HP 419A repair - Page 1
Forum section: electronics / general

author: Rax
author_rank: Super Contributor
timestamp: March 04, 2023, 07:04:36 pm
content: Hi all,Got one of these in a "lend-for-purchase" sort of thing from a good friend. I'm kind of going through an assessment of condition, and after removing the crud from very extended storage (probably decades) - which makes it pretty handsome at this point - and turning it on, there's needle movement and other good signs, but really, that's as far as I got with the operation kind of thing. I am not probably going to go further than that with operating it before I do a full recap job.I opened it to see about the internal batteries, and as expected, they leaked. It doesn't look like it's very messy inside (probably, all had well dried out before I was born), but, strangely, it seems the leak essentially decomposed the plastic mount for the batteries, which is crumbling. I am literally picking up pieces with tweezers from the inside.I don't seem to find very good data on repair of these - I am wondering what have others replaced the batteries with? I am assuming maybe NiMH chemistry, or maybe some have gone further afield and redid the charging circuitry and went Li-ion?Thank you for your pointers and feedback in advance.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 04, 2023, 07:34:51 pm
content: They were NiCads, like everything of that vintage. I've got two that need work and are in the "when I get a roundtuit" catagory. Not much info other than the manual, which you should be able to download.

author: Rax
author_rank: Super Contributor
timestamp: March 04, 2023, 08:09:53 pm
content: Quote from: Conrad Hoffman on March 04, 2023, 07:34:51 pmThank you, Conrad. That's why assumed NiMH would be a drop-in swap.I am seeing an HP part# 1420-0015 for BT1 through 4 (6.25V) and 1420-0004 for BT5 (1.35V... mercury!). I've seen a thread where the latter was replaced with a little module, so that's one option. It also looks like the mount for BT5 was different, so a different form factor it must be.I don't seem to find much more than this. Keysight part number lookup finds the former, but not the latter, and there's no replacement listed.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 04, 2023, 08:43:02 pm
content: For the mercury battery a similar sized siver cell is the best direct 1:1 replacement available. It is a bit less stable and higher voltage, but this should not make such a big difference for most uses.For tha main batteries NiMH may work, though the primitive charge circuit is not ideal. NiCd were a bit more tolerant to primitive over charging.One may opt for just remove the battery for charging.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 05, 2023, 04:29:52 am
content: Yeah, NiMh doesn't usually like trickle charging and should have a more sophisticated circuit. Still, at low rates they might work OK. Something that works in old camera equipment for the mercury cells is a zinc-air battery. Life is short, but they don't cost much either. I didn't remember that my units had anything other than the NiCads, but it's been a while.

author: Rax
author_rank: Super Contributor
timestamp: March 05, 2023, 04:53:44 pm
content: Not a very easy to work on unit. Also, the SM seems to be missing entirely any direction on replacing the batteries.BT5 seems to be placed in just about the most inaccessible place possible. Is its maintenance and servicing supposed to happen through the underside? I think I was able o locate silver oxide chemistry batteries (357), and a holder for them (finding this pair is awfully tedious...) at my usual parts store.One concerning aspect which I can't make sense of is some sort of oxidation I am seeing on the other side of the aluminum panel the Hg battery is affixed to. Has anyone seen a similar type of interaction? If yes, any thoughts on what this could be? I am worried this may be some sort of mercury contamination caused by that battery.Pictures below to depict this.

author: guenthert
author_rank: Frequent Contributor
timestamp: March 05, 2023, 05:28:27 pm
content: I don't think the batteries were meant to be serviceable, but rather intended for lifetime. On one of my units the NiCd batteries hadalmostenough life in them when I got it (decades after manufacturing).To remove the mercury battery you probably have to remove the underside and the side panel. I actually like the 419A's mechanical construction with generous dimensions (those were the times), Fluke's 845 seems to be more difficult to work on.Do yourself a favor and forget about replacing batteries for now. After removing the dead ones, put it on a lab power supply to test whether (or rather to which degree) it is still working. The Neon driving CdSe cell based modulator/demodulator is the weak point. If the Neons are flickering arhythmic then you're trouble and the batteries are your least concern.

author: Rax
author_rank: Super Contributor
timestamp: March 05, 2023, 05:57:46 pm
content: Quote from: guenthert on March 05, 2023, 05:28:27 pmThe problem is if that oxidized area is anything near suspicious of being contaminated with mercury (salts or whatever), I don't think I want this project on my bench, period.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 05, 2023, 07:51:00 pm
content: The critical part is if the LDRs are still good. AFAIK they can get slower and this degrades the performance. This is a special type of LDR that is no longer available.The unit has 2 pairs of LDRs and if really deparate, one could replace the pair used for demodulation with a different circuit (FETs). That part is a bit less critical. So only 2 good LDRs would really be needed.The neons can also fail, but here chances for a replacement are still there though also tricky.The battery cells are the more predictable part.Leaking mercury cells are usually not such a big problem - usually ther mercury should stay inside, even if the electrolyte leaks. Neither metallic mercury nor the oxide is water soluble.

author: Rax
author_rank: Super Contributor
timestamp: March 06, 2023, 02:44:24 am
content: My item of highest concern remains the strange areas of oxidation (?) on the aluminum panel.Has anyone seen this? Any thoughts as to what exactly it is?...

author: David Aurora
author_rank: Frequent Contributor
timestamp: March 06, 2023, 03:39:44 am
content: Fair bit of info in this video if memory serves:I picked one up in pretty ratty condition a couple years ago, got it basically working and cleaned up except that the leakage had gotten to and destroyed one switch mechanism (the read null button from memory)and I've never managed to find a replacement. It's on the to-do list, but kind of low priority

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 11, 2023, 07:16:09 pm
content: Could be wrong, but I vaguely remember that the mercury battery had to do with the variable null function. Never used it and didn't worry about not having a battery installed.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 11, 2023, 07:52:51 pm
content: The problem with the mercury cell is not getting a replacement - a silver based coin cell or with a slight mode to the divider a 3 V Li cell are reasonable easy.The question is how much mercury is in the leakage that may have come from the mercury cell.I personally would not worry that much, as mercury metal would hardly leak out. Mercury salts, if they are water soluble at all should be not that dangerrous, unless you eat them. So a good clean up and maybe get rid of the wipes with the collection of hazzardous waste should be OK. I don't see much chance for a mercury residue that would be mobile.The topic of leaking merury cells was up in the forum before:https://www.eevblog.com/forum/repair/clean-up-corroded-mercury-oxide-batteries/