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.