3478A combination feels-bad moment and stuff learned - Page 1
Forum section: electronics / repair

author: neverendingstudent
author_rank: Regular Contributor
timestamp: June 22, 2025, 01:45:47 am
content: So, I had a recent batch of test equipment arrive from an auction - Keithley Electrometers, Stanford Research 2.5KV supplies, some other miscellany and 3 quite old Hewlett Packard 3478A multimeters in (after cleaning) quite handsome condition.I lovingly cleaned the first one, tested the internal battery, did accuracy checks against my sources and found literal perfect DC volts accuracy after full warmup - 16.0000V reading from my Advantest R6144 - just beautiful. Lovely amounts of shiny gold in the board inside, make 'like they used to'.Had finished documenting it, writing up a quite glowing summary of its condition and functionality, then posted on eBay... and only then did I learn about RIFA capacitors and how they are practically a guaranteed horrifically bad-smelling failure ticking on a countdown to betray the board they are sworn to protect. That and then I learned about the near-vertical dropoff of the CAL RAM battery performance curve, meaning that I should replace that as well if I'm doing a proper refurb.Sigh.Plus side, I learned about RIFA capacitors without having to smell one. That and the voltage cliff that Lithium Sulfur Dioxide battery chemistry jumps off at end of life.Down side, it feels bad to put all that time into getting one of these ready for sale only to realize I was about to sell a smelly time-bomb waiting to happen.

author: chilternview
author_rank: Frequent Contributor
timestamp: June 22, 2025, 08:05:59 am
content: RIFA caps aren't hard to change, and there aren't that many of them. You've not lived till you've smelt that smell!As to batteries, well, yeah, they don't last forever. They would certainly be on my list to change.

author: neverendingstudent
author_rank: Regular Contributor
timestamp: June 22, 2025, 02:35:27 pm
content: I'm eyeballing and checking the other 2 so I know for sure what and how many parts I need to order, and in checking the battery of the second unit it read as 3.4V.The battery is labeled as 3V (which is what I expected), and it shouldn't be producing more than a tiny bit above that if it is the correct Li-SO2 battery chemistry. I guess the CAL RAM tolerates that voltage well enough since it is functional and accurate, but it was unexpected to find.I also just found out that there are 4 RIFA caps instead of the 2 larger/more obvious ones I already noticed (saw the 22nF caps right at the transformer first, only just now noticed the 2 1000pF (not sure why it isn't labeled 1nF) under the HPIB/GPIB cable.Quote from: chilternview on June 22, 2025, 08:05:59 amGiven I've read the smell gets compared to Surströmming I humbly choose not to have that experience if at all possible.

author: J-R
author_rank: Super Contributor
timestamp: June 23, 2025, 06:36:22 am
content: 3.4V does seem a little odd for a 3V lithium primary. Typically it's closer to 3.2V. Maybe it's a custom chemistry mix to reach a slightly higher nominal voltage? Or a previous owner inadvertently used a lithium ion? Photos of it? Does the voltage change when the unit is powered off vs. on?It's not really possible to tell the remaining capacity of these cells, so for resale it would make sense to replace them along with the Rifas.I also assume you are going to back up the calibration data before the battery replacement? Providing that backup to the buyer would also be a nice gesture.The accuracy of these is somewhat expected since they are only 5.5 digits and use an LM399 reference. However, they don't seem to command a very high price due to various issues as you've seen, and also due to a very limited feature set. I've seen others suggest these for digit hunters on a tight budget and that seems reasonable to me as well especially if you can verify each unit's performance.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 23, 2025, 07:36:55 am
content: The 3478 are nice, as they have a 30 mV range and thus way can resolve 0.1 µV. So they are nice to use with small voltages like thermocouples and similar.

author: pqass
author_rank: Super Contributor
timestamp: June 23, 2025, 08:32:17 pm
content: Here are some useful links for the 3478A.RIFA cap replacements: the caps I purchasedhere, or readthis thread.One method to change the 3.6V batteryhere, and part 2here. I explain how to extract (via GPIB interface) and verify (use latest script link below) the cal data FIRST before touching the battery.Care must be taken such that your grounded soldering iron tip doesn't short the battery, removing power to the cal RAM.How to make a GPIB to serial (USB) with just an Arduino UNO and an IEEE488 connector. See the AR488 Project discussedhere(follow the github link in the message).A minimalist way to read/write the cal data, seehere. Or usethisWindows GUI program instead.Verify the extracted cal data with my latest scripthere. Examplehere.

author: krish2487
author_rank: Frequent Contributor
timestamp: June 23, 2025, 09:17:16 pm
content: To make it a more permanent fix.. look at this thread.https://www.eevblog.com/forum/projects/3478a-fram-modification/?allLook at the gerbers and boards by@Dan NIIRC there should be a link to OSH park project somewhere there .. or I can dig it up..As others have mentioned.. first backup the cal data.. then play around with the replacements... and I cant stress this enough.. make the FRAM mod..I made the change to my 3478A and I m a happy guy..

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: June 24, 2025, 04:23:37 am
content: Quote from: chilternview on June 22, 2025, 08:05:59 amHoo-hoo that smell!

author: neverendingstudent
author_rank: Regular Contributor
timestamp: June 24, 2025, 06:21:42 am
content: J-R: the battery looks like it has been previously user-replaced, going by the differing look of the solder joins. It is a yellow battery labeled 'MATSUSHITA ELECTRIC JAPAN' and 'P/N 227322'. Other than 'warning don't do stuff to make this go boom' label, the other relevant portion is face down on the board, but I did find a picture of the same battery from someone working on an old camera:http://www.jollinger.com/photo/cam-coll/images/kodak/disc/kodak_disc_8000-batteries.jpgInterestingly, in that picture it IS labeled as 3V, but no definite display of what battery chemistry. Other than that one page about the camera (http://www.jollinger.com/photo/cam-coll/cameras/Kodak_disc8000.html) I really cannot find ANY data on that specific battery.I'll attach my pictures of it at the bottom of this post.Plan is to swap the caps and batteries just so I can put a sticker label on the new batteries with the date code of installation.Also planning on both backing up cal data, and supplying 3V to where-ever it is I need to hook it up to keep the CAL RAM live during the replacement - the cal backup will be a... well, backup plan.Do these use LM399? I'm not so sure. The voltage reference is pretty obvious visually and that isn't what it is labeled as, I have:'some mfg symbol I'm not familiar with, maybe CT?''1826-''1249-5''8811'I'll include a picture of that as well.As for price, I figure I can ask quite a bit more than the average so long as I demonstrate all the issues these have, that they have all been addressed, and the accuracy of the units. I've had a pretty good track record with providing much higher than average quality in exchange for getting higher than average prices.EDIT: removed 1 incorrect picture of a different battery.

author: neverendingstudent
author_rank: Regular Contributor
timestamp: June 24, 2025, 06:28:58 am
content: Sorry, uploaded the wrong picture of a normal battery from one of the other units instead of the voltage reference.Here is the reference (pretty sure).

author: Kleinstein
author_rank: Super Contributor
timestamp: June 24, 2025, 06:41:56 am
content: They use a LM399 (not sure about A version of possible selection) with a HP custom part number. So the reference is quite a bit better than typical for a 5.5 digit meter.The manufacturer symbol on the reference is Linear Technology (LT), so the normal source for a LM399.On the downside the display is a LCD with relatively poor contrast. At least it is not getting dim over time as old VFDs.

author: neverendingstudent
author_rank: Regular Contributor
timestamp: June 24, 2025, 06:48:12 am
content: Quote from: Kleinstein on June 24, 2025, 06:41:56 amThank you for the tidbit about it being a differently-labeled 399! I really appreciate learning that.I've seen a backlight mod for these meters, but as far as I like to go to sell nice things in good condition, I feel that would be a step over the line for me in terms of labor and materials vs what I could charge as the maximum realistic price, no matter how nice I make it. But that said, it is worth noting for the hardcore enthusiast that there is a documented backlight mod to be had.https://www.eevblog.com/forum/repair/hp-3478a-dmm-lcd-lighting/https://www.eevblog.com/forum/testgear/hp3478a-backlight-my-version/msg1506655

author: TERRA Operative
author_rank: Super Contributor
timestamp: June 24, 2025, 04:11:19 pm
content: Have you seen this?https://www.eevblog.com/forum/testgear/hp3548-hp3457-oled-display/I have a few displays left from my last PCB assembly run if this sort of thing interests you..

author: neverendingstudent
author_rank: Regular Contributor
timestamp: June 25, 2025, 12:02:05 am
content: Quote from: TERRA Operative on June 24, 2025, 04:11:19 pmI had not seen that before. Very handsome! Probably also on my 'not fiscally viable to install in for-sale units', but still, very nice.

author: Smokey
author_rank: Super Contributor
timestamp: June 25, 2025, 12:14:45 am
content: All this stuff still for sale? what's your ebay?

author: TERRA Operative
author_rank: Super Contributor
timestamp: June 25, 2025, 12:26:35 am
content: Quote from: Smokey on June 25, 2025, 12:14:45 amThe OLED display?I don't have it listed on ebay, but PM me if you are interested in one.

author: Smokey
author_rank: Super Contributor
timestamp: June 25, 2025, 01:08:45 am
content: Sorry, I meant the OP.Quote from: neverendingstudent on June 22, 2025, 01:45:47 am

author: neverendingstudent
author_rank: Regular Contributor
timestamp: June 25, 2025, 01:23:46 am
content: Quote from: Smokey on June 25, 2025, 01:08:45 amFor the 3 meters I'm currently working on I ordered parts last night from Digikey (replacement Y2 safety caps and batteries) and they shipped this morning, so I should start having them available to document and post on eBay in a few days. The rest of the lot is in the queue but not yet tested/cleaned/listed.My eBay store is here:https://www.ebay.com/usr/mehlisaaCurrently on my bench is one of my remaining 3 Sony/Tektronix AFG310 function generators. Recently had a Facebook Marketplace customer buy a scope from me and when I mentioned I have a ton of T&M and asked if he needed anything else, he mentioned wanting a function generator so I serviced one and sold it to him (front panel membrane buttons were worn / the conductive backing was not so conductive anymore). Since that was reasonably simple and didn't need any parts ordered, I'm going to service those 3 next, keep 1 for the bench (they are rather nice units) and sell the other 2. Oh, correction, I will be getting parts - they have some missing rubber feet I'm ordering off AliExpress.I recently posted about the AFG310 units here:https://www.eevblog.com/forum/repair/best-method-to-restore-conductive-keypads/msg5957011/#msg5957011EDIT:Oh, and sadly the older of the 2 Keithly electrometers (a beautiful 610C) did not survive shipping - the transformer got ripped away from its mounting and pulled all the wires from the board. I was really sad about that. Ended up getting a 50% refund on the whole lot since the auction house shipped everything in the exact way I requested they not do so, so kind of a wash I guess. Maybe I can find a schematic on the 610C to reconnect the transformer and see if it is something I can bring back to life. And no, neither electrometer came with cables - I only have the one uncertain-quality Chinese cable I bought for my other 617.EDIT 2:Smokey's question about eBay made me think I should add that to my profile / post signature, so I did. If anyone looking back on this wonders why they didn't notice it, it's because it wasn't there yet.