I recently got an old Fluke 8120a multimeter, but it seems to not be working properly. All the tubes light up and display numbers, but all of the readings seem to be off by about 60%. I try the calibration procedure in the manual, but none of the potentiometers seem to have any change in what the tubes are displaying. Depending on what mode the meter is on, sometimes the tubes will flash the number “4”. I’m new to electronic repair, so maybe I am overlooking something obvious. Would it be wrong to assume one or more of the ICs are bad? (The amplifier, Binary counter, or decoder driver?) Thanks!
Hi, for a multi-meter of its age (pretty old!) anything is possible at this point but i am positive it can still be repaired with lots of patience and a more methodical approach. Any repair of any piece of equipment have to start with 2 basic steps. 1: very close visual inspection of every single component and wiring and a close-up (sometimes even under the magnifying glass) of all the soldering points on the soldered side of the PCB. A visual inspection most of the time can tell you a whole lot of whats actually wrong with it. 2: Consult the schematic and/or the service manual and test the supply voltages (presence and levels) to see if the match the schematic. A more in depth verification would then be to verify the level of the AC ripple across each voltage rail (using either a multi-meter on AC+DC mode or an oscilloscope). A general rule of thumb would be that at this point it is 99% likely that all supply filtering capacitors need replacing unless someone has already done that before you had acquired the meter. And yes Logic ICs (in the decoding/counting stages) do go bad over time as well. You are not wrong in suspecting that either but, if they are socket-ed then it makes life much easier to simply swap them than having to carefully de-solder them without damaging traces in the process !
PS: dirty switches (rotary, toggle, slide or piano) and potentiometers can also be problematic and need a proper and thorough cleaning as well.
@ovi4 Just thought I would fill everyone in on the solution, just so its documented. The ICs were indeed socketed, so I decided to just replace them and see what happens. It ended up being the binary counter IC. Swapped that out and now it works like a dream. I guess I got lucky. Thanks everyone!!