Fluke 789 repair - safety implications? - Page 1
Forum section: electronics / repair

author: atsthng
author_rank: Contributor
timestamp: February 11, 2015, 04:43:14 pm
content: I purchased a parts unit Fluke 789 off eBay with the intent of repairing it for own use or resale. The meter has been restored to full operation but I am wondering about the safety implications of the fix I implemented. Hope board members can weigh in on this.This Fluke 789 had current reading and sourcing failures. There are two current input jacks on this meter (A, mA) which are also used to source and regulate currents to test 4-20mA current loops for industrial sensor circuits.The failure was characterized to the 'A' input jack; the 'mA' input jack worked. Per the Fluke user manual, the impedance from the 'A' input jack to common was incorrect; it is supposed to measure in the order of 1.4 ohms but measured as several hundred ohms. This test is supposed to indicate whether the fuse protecting the input is OK or not.Inspection showed that the fuse was of the wrong value; it is supposed to be 440mA but the inserted fuse had a rating of 10A. Attempting to trace the circuit from the input jack led to the discovery of a blown trace in an internal PCB layer after the HRC holder. The trace connects the fuse holder to the rotary switch contacts, which then leads to the current sense resistors.The repair comprised of bypassing the blown trace with a short length of insulated wire (solid core 1.8A current rating 1kV insulation rating), replacing the sidactor before the blown trace, and installing the proper rated HRC fuse in place of the 10A one. The bodge wire roughly follows the path of the internal trace.In your opinion, does this fix compromise the safety of the user, or is it sufficiently robust?Personally I would be comfortable to use this on the bench since the primary means of protection - the fuse - is present before the wire. However, I would be wary of using it in an industrial setting. Let me know otherwise :-)Note that this fix only affects the 'A' input; the other inputs are not modified in any way.Pictures attached showing the physical routing of the wire used for the fix; where it crosses the blast wall for the HRC fuse, overall layout of the board and input circuit path.

author: ModemHead
author_rank: Frequent Contributor
timestamp: February 12, 2015, 01:40:36 am
content: The repair looks good to me. Offhand, I can't think of any safety issues with it.

author: retiredcaps
author_rank: Super Contributor
timestamp: February 12, 2015, 07:29:51 am
content: The repair looks okay to me, but I doubt Fluke would say it is okay for liability reasons especially since one of the internal pcb traces is compromised.I can't find which video it is exactly, but I seem to recall that the pace worldwide videos had an excellent repair video for multilayer pcb repairs (one of the rework and repair)?https://www.youtube.com/user/paceworldwide/videos

author: Shock
author_rank: Super Contributor
timestamp: February 12, 2015, 08:42:57 am
content: It could be viewed as more unsafe if one of those joints fail and the end of the jumper wire is floating around, so even better if you secure the wire in a few locations with an suitable adhesive or capton tape.It just doesn't get you out of liability, if anything happened the odds are you would be blamed. So if you are selling it you should disclose that the pcb has a repair, and that while the unit is fully functional the repair and unit is sold without any warranty and not guaranteed in any way.

author: atsthng
author_rank: Contributor
timestamp: February 12, 2015, 01:20:41 pm
content: Thanks for the good advice. I'll use some hot glue to hold the bodge wire in case the solder joint fails.I think the main risk with this fix occurs if someone replaces the HRC fuse with one that is incorrectly rated - given the relative thickness of the wire versus the original trace (from what I can see), there's going to be a lot more energy going through the wire compared to the trace - and what happens then I can't predict for sure. Likely the some trace downstream will blow up, but who knows without testing...If I did sell this in the future, the details of the repair would be disclosed. However, I've decided to keep the unit for my own use and clearly label it as not suitable for field use.

author: meeder
author_rank: Regular Contributor
timestamp: March 26, 2015, 02:30:54 pm
content: Repair looks good, some hotmelt to keep the wire in place will prevent issues.It is a great device though, used one for about 6 years and it really is a rock solid device, the current source and the ability to use it to power loop powered devices is a fantastic feature. I used it extensively when calibrating pressure and temperature sensors which work of a 4-20mA loop, simply the best multimeter I ever used in the field. The only nag was that there is no warning when the leads are in the wrong socket which is pretty inexcusable for a device that is this expensive (around €1100 if I am right).

author: atsthng
author_rank: Contributor
timestamp: March 26, 2015, 03:13:50 pm
content: Attached is a photo of the wire epoxied down to the PCB.A thread on StackExchange mentions that hot glue eventually fails, and the recommendation was either a neutral silicone rubber or epoxy.http://electronics.stackexchange.com/questions/18525/what-kind-of-glue-should-i-use-for-pcb-mounted-components-to-avoid-vibrationsI used a high strength epoxy from RS.https://sg.rs-online.com/web/p/epoxy-resins-adhesives/1593957/