HP 3478A RIFA Capacitor Replacement - Page 1
Forum section: electronics / repair

author: steve1515
author_rank: Regular Contributor
timestamp: May 12, 2019, 07:23:24 pm
content: I'm planning to replace the battery in my HP 3478A and also the 4 RIFA capacitors. I noticed that there are also 4 electrolytics in the meter as well. I figure since all the caps and battery are about 40 years old, I should replace them. I've already backed up my calibration, so I should be good to go, but I had some questions...1.) If I replace the electrolytic caps and RIFA caps could it change the calibration? I figure no, since they are just on the power supply side. (Electrolytics circled in green and RIFA in red in attached image.)2.) Would Panasonic FC series be a suitable replacement for the electrolytic caps?3.) For the RIFA caps, I was planning on using the following. Do these look right?https://www.digikey.com/product-detail/en/epcos-tdk/B32021A3102K000/495-4781-ND/3489458https://www.digikey.com/product-detail/en/epcos-tdk/B32022A3223M000/495-3794-ND/1648111I've never had to replace line side caps before, so I wanted to make sure I was getting the right ones and they would be safe.4.) Bonus question... What are the C765 and C766 caps being used for? I'm not exactly sure based on their configuration. They seem to get bypassed when the power is on.Thanks for any advice! Hope to make this meter last another 40 years.

author: bd139
author_rank: Super Contributor
timestamp: May 12, 2019, 07:41:51 pm
content: 1. No it will be fine2. Yes they look fine. As long as pitch is correct. 3478A only has Y caps so that’s cool.3. Probably fine. I wouldn’t replace them though. Mine are fine.4. They absorb any switch transients and arcs. Snubbers basically.Big thing you’re missing is the calibration battery in it. They are definitely near EOL and are a pain to replace without losing cal data. Have a search around in the forum for other people who have done it. I lost the calibration data on mine. Oops.

author: steve1515
author_rank: Regular Contributor
timestamp: May 12, 2019, 08:05:25 pm
content: Thanks for the help.My RIFA caps are starting to crack a little, so I'd like to replace them. I do plan to replace the battery also. I downloaded the calibration data via GPIB, so hopefully that's all I need to restore it back after the battery is swapped out.

author: bd139
author_rank: Super Contributor
timestamp: May 12, 2019, 08:06:51 pm
content: Good to hear. Good luck

author: Miti
author_rank: Super Contributor
timestamp: May 12, 2019, 11:04:06 pm
content: I agree wit BD139 but I think he has 2. and 3. reversed. I even measured the ESR on mine. They are almost like new, don't bother replacing the electrolytics.I replaced my RIFA caps with:https://www.digikey.ca/product-detail/en/kemet/PME271Y410MR19T0/399-7491-1-ND/3459465https://www.digikey.ca/product-detail/en/kemet/PME271Y522MR30/399-5413-ND/1927358Cheers,Miti

author: steve1515
author_rank: Regular Contributor
timestamp: May 20, 2019, 11:28:00 pm
content: Hmm...I just finished replacing the caps and battery on my 3478A. I didn't do it with battery power to the SRAM and expected to lose the calibration since I already had a backup done earlier. I was ok with having to reload the calibration.Now the strange part... I power it up and the calibration is still there. I definitely had the battery out for a few minutes, so the SRAM voltage dropped to zero. I even reread it with GPIB and all is equal. Is this normal? I've never heard of this happening to anyone else. How can this be?Anyway... the operation was a success!

author: bitseeker
author_rank: Super Contributor
timestamp: May 21, 2019, 02:29:54 am
content: That's very interesting. I've never heard of that happening. Normally, you either have to solder in an aux battery or do the swap with the power on (and a non-grounded soldering iron). What are the first four digits of the serial number on your DMM?

author: xrunner
author_rank: Super Contributor
timestamp: May 21, 2019, 02:35:48 am
content: Quote from: steve1515 on May 20, 2019, 11:28:00 pmDid they look like these, which came out of my 3478A?I dunno about the memory being retained. Hmmm ....

author: steve1515
author_rank: Regular Contributor
timestamp: May 21, 2019, 03:02:02 am
content: @bitseeker: The first 4 of my serial are 2619. Does that provide any clues?@xrunner: Yup... Mine had 4 RIFA caps and they all had cracks just like yours. I was really surprised when I didn't see the "UNCALIBRATED" message. Not sure how I pulled it off, but all 256 nibbles in SRAM were retained...

author: bitseeker
author_rank: Super Contributor
timestamp: May 21, 2019, 03:20:42 am
content: Well, it's the newer 1989 design with the plastic front bezel and optocoupler on the main board. The earlier 80's design has an aluminum front bezel and toroidal transformers for isolation. I hadn't looked for other differences earlier. Perhaps there's something different about the cal RAM.

author: helius
author_rank: Super Contributor
timestamp: May 21, 2019, 05:03:52 am
content: SRAM can experience a "remanence" effect where each cell develops a physical bias towards the polarity that it held over a long period. When power is removed, the physical bias remains, and it can influence the polarity with which the cell pulls when power is restored.It's also possible that this revision of the hardware replaced the SRAM with a non-volatile memory.

author: bd139
author_rank: Super Contributor
timestamp: May 21, 2019, 06:59:08 am
content: Probably a 6T cell. They can retain state for several minutes after power down. Especially if they are cold.

author: bitseeker
author_rank: Super Contributor
timestamp: May 21, 2019, 07:09:44 pm
content: I guess I'll have to open 'em up and see if they use different SRAM. I have both generations.

author: pinpassion
author_rank: Contributor
timestamp: July 11, 2020, 01:15:22 pm
content: Hi Gang,I want to replace my 4 Rifa caps in my HP 3478A multimeter. I have the older designed meter that has the torroid couplers and vertically mounted TO5 regulator. I already replaced the old battery, no problems. Upon looking at the Rifa caps on the board, the ones hiding under the ribbon cable that connects the HP-IB interface to the board (C720 & C721) are not to hard to get at. Removing the socketed ribbon cable, and the safety shield on the board bottom will allow them to be removed and replaced.The other two caps (C765 & C766) are hard to even see, let alone replace. They are grouped around the power switch, the TO5 regulator, the power transformer, and wires connecting the regulator to the main board.If anyone has any advice on getting at the caps easily, please let me know. Typically after doing a job, you find the "easy way" to do it that might not have been apparent initially. It looks to me like I need to remove the power switch. It is connected mechanically to the front panel by a long plastic shaft. It also looks like I will need to remove the regulator assembly that is screwed on the the chassis.I want to disturb the components as little as possible, so if experience dictates a better way of doing the replacement, please feel free to chime in!!Thanks,Mike

author: bd139
author_rank: Super Contributor
timestamp: July 11, 2020, 01:32:21 pm
content: I removed the power switch on mine. Just be careful with the plastic shaft on the switch itself as it goes quite brittle. Yes I broke mine

author: pinpassion
author_rank: Contributor
timestamp: July 11, 2020, 02:13:27 pm
content: Thanks BD139. I would not have expected that. The plastic shaft "looks" stout, but I suppose years have not been kind to the plastic. I appreciate you mentioning it. I will be very careful with it.Mike