Repairing Fluke 8845A/8846A Multimeters - Part identification - Page 1
Forum section: electronics / testgear

author: Orting
author_rank: Newbie
timestamp: November 02, 2024, 04:50:09 am
content: Newbie here with an issue identifying parts on a Fluke 8845A. Everyone knows Fluke is not helpful in this matter, so desperatley in need of assistance. Have worked for many years with analog equipment especially tubes, so repairing a DMM is somewhat new to me. The 8845A does work, but no display. The PT shorted and fried several parts in the +60v rail including the power for the display. The Display works when connected to a working 8845A as well. Currently have +12volts on the +60v rail. Looking at posts here, I have discovered another user with the same issue in 2017, but didn’t see the resolution to the issue. Would like to replace all smd’s in the general area (7 devices), but need to translate the codes to order from mouser.Q5 TN4CWQ6 SAwQ7CR1 A11 1 (or could be an I)CR3 IL44UVR1 8J jVR2 N34Please find attached a picture of the affect area and components.Thanks in advance for help you guys can provide,Eric

author: AVGresponding
author_rank: Super Contributor
timestamp: November 02, 2024, 09:24:43 am
content: Not sure shotgunning all the parts will help, particularly. They are unlikely toallbe dead. I'd focus on the V-reg (8J most likely) and the MLCC caps first; if you remove them, does the short disappear?

author: Orting
author_rank: Newbie
timestamp: November 02, 2024, 04:54:23 pm
content: Thanks for the reply. The picture is of a good board and not the unit in question. For my unit there is a little more here. I have robbed parts from another Fluke 8845A and need to replace them too. Pulling parts from one board to another is haphazard at best and can damage the parts beyond being useful. At least one was damaged this way. When the PT shorted it took out several components as well with holes in the parts. One cap burnt thru to the middle layer of the pcb, leaving carbon behind that had to be dug out of the board. Hence the need to identify the parts with a part number that can be ordered. Is there a guide for part identification?I wouldn’t be this concerned, but the unit works. Relays function when pushing the display buttons and it can be shut down from the front panel. I even get relay indications when touching the leads together in continuity mode.

author: floobydust
author_rank: Super Contributor
timestamp: November 02, 2024, 06:37:32 pm
content: I found some doing a quick check, it helps to add the package when searching.CR1MMBD1501ALow Leakage diode "A11"CR3 BAT54S Dual Schottky Diode "IL44"VR1 MMBZ5234B 6.2V Zener "8J" SOT-23VR2MMSZ526356V Zener "N3" SOD-123Q2Si4532ADual Mosfet P-N 30V SOIC-8Q5TN2425N-Channel Enhancement-Mode Vertical DMOS FET 250V "TN4C" SOT-89Q6BSS123Mosfet N-ch SOT-23"SA"Q7 MMBT3904 NPN BJT "1AM"T1 PE-65728 Pulse Electronics AUI TransformerU2, U6NC7SZ14TinyLogic UHS Inverter with Schmitt Trigger Input SOT-23-5 OnsemiU3 not sure markingsedit:The circuit looks like a discrete HV linear regulator. For 60VDC out?Q7/R35 for current-limitingQ5 series-pass transistorI would measure the voltages on Q5 to see what is going on, I would expect it be a vulnerable part.

author: Orting
author_rank: Newbie
timestamp: November 02, 2024, 08:10:32 pm
content: You my friend are awesome!!!!!!!!! Now is this from your experience or is there something you researched to find the part numbers? I really appreciate your help, just would love to know how to find the numbers myself.Thank you for your help!!!!Eric

author: floobydust
author_rank: Super Contributor
timestamp: November 03, 2024, 06:50:35 am
content: Using the entire SMD marking code- it does not search well. Finding the root marking code, by removing a possible data code/manufacturing plant number etc. helps a lot.Another aspect that can help is a search query including the package name.It's mostly experience that tells me what type of part it could be, based on the circuit sections I can estimate.

author: Orting
author_rank: Newbie
timestamp: November 03, 2024, 06:47:26 pm
content: Okay, humor me here! I’m a hardheaded 67 year old medical retiree. Learned electronics in the Navy way too many moons ago! I’ve repaired and built amps for musicians for years and still work on things that I buy dead and repair them, however, trying to repair these DMMs is a royal pain! I just don’t have your experience, wish I did at this point.I didn’t ask about U2, but upon looking closely, the nomenclature looks like uA 6. The number 6 is at 90 degrees to the uA. The u is the Manufacturer correct, but seem to remember seeing uA as a manufacturers code. The only thing left is the 6, so how do I find a part with that?Dazed and confused!!!

author: floobydust
author_rank: Super Contributor
timestamp: November 04, 2024, 02:15:43 am
content: This is supposed to be fun, 3-10x magnifier and bright light includedPeople here will help out.SMT markings use the alphabet/numbers (not Greek) so U3 is possibly "HA" or "MA". It's a SOT-23-6 package. Not important right now, it seems to be part of the isolator circuit. I don't know this model Fluke really well but worthy of a repair. When I'm more awake I can look for the IC and let you know what steps I took.You don't need to know all the parts until you find something wrong and need to trace up or down stream.For now I would carefully (don't slip and short something out) on DCV measure the voltages on Q5 with needle probes. If you can't fit a probe there, then use nearby traces to other parts, or the vias etc. as a place to poke.TP5 looks like DGND, a place top keep the -ve probe of your working multimeter.

author: AVGresponding
author_rank: Super Contributor
timestamp: November 04, 2024, 06:16:48 am
content: Definitely HA. Worth noting also that the solder on that probable pass transistor looks like it might have seen some unscheduled heating

author: Kean
author_rank: Unknown
timestamp: November 04, 2024, 09:58:24 am
content: On U3 (which I think you meant rather than U2), I read it as HA or MA, followed by rotated 6, and a SOT-23-6 package. A better photo might help to confirm the first letter.One website that can help for looking up SMD part codes ishttps://smd.yooneed.one/- you start with the first two characters and then filter by package type.Except for diodes and transistors, I've found it rarely gets you direct to the part - there is just too many options and many manufacturers use shorter codes so they can include batch info. You often need to analyse the circuit and pin connections, as well as surrounding components, to narrow down what type of device it might be. Then start looking at datasheets of potential candidates and hope the datasheet actually mentions the SMD markings. In several cases I've seen the marking information get lost in newer datasheet revisions.Note that sometimes it isn't the first two characters that are significant, as manufacturers may use the leading characters to indicate batch or manufacturing location. More an more manufacturers do seem to be rotating these characters to help differentiate them (like the 6 on U3). Sometimes they will underline (or overline) certain characters - but you need to look at the datasheet, or the specific manufacturer part marking standards to understand what they denote. On these small parts, there is generally no indication of manufacturer, although sometime you do recognise a marking standard.