm
content: Quote from: robert.rozee on February 22, 2025, 12:07:32 pmYou never heard of Nippon Chemi-Con?https://chemi-con.com/

author: robert.rozee
author_rank: Frequent Contributor
timestamp: February 23, 2025, 01:44:55 am
content: i have heard of nippon, just not chemi-con. i presume one took over the other?cheers,rob :-)

author: wraper
author_rank: Unknown
timestamp: February 23, 2025, 02:06:17 am
content: Quote from: robert.rozee on February 23, 2025, 01:44:55 amNo, LMAO. Nippon means Japan. Nippon Chemi-con is Japanese company. United Chemi-con is their US division.

author: Fungus
author_rank: Super Contributor
timestamp: February 23, 2025, 03:07:33 am
content: A highly respected maker of capacitors....

author: robert.rozee
author_rank: Frequent Contributor
timestamp: February 23, 2025, 10:46:15 am
content: looks like i may be showing my age!i would have sworn that "nippon" was the brand name associated with capacitors, but it has been a couple of decades since i was last involved in parts selection in a commercial environment. out of interest i had a look online, and indeed can see "nippon chemi-con" used in capacitor branding. also "nippon chemi" in some places. the one definitive example of "nippon" alone is here:https://www.nippon-chemi.com/About_nippon/where there is a section titled "About NIPPON". oh, and a few fringe resellers with dubious branding, such as:http://www.jjyaudio.com/productview.asp?sid=3663cheers,rob :-)

author: factory
author_rank: Super Contributor
timestamp: February 23, 2025, 12:08:25 pm
content: There is also Korea Chemi-con, no idea if they are part of the same group or not, I suspect not.And SamYoung were using the Chemi-con logo with a dot in the middle, these are known to be crap.David

author: wraper
author_rank: Unknown
timestamp: February 23, 2025, 01:42:20 pm
content: Quote from: factory on February 23, 2025, 12:08:25 pmSamyoung was established as a joint venture with Nippon Chemi-con. Dunno if there is any ownership still left. They're fine. They had a few bad series when capacitor plague happened, But Chemi-con had at least two bad series too.

author: David Hess
author_rank: Super Contributor
timestamp: February 23, 2025, 02:14:51 pm
content: The identity of each company making capacitors recently came up for me when I noticed that Illinois Capacitor, about as an American sounding name as you can get, had tariffs, but Chemi-con did not. These tariffs came from Biden and not Trump.

author: Kean
author_rank: Unknown
timestamp: February 23, 2025, 02:45:08 pm
content: Quote from: David Hess on February 23, 2025, 02:14:51 pmAs I understand it, any tarrifs will depend on the source you are buying from (or the distributor has sourced them from), rather than the company headquarters. Illinois Cap had headquarters in North America (Illinois surprisingly), but also a Asia (HK). In any case, Ill Cap were bought out by Cornell Dubilier about 10 years ago, and then by Knowles in 2023 (with HQ in Illinois!).In the case of Chemi-con, I would presume you were buying parts that came from United Chemi-con (also with HQ in ... yes, Illinois), and thus were either wholly made locally or more likely were already imported (as raw materials or finished goods) and had any tarrifs absorbed into the price.

author: David Hess
author_rank: Super Contributor
timestamp: February 23, 2025, 06:18:06 pm
content: Quote from: Kean on February 23, 2025, 02:45:08 pmThat was what I concluded when I looked into it. Even with the tariffs, the foreign sources parts were less expensive than the American ones.

author: amaschas
author_rank: Regular Contributor
timestamp: February 23, 2025, 07:29:38 pm
content: Thanks everyone, this was all super helpful! Just put together an order. One of the blown caps burned the board pretty badly, not entirely through but sufficient to see the internal weave of the substrate, so it looks like I'm going to have to do some trace surgery/epoxy fill as well.

author: nctnico
author_rank: Super Contributor
timestamp: February 23, 2025, 07:57:13 pm
content: I'd go for the modern polymer tantalums in this case. From what I have gathered, these have a much less dramatic failure mode compared to the MnO types. I wouldn't replace tantalums with electrolytic (and vice versa) as the circuit may rely on a specific ESR range.

author: David Hess
author_rank: Super Contributor
timestamp: February 24, 2025, 09:43:38 am
content: Quote from: nctnico on February 23, 2025, 07:57:13 pmIf the circuit relies on a specific ESR range, then the much lower ESR of polymer tantalums is also a problem.

author: amaschas
author_rank: Regular Contributor
timestamp: February 27, 2025, 09:04:24 pm
content: Figure I will just document my repair here for future interest. This board appears to have had _two_ previous tantalum cap failures in addition to the one that happened when I was testing it. Hence my sense that all of the tantalums should be replaced. Seems like C312 and C323 had previously failed and were (poorly) replaced, and then 322 failed during my tests and took CR305 with it. I will probably replace CR307 for good measure. I'm also suspicious that the U552 transistor was replaced at some point. The board is absolutely coated with some gunk—looks like someone plastered flux everywhere. My plan is to dremel out as much of the burned stuff as I can, then fill the resulting holes with conformal epoxy, manufacture some new pads and get it looking as good as possible with some isopropyl and a toothbrush.I have high hopes for this machine since as ugly as it looks, it was actually still functioning with the burned out caps and showing very accurate voltage measurements.

author: RolandK
author_rank: Regular Contributor
timestamp: July 15, 2025, 01:34:45 pm
content: The 34401a makes 15V just from the 18V by some zener diodes (+/-15V). Therefore every ripple from the 18V side is transfered to the 15V side.The 18V side is stabilised with electrolytics, which are heated by the linear voltage regulators. If the 34401 spends it's life in a hot rack the electrolytics will degrade over time.Then there is some ripple, which is transfered via the zener diodes to the tantals. Tantals don't like ripple. They may fall with a short and kill the zener diode and/or one of the opamps at the 15v source.So if you have a defect tantal allways check the voltages at all electrolytics for AC ripple. I just use an ohmmeter at the electrolytics (care for polarity). The resistance should rise slow to some kOhm (inverse proportional to it's capacitance). If it goes to only some 10 Ohm you have a leakage somewhere. If it raises quick, you have low capacitance. Capacitance is easier to check with a C-Meter.

author: David Hess
author_rank: Super Contributor
timestamp: July 15, 2025, 04:02:56 pm
content: Quote from: RolandK on July 15, 2025, 01:34:45 pmIf zener diodes are being used as shunt regulators, then there is a series resistance limiting the current and ripple. Between the resistance and zener diode, incoming ripple is considerably attenuated, and the tantalum capacitors will not have a problem with that.Tantalum capacitors handle moderate levels of ripple fine. What they do not handle is excessive dE/dT, which leads to high surge current when charging, like if they are attached directly to a DC input by a low impedance switch. For this reason they are not recommended as input capacitors where the input power may be applied suddenly. As part of a zener shunt regulator powered from an earlier stage which has high capacitance, they should be fine.The problem may be due to not enough voltage derating of the solid tantalum capacitors.

author: iMo
author_rank: Super Contributor
timestamp: July 15, 2025, 04:17:52 pm
content: The zeners are not wired as the shunt regs, but as simple serial voltage droppers..

author: David Hess
author_rank: Super Contributor
timestamp: July 15, 2025, 09:22:53 pm
content: Quote from: iMo on July 15, 2025, 04:17:52 pmThey used 3 volt zener diodes?That is the sort of thing I would expect from +/-18 volt regulated supplies and a mix of 40/44 volt linear and 36 volt linear ICs, although it brings up why the higher supply voltage was used at all.

author: RolandK
author_rank: Regular Contributor
timestamp: July 16, 2025, 07:19:34 am
content: Exactly this is what they do. They have eg. a 1000 µF electrolytic behind the bridge rectifier, a linear voltage regulator for 18V, a 1 µF tantal to stabilize the 18V, then a 3,3V Zener voltage drop and a 22µF tantal at the 15V. If the 1000µF fails, the tantals get the voltage ripple - exactly that excessive dE/dT they don't like.If the tantals in this circuit fail - check the electrolytics.The +/- 18V are for the input stages with 10V range. There are three +/-15 V sets: two for the AC Circuits incl. RMS converter, one for the AD converter. The DC amplifier uses a 4.7V zener drop for the supply of two AD706.

author: iMo
author_rank: Super Contributor
timestamp: July 16, 2025, 08:20:26 am
content: The zeners are 3pins smd one, 3.2V (perhaps 3.3V in some newer revisions). Pretty shocking to see how the +/-15V for the ADC (etc.) have been created in a 6.5digit HP meter..PS: the "+/-18V" is somehow higher, like +/-18.5V (set by the two LM317/LM337). So tantalums for 35V would be the way to go..

author: David Hess
author_rank: Super Contributor
timestamp: July 16, 2025, 01:27:23 pm
content: Quote from: RolandK on July 16, 2025, 07:19:34 amThat level of voltage ripple is not enough.