Fluke 8846A repair attempt - Page 1
Forum section: electronics / repair

author: giosif
author_rank: Frequent Contributor
timestamp: June 28, 2023, 09:30:37 pm
content: Hi all,I recently scored a Fluke 8846A off the bay, obviously, in non-working condition.Upon opening it up, I was greeted with several "beautiful landscapes", the most extensive one being shown in the photo attached.I would like to bring this meter back to some functioning state, so I've started looking for the components which are most obviously damaged.Some of them are burnt/damaged beyond recognition, or have simply been vaporized.But, with the help from TiN's very nice high-res photos from one of his repairs, I was able to build a list of components needing replacement (please see below).Suffice to say I expect the list to grow as I'm progressing with the investigation, but it's a start.I know this may be a tall order but, in case someone happens to have another unit opened up, I would appreciate it if they could check & confirm the identification of at least some of the components in the below list.Also, if someone happens to have a spare Fluke 2464674 resistor network or even an entire board from a broken 8846A, I would be interested.And here is the list (grouped per proximity of placement on the board):CR16 - OnSemi BAV99LR70 - 1kOhmC67 - ?U24 - Ti LF353M op-ampVR4 - OnSemi MMBZ5230BLT1GR194 - 200 OhmK2 - Panasonic/NAIS DS2E-SL2-DC5V relayZ1 - Fluke 2464674 resistor networkQ25 - OnSemi/Fairchild MMBF4117CR30 - OnSemi BAV99LR103 - 100 OhmQ54 - OnSemi MMBT3904LR582 - 100 OhmThanks!

author: TaylorD93
author_rank: Regular Contributor
timestamp: June 29, 2023, 11:43:51 am
content: Have you reached out to Fluke themselves? they may be able to confirm some of the component values if youre not sure.

author: giosif
author_rank: Frequent Contributor
timestamp: June 29, 2023, 05:13:54 pm
content: I had not but, following your suggestion, I have contacted them.I don't hold much hope, but it is worth a try.Thanks for the idea!

author: giosif
author_rank: Frequent Contributor
timestamp: July 01, 2023, 01:31:33 pm
content: Just to provide an update that I contacted Fluke support and I'm still waiting for them to respond.

author: giosif
author_rank: Frequent Contributor
timestamp: July 01, 2023, 01:40:57 pm
content: In the meantime, doing more troubleshooting, I determined the OnSemi NTP18N06L mosfet used for the -15V power rail has failed.That specific mosfet model seems to be obsolete, so I am looking for an equivalent replacement.And, so far, I came up with the following candidates:-Diodes DMT6009LCT-ST Micro STP80NF55-06-Vishay IRLZ24However, I am not that confident in my abilities to identify the main parameters when looking for substitutes for mosfets, so I would appreciate it if someone could validate the above list or suggest a more appropriate replacement.Thanks!

author: bdunham7
author_rank: Super Contributor
timestamp: July 01, 2023, 06:52:35 pm
content: Can you boot it up and take a photo of the front panel showing the system information (press INSTRUMENT SETUP, then SYSTEMS, then VERSIONS) ? You'll want to get your information from someone with the same hardware version if possible, although many things may be the same. If yours matches mine, I can pop the top and have a look.I'd say that if there has been an input overload severe enough to propagate to other sections of the board, it might be pretty much unrepairable. These are not easy beasts to fix in the first place and if the jolt was big enough to fry the input protection--which is fairly robust--and still pop those diode clamps, then there may be quite some mayhem elsewhere.If you can boot it up and get me that info, I'd also suggest removing the remains of R194 and cleaning the area with IPA and a Q-tip. It looks like the other components just have some soot on them. If Z1 is damaged, then I would think it is beyond economic repair.

author: giosif
author_rank: Frequent Contributor
timestamp: July 02, 2023, 10:04:37 pm
content: Quote from: bdunham7 on July 01, 2023, 06:52:35 pmMuch appreciated. I will do that after restoring the -15V rail.Quote from: bdunham7 on July 01, 2023, 06:52:35 pmI am aware my chances of bringing this back to full functionality are slim, but I have to try.Quote from: bdunham7 on July 01, 2023, 06:52:35 pmI've done that already. This was a photo I took when opening the meter for the first time.Quote from: bdunham7 on July 01, 2023, 06:52:35 pmYeah. Z1 has definitely suffered some abuse (not just soot from R194). I'm not entirely sure if it's damaged beyond repair, but I put it on my list of parts to look for, in case everything else is restored and this is the last item to fix.

author: bdunham7
author_rank: Super Contributor
timestamp: July 02, 2023, 10:14:25 pm
content: Quote from: giosif on July 02, 2023, 10:04:37 pmIs that an inguard or outguard power rail? If inguard, what does the meter do now when you turn it on? Also, can you tell what input circuit R194 is on? I'm just curious exactly how something like this happens--it certainly must have been sparkly. But the MOVs aren't incinerated It's too bad there's no schematic, as I'm sure you'll agree at this point.

author: giosif
author_rank: Frequent Contributor
timestamp: August 07, 2023, 08:38:51 pm
content: Coming back to trying to repair this meter, I was wondering if anyone might have traced the input circuit, especially the area from the original picture above (i.e. R194, RT1, RT2, Z1), area which, in my case, has suffered significant damage.The connection between R194 and RT1 seems flaky in my meter (I think they should be directly connected on one end) and I wonder what other connections might not work as intended/be reliable.I did take a picture of the area after removing the burned PCB material (and before filling the area with solder mask), but that's not revealing all the traces - please see attached photo.In hindsight, I should have taken a similar photo of the other side of the PCB as well.Quote from: bdunham7 on July 01, 2023, 06:52:35 pmTook a while, but here is the information:OutG SW = 1.0.649.16InG SW = ad000130OutG HW = 7b27e634InG HW = e4112603

author: giosif
author_rank: Frequent Contributor
timestamp: August 16, 2023, 08:43:17 pm
content: Just wondering if anyone might have the (reverse engineered) schematics for that area of the board already available.Alternatively, if someone based in the UK has the same meter or just its PCB (doesn't have to be in working state, as long as the input area is in good condition), if they would be willing to lend it for me to trace that area of the board, that would be much appreciated (would share the results here, of course).

author: Shonky
author_rank: Frequent Contributor
timestamp: August 16, 2023, 09:53:12 pm
content: Have you seenhttps://xdevs.com/fix/f8846a/? No schematics but some good photos and and internal schematic of Z1. Should be enough to validate if it's in the ballpark at least.

author: giosif
author_rank: Frequent Contributor
timestamp: August 17, 2023, 06:45:17 pm
content: Quote from: Shonky on August 16, 2023, 09:53:12 pmYes, I've read that article and the related ones, but that doesn't provide that many details on the input circuit.